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RFC 1142 - OSI IS-IS Intra-domain Routing Protocol


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Network Working Group                                     D. Oran, Editor
Request for Comments: 1142                        Digital Equipment Corp.
                                                            February 1990

	        OSI IS-IS Intra-domain Routing Protocol

Status of this Memo

   This RFC is a republication of ISO DP 10589 as a service to the
   Internet community.  This is not an Internet standard.
   Distribution of this memo is unlimited.

NOTE:  This is a bad ASCII version of this document.  The official
document is the PostScript file, which has the diagrams in place.
Please use the PostScript version of this memo.

ISO/IEC DIS 10589

Information technology Telecommunications and information exchange
between systems Interme diate system to Intermediate system
Intra-Domain routeing exchange protocol for use in Conjunction with
the Protocol for providing the Connectionless- mode Network Service
(ISO 8473) Technologies de l'information Communication de donnies et
ichange d'information entre systhmes Protocole intra-domain de routage
d'un systhme intermediare ` un systhme intermediare ` utiliser
conjointement avec le protocole fournissant le service de riseau en
mode sans connexion (ISO 8473) UDC 00000.000 : 000.0000000000
Descriptors:

Contents
	Introduction		iv
	1 	Scope and Field of Application	1
	2 	References	1
	3 	Definitions	2
	4 	Symbols and Abbreviations 	3
	5 	Typographical Conventions	4
	6 	Overview of the Protocol	4
	7 	Subnetwork Independent Functions	9
	8 	Subnetwork Dependent Functions	35
	9 	Structure and Encoding of PDUs	47
	10 	System Environment	65
	11 	System Management 	67
	12 	Conformance	95
	Annex A 	PICS Proforma	99
	Annex B 	Supporting Technical Material	105
	Annex C 	Implementation Guidelines and Examples	109
	Annex D 	Congestion Control and Avoidance	115

Introduction

This Protocol is one of a set of International Standards produced to
facilitate the interconnection of open systems. The set of standards
covers the services and protocols re quired to achieve such
interconnection.  This Protocol is positioned with respect to other
related standards by the layers defined in the ISO 7498 and by the
structure defined in the ISO 8648. In particular, it is a protocol of
the Network Layer. This protocol permits Intermediate Systems within a
routeing Domain to exchange configuration and routeing information to
facilitate the operation of the route ing and relaying functions of
the Network Layer.  The protocol is designed to operate in close
conjunction with ISO 9542 and ISO 8473.  ISO 9542 is used to establish
connectivity and reachability between End Systems and Inter mediate
Systems on individual Subnetworks. Data is carried by ISO 8473.  The
related algo rithms for route calculation and maintenance are also
described.  The intra-domain ISIS routeing protocol is intended to
support large routeing domains consisting of combinations of many
types of subnetworks. This includes point-to-point links, multipoint
links, X.25 subnetworks, and broadcast subnetworks such as ISO 8802
LANs.  In order to support large routeing domains, provision is made
for Intra-domain routeing to be organised hierarchically. A large
domain may be administratively divided into areas.  Each system
resides in exactly one area. Routeing within an area is referred to as
Level 1 routeing. Routeing between areas is referred to as Level 2
routeing.  Level 2 Intermediate systems keep track of the paths to
destination areas. Level 1 Intermediate systems keep track of the
routeing within their own area. For an NPDU destined to another area,
a Level 1 Intermediate system sends the NPDU to the nearest level 2 IS
in its own area, re gardless of what the destination area is. Then the
NPDU travels via level 2 routeing to the destination area, where it
again travels via level 1 routeing to the destination End System.

Information technology

Telecommunications and information exchange between systems
Intermediate system to Intermediate system Intra-Domain routeing
exchange protocol for use in Conjunction with the Protocol for
providing the Connectionless-mode Network Service (ISO 8473)

1 Scope and Field of Application

This International Standard specifies a protocol which is used by
Network Layer entities operating ISO 8473 in In termediate Systems to
maintain routeing information for the purpose of routeing within a
single routeing domain. The protocol herein described relies upon the
provision of a connectionless-mode underlying service.11See ISO 8473
and its Addendum 3 for the mechanisms necessary to realise this
service on subnetworks based on ISO 8208, ISO 8802, and the OSI Data
Link Service.

This Standard specifies: 

a)procedures for the transmission of configuration and 
routeing information between network entities resid
ing in Intermediate Systems within a single routeing 
domain; 

b)the encoding of the protocol data units used for the 
transmission of the configuration and routeing infor
mation; 

c)procedures for the correct interpretation of protocol 
control information; and 

d)the functional requirements for implementations 
claiming conformance to this Standard.

The procedures are defined in terms of: 

a)the interactions between Intermediate system Network 
entities through the exchange of protocol data units; 
and 

b)the interactions between a Network entity and an un
derlying service provider through the exchange of 
subnetwork service primitives.

c)the constraints on route determination which must be 
observed by each Intermediate system when each has 
a routeing information base which is consistent with 
the others.

2 References

2.1  Normative References

The following standards contain provisions which, through reference in
this text, constitute provisions of this Interna tional Standard.  At
the time of publication, the editions in dicated were valid. All
standards are subject to revision, and parties to agreements based on
this International Stan dard are encouraged to investigate the
possibility of apply ing the most recent editions of the standards
listed below.  Members of IEC and ISO maintain registers of currently
valid International Standards.  ISO 7498:1984, Information processing
systems Open Systems Interconnection Basic Reference Model.  ISO
7498/Add.1:1984, Information processing systems Open Systems
Interconnection Basic Reference Model Addendum 1: Connectionless-mode
Transmission.  ISO 7498-3:1989, Information processing systems Open
Systems Interconnection Basic Reference Model Part 3: Naming and
Addressing.  ISO 7498-4:1989, Information processing systems Open
Systems Interconnection Basic Reference Model Part 4: Management
Framework.  ISO 8348:1987, Information processing systems Data
communications Network Service Definition.  ISO 8348/Add.1:1987,
Information processing systems Data communications Network Service
Definition Addendum 1: Connectionless-mode transmission.  ISO
8348/Add.2:1988, Information processing systems Data communications
Network Service Definition Addendum 2: Network layer addressing.  ISO
8473:1988, Information processing systems Data communications Protocol
for providing the connectionless-mode network service.  ISO
8473/Add.3:1989, Information processing systems Telecommunications and
information exchange between
systems  Protocol for providing the connectionless-
mode network service  Addendum 3: Provision of the 
underlying service assumed by ISO 8473 over 
subnetworks which provide the OSI data link service.
ISO 8648:1988,  Information processing systems  Open 
Systems Interconnection  Internal organisation of the 
Network Layer.
ISO 9542:1988, Information processing systems  Tele
communications and information exchange between sys
tems  End system to Intermediate system Routeing ex
change protocol for use in conjunction with the protocol 
for providing the connectionless -mode network service 
(ISO 8473).
ISO 8208:1984, Information processing systems  Data 
communications  X.25 packet level protocol for Data 
terminal equipment
ISO 8802:1988, Information processing systems  Tele
communications and information exchange between sys
tems  Local area networks.
ISO/TR 9575:1989, Information technology   Telecom
munications and information exchange between systems 
 OSI Routeing Framework.
ISO/TR 9577:1990, Information technology   Telecom
munications and information exchange between systems 
 Protocol Identification in the Network Layer.
ISO/IEC DIS 10165-4:, Information technology  Open 
systems interconnection  Management Information Serv
ices  Structure of Management Information Part 4: 
Guidelines for the Definition of Managed Objects.
ISO/IEC 10039:1990, IPS-T&IEBS  MAC Service Defini
tion.

2.2 Other References

The following references are helpful in describing some of 
the routeing algorithms: 

McQuillan, J. et. al., The New Routeing Algorithm for the 
ARPANET, IEEE Transactions on Communications, May 
1980.

Perlman, Radia, Fault-Tolerant Broadcast of Routeing In
formation, Computer Networks, Dec. 1983. Also in IEEE 
INFOCOM 83, April 1983.

Aho, Hopcroft, and Ullman, Data Structures and Algo
rithms, P204208  Dijkstra algorithm.

3 Definitions

3.1 Reference Model definitions

This International Standard  makes use of the following 
terms defined in ISO 7498:

a)Network Layer
b)Network Service access point
c)Network Service access point address
d)Network entity
e)Routeing
f)Network protocol
g)Network relay
h)Network protocol data unit

3.2 Network Layer architecture 
definitions

This International Standard makes use of the following 
terms defined in ISO 8648:

a)Subnetwork
b)End system
c)Intermediate system
d)Subnetwork service
e)Subnetwork Access Protocol 
f)Subnetwork Dependent Convergence Protocol 
g)Subnetwork Independent Convergence Protocol

3.3 Network Layer addressing 
definitions

This International Standard makes use of the following 
terms defined in ISO 8348/Add.2:

a)Subnetwork address
b)Subnetwork point of attachment
c)Network Entity Title
3.4 Local Area Network Definitions
 This International Standard makes use of the following 
terms defined in ISO 8802: 
a)Multi-destination address 
b)Media access control
c)Broadcast medium
3.5 Routeing Framework Definitions
 This document makes use of the following terms defined in 
ISO/TR 9575: 
a)Administrative Domain 
b)Routeing Domain 
c)Hop 
d)Black hole 

3.6 Additional Definitions
For the purposes of this International Standard, the follow
ing definitions apply: 
3.6.1   
Area: A routeing subdomain which maintains de
tailed routeing information about its own internal 
composition, and also maintains routeing informa
tion which allows it to reach other routeing subdo
mains. It corresponds to the Level 1 subdomain. 
3.6.2   
Neighbour: An adjacent system reachable by tra
versal of a single subnetwork by a PDU. 
3.6.3   
Adjacency: A portion of the local routeing infor
mation which pertains to the reachability of a sin
gle neighbour ES or IS over a single circuit.
Adjacencies are used as input to the Decision Proc
ess for forming paths through the routeing domain.
A separate adjacency is created for each neighbour 
on a circuit, and for each level of routeing (i.e. 
level 1 and level 2) on a broadcast circuit.
3.6.4   
Circuit: The subset of the local routeing informa
tion base pertinent to a single local SNPA. 
3.6.5   
Link: The communication path between two 
neighbours. 
A Link is up when communication is possible 
between the two SNPAs.
3.6.6   
Designated IS: The Intermediate system on a 
LAN which is designated to perform additional du
ties. In particular it generates Link State PDUs on 
behalf of the LAN, treating the LAN as a 
pseudonode. 
3.6.7   
Pseudonode: Where a broadcast subnetwork has n 
connected Intermediate systems, the broadcast 
subnetwork itself is considered to be a 
pseudonode. 
The pseudonode has links to each of the n Interme
diate systems and each of the ISs has a single link 
to the pseudonode (rather than n-1 links to each of 
the other Intermediate systems). Link State PDUs 
are generated on behalf of the pseudonode by the 
Designated IS. This is depicted below in figure 1.
3.6.8   
Broadcast subnetwork: A subnetwork which sup
ports an arbitrary number of End systems and In

termediate systems and additionally is capable of 
transmitting a single SNPDU to a subset of these 
systems in response to a single SN_UNITDATA 
request.  
3.6.9   
General topology subnetwork: A subnetwork 
which supports an arbitrary number of End sys
tems and Intermediate systems, but does not sup
port a convenient multi-destination connectionless 
trans

mission facility, as does a broadcast sub

net

work. 
3.6.10   
Routeing Subdomain: a set of Intermediate sys
tems and End systems located within the same 
Routeing domain.
3.6.11   
Level 2 Subdomain: the set of all Level 2 Inter
mediate systems in a Routeing domain.
4 Symbols and Abbreviations 
4.1 Data Units
PDU	Protocol Data Unit
SNSDU	Subnetwork Service Data Unit
NSDU	Network Service Data Unit
NPDU	Network Protocol Data Unit
SNPDU	Subnetwork Protocol Data Unit

4.2 Protocol Data Units
ESH PDU	ISO 9542 End System Hello Protocol Data 
Unit 
ISH PDU	ISO 9542 Intermediate System Hello Protocol 
Data Unit
RD PDU	ISO 9542 Redirect Protocol Data Unit
IIH	Intermediate system to Intermediate system 
Hello Protocol Data Unit
LSP	Link State Protocol Data Unit 
SNP	Sequence Numbers Protocol Data Unit
CSNP	Complete Sequence Numbers Protocol Data 
Unit
PSNP	Partial Sequence Numbers Protocol Data Unit

4.3 Addresses
AFI	Authority and Format Indicator
DSP	Domain Specific Part
IDI	Initial Domain Identifier
IDP	Initial Domain Part
NET	Network Entity Title
NSAP	Network Service Access Point
SNPA	Subnetwork Point of Attachment

4.4 Miscellaneous
DA	Dynamically Assigned
DED	Dynamically Established Data link
DTE	Data Terminal Equipment
ES	End System
IS	Intermediate System
L1	Level 1
L2	Level 2
LAN	Local Area Network
MAC	Media Access Control
NLPID	Network Layer Protocol Identifier
PCI	Protocol Control Information
QoS	Quality of Service
SN	Subnetwork
SNAcP	Subnetwork Access Protocol
SNDCP	Subnetwork Dependent Convergence Protocol
SNICP	Subnetwork Independent Convergence Proto
col
SRM	Send Routeing Message
SSN	Send Sequence Numbers Message
SVC	Switched Virtual Circuit
5 Typographical Conventions
This International Standard makes use of the following ty
pographical conventions:
a)Important terms and concepts appear in italic type 
when introduced for the first time;
b)Protocol constants and management parameters appear 
in sansSerif type with multiple words run together. 
The first word is lower case, with the first character of 
subsequent words capitalised;
c)Protocol field names appear in San Serif type with 
each word capitalised.
d)Values of constants, parameters, and protocol fields 
appear enclosed in double quotes.

6 Overview of the Protocol
6.1 System Types
There are the following types of system:
End Systems: These systems deliver NPDUs to other sys
tems and receive NPDUs from other systems, but do 
not relay NPDUs. This International Standard does 
not specify any additional End system functions be
yond those supplied by ISO 8473 and ISO 9542.
Level 1 Intermediate Systems: These systems deliver and 
receive NPDUs from other systems, and relay 
NPDUs from other source systems to other destina
tion systems. They route directly to systems within 
their own area, and route towards a level 2 Interme
diate system when the destination system is in a dif
ferent area.
Level 2 Intermediate Systems: These systems act as Level 1 
Intermediate systems in addition to acting as a sys
tem in the subdomain consisting of level 2 ISs. Sys
tems in the level 2 subdomain route towards a desti
nation area, or another routeing domain.
6.2 Subnetwork Types
There are two generic types of subnetworks supported.
a)broadcast subnetworks: These are multi-access 
subnetworks that support the capability of addressing 
a group of attached systems with a single NPDU, for 
instance ISO 8802.3 LANs.
b)general topology subnetworks: These are modelled as 
a set of point-to-point links each of which connects 
exactly two systems.
There are several generic types of general topology 
subnetworks:
1)multipoint links: These are links between more 
than two  systems, where one system is a primary  
system, and the remaining systems are secondary 
(or slave) systems. The primary is capable of direct 
communication with any of the secondaries, but 
the secondaries cannot communicate directly 
among themselves. 
2)permanent point-to-point links: These are links 
that stay connected at all times (unless broken, or 
turned off by system management), for instance 
leased lines or private links.
3)dynamically established data links (DEDs): these 
are links over connection oriented facilities, for in
stance X.25, X.21, ISDN, or PSTN networks.
Dynamically established data links can be used in one 
of two ways:
i)static point-to-point (Static): The call is estab
lished upon system management action and 

cleared only on system management action (or 
failure).
ii)dynamically assigned (DA): The call is estab
lished upon receipt of traffic, and brought 
down on timer expiration when idle. The ad
dress to which the call is to be established is  
determined dynamically from information in 
the arriving NPDU(s). No ISIS routeing 
PDUs are exchanged between ISs on a DA cir
cuit.
All subnetwork types are treated by the Subnetwork Inde
pendent functions as though they were connectionless 
subnetworks, using the Subnetwork Dependent Conver
gence functions of ISO 8473 where necessary to provide a 
connectionless subnetwork service. The  Subnetwork De
pendent functions do, however, operate differently on 
connectionless and connection-oriented subnetworks.
6.3 Topologies
A single organisation may wish to divide its Administrative 
Domain into a number of separate Routeing Domains. 
This has certain advantages, as described in ISO/TR 9575. 
Furthermore, it is desirable for an intra-domain routeing 
protocol to aid in the operation of an inter-domain routeing 
protocol, where such a protocol exists for interconnecting 
multiple administrative domains.
In order to facilitate the construction of such multi-domain 
topologies, provision is made for the entering of static 
inter-domain routeing information. This information is pro
vided by a set of Reachable Address Prefixes entered by 
System Management at the ISs which have links which 
cross routeing domain boundaries. The prefix indicates that 
any NSAPs whose NSAP address matches the prefix may 
be reachable via  the SNPA with which the prefix is associ
ated. Where the subnetwork to which this SNPA is con
nected is a general topology subnetwork supporting dy
namically established data links, the prefix also has associ
ated with it the required subnetwork addressing 
information, or an indication that it may be derived from 
the destination NSAP address (for example, an X.121 DTE 
address may sometimes be obtained from the IDI of the 
NSAP address).
The Address Prefixes are handled by the level 2 routeing al
gorithm in the same way as information about a level 1 area 
within the domain. NPDUs with a destination address 
matching any of the prefixes present on any Level 2 Inter
mediate System within the domain can therefore be relayed 
(using level 2 routeing) by that IS and delivered out of the 
domain. (It is assumed that the routeing functions of the 
other domain will then be able to deliver the NPDU to its 
destination.) 
6.4 Addresses
Within a routeing domain that conforms to this standard, 
the Network entity titles of Intermediate systems shall be 
structured as described in 7.1.1.
All systems shall be able to generate and forward data 
PDUs containing NSAP addresses in any of the formats 
specified by ISO 8348/Add.2. However,  NSAP addresses 

of End systems should be structured as described in 7.1.1 in 
order to take full advantage of ISIS routeing. Within such 
a domain it is still possible for some End Systems to have 
addresses assigned which do not conform to 7.1.1, provided 
they meet the more general requirements of 
ISO 8348/Add.2, but they may require additional configura
tion and be subject to inferior routeing performance.
6.5  Functional Organisation
The intra-domain ISIS routeing functions are divided into 
two groups
-Subnetwork Independent Functions 
-Subnetwork Dependent Functions
6.5.1 Subnetwork Independent Functions
The Subnetwork Independent Functions supply full-duplex 
NPDU transmission between any pair of neighbour sys
tems. They are independent of the specific subnetwork or 
data link service operating below them, except for recognis
ing two generic types of subnetworks: 
-General Topology Subnetworks, which include 
HDLC point-to-point, HDLC multipoint, and dynami
cally established data links (such as X.25, X.21, and 
PSTN links), and  
-Broadcast Subnetworks, which include ISO 8802 
LANs.
The following Subnetwork Independent Functions are iden
tified
-Routeing. The routeing function determines NPDU 
paths. A path is the sequence of connected systems 
and links between a source ES and a destination ES.
The combined knowledge of all the Network Layer 
entities of all the Intermediate systems within a route
ing domain is used to ascertain the existence of a path, 
and route the NPDU to its destination. The routeing 
component at an Intermediate system has the follow
ing specific functions:
7It extracts and interprets the routeing PCI in an 
NPDU. 
7It performs NPDU forwarding based on the desti
nation address. 
7It manages the characteristics of the path. If a sys
tem or link fails on a path, it finds an alternate 
route.  
7It interfaces with the subnetwork dependent func
tions to receive reports concerning an SNPA 
which has become unavailable, a system that has 
failed, or the subsequent recovery of an SNPA or 
system.  
7It informs the ISO 8473 error reporting function 
when the forwarding function cannot relay an 
NPDU, for instance when the destination is un
reachable or when the NPDU would have needed 

to be segmented and the NPDU requested no seg
mentation.
-Congestion control. Congestion control manages the 
resources used at each Intermediate system. 
6.5.2 Subnetwork Dependent Functions 
The subnetwork dependent functions mask the characteris
tics of the subnetwork or data link service from the 
subnetwork independent functions. These include:
-Operation of the Intermediate system functions of 
ISO 9542 on the particular subnetwork, in order to
7Determine neighbour Network entity title(s) and 
SNPA address(es) 
7Determine the SNPA address(s) of operational In
termediate systems 
-Operation of the requisite Subnetwork Dependent 
Convergence Function as defined in ISO 8473 and its 
Addendum 3, in order to perform 
7Data link initialisation 
7Hop by hop fragmentation over subnetworks with 
small maximum SNSDU sizes 
7Call establishment and clearing on dynamically es
tablished data links
6.6 Design Goals
This International Standard supports the following design 
requirements. The correspondence with the goals for OSI 
routeing stated in ISO/TR 9575 are noted.
-Network Layer Protocol Compatibility. It is com
patible with ISO 8473 and ISO 9542. (See clause 7.5 
of ISO/TR 9575),
-Simple End systems: It requires no changes to end 
systems, nor any functions beyond those supplied by 
ISO 8473 and ISO 9542. (See clause 7.2.1 of ISO/TR 
9575),
-Multiple Organisations: It allows for multiple route
ing and administrative domains through the provision 
of static routeing information at domain boundaries. 
(See clause 7.3 of ISO/TR 9575),
-Deliverability It accepts and delivers NPDUs ad
dressed to reachable destinations and rejects NPDUs 
addressed to destinations known to be unreachable. 
-Adaptability. It adapts to topological changes within 
the routeing domain, but not to traffic changes, except 
potentially as indicated by local queue lengths. It 
splits traffic load on multiple equivalent paths. (See 
clause 7.7 of ISO/TR 9575),
-Promptness. The period of adaptation to topological 
changes in the domain is a reasonable function of the 
domain diameter (that is, the maximum logical dis

tance between End Systems within the domain) and 
Data link speeds. (See clause 7.4 of ISO/TR 9575),
-Efficiency. It is both processing and memory effi
cient. It does not create excessive routeing traffic 
overhead. (See clause 7.4 of ISO/TR 9575),
-Robustness. It recovers from transient errors such as 
lost or temporarily incorrect routeing PDUs. It toler
ates imprecise parameter settings. (See clause 7.7 of 
ISO/TR 9575),
-Stability. It stabilises in finite time to good routes, 
provided no continuous topological changes or con
tinuous data base corruptions occur. 
-System Management control. System Management 
can control many routeing functions via parameter 
changes, and inspect parameters, counters, and routes. 
It will not, however, depend on system management 
action for correct behaviour. 
-Simplicity. It is sufficiently simple to permit perform
ance tuning and failure isolation. 
-Maintainability. It provides mechanisms to detect, 
isolate, and repair most common errors that may affect 
the routeing computation and data bases. (See clause 
7.8 of ISO/TR 9575),
-Heterogeneity. It operates over a mixture of network 
and system types, communication technologies, and 
topologies. It is capable of running over a wide variety 
of subnetworks, including, but not limited to: ISO 
8802 LANs, ISO 8208 and X.25 subnetworks, PSTN 
networks, and the OSI Data Link Service. (See clause 
7.1 of ISO/TR 9575),
-Extensibility. It accommodates increased routeing 
functions, leaving earlier functions as a subset. 
-Evolution. It allows orderly transition from algorithm 
to algorithm without shutting down an entire domain.
-Deadlock Prevention. The congestion control compo
nent prevents buffer deadlock.
-Very Large Domains. With hierarchical routeing, and 
a very large address space, domains of essentially un
limited size can be supported. (See clause 7.2 of 
ISO/TR 9575),
-Area Partition Repair. It permits the utilisation of 
level 2 paths to repair areas which become partitioned 
due to failing level 1 links or ISs. (See clause 7.7 of 
ISO/TR 9575),
-Determinism. Routes are a function only of the physi
cal topology, and not of history. In other words, the 
same topology will always converge to the same set of 
routes. 
-Protection from Mis-delivery. The probability of  
mis-delivering a NPDU, i.e. delivering it to a Trans
port entity in the wrong End System, is extremely low. 

-Availability. For domain topologies with cut set 
greater than one, no single point of failure will parti
tion the domain. (See clause 7.7 of ISO/TR 9575),
-Service Classes. The service classes of transit delay, 
expense22Expense is referred to as cost in ISO 8473. The latter term is
not used here because of possible confusion with the more general usage
of the term to 
indicate path cost according to any routeing metric.
, and residual error probability of ISO 8473 
are supported through the optional inclusion of multi
ple routeing metrics.
-Authentication. The protocol is capable of carrying 
information to be used for the authentication of Inter
mediate systems in order to increase the security and 
robustness of a routeing domain. The specific mecha
nism supported in this International Standard how
ever, only supports a weak form of authentication us
ing passwords, and thus is useful only for protection 
against accidental misconfiguration errors and does 
not protect against any serious security threat. In the 
future, the algorithms may be enhanced to provide 
stronger forms of authentication than can be provided 
with passwords without needing to change the PDU 
encoding or the protocol exchange machinery.
6.6.1 Non-Goals
The following are not within the design scope of the intra-
domain ISIS routeing protocol described in this Interna
tional Standard:
-Traffic adaptation. It does not automatically modify 
routes based on global traffic load.
-Source-destination routeing. It does not determine 
routes by source as well as destination.
-Guaranteed delivery. It  does not guarantee delivery 
of all offered NPDUs. 
-Level 2 Subdomain Partition Repair. It will not util
ise Level 1 paths to repair a level 2 subdomain parti
tion. For full logical connectivity to be available, a 
connected level 2 subdomain is required.
-Equal treatment for all ES Implementations. The 
End system poll function defined in 8.4.5 presumes 
that End systems have implemented the Suggested ES 
Configuration Timer option of ISO 9542. An End sys
tem which does not implement this option may experi
ence a temporary loss of connectivity following cer
tain types of topology changes on its local 
subnetwork.
6.7 Environmental Requirements
For correct operation of the protocol, certain guarantees are 
required from the local environment and the Data Link 
Layer. 
The required local environment guarantees are:
a)Resource allocation such that the certain minimum re
source guarantees can be met, including

1)memory (for code, data, and buffers)
2)processing;
See 12.2.5 for specific performance levels required for 
conformance
b)A quota of buffers sufficient to perform routeing func
tions;
c)Access to a timer or notification of specific timer expi
ration; and
d)A very low probability of corrupting data.
The required subnetwork guarantees for point-to-point links 
are: 
a)Provision that both source and destination systems 
complete start-up before PDU exchange can occur;
b)Detection of remote start-up;
c)Provision that no old PDUs be received after start-up 
is complete;
d)Provision that no PDUs transmitted after a particular 
startup is complete are delivered out of sequence;
e)Provision that failure to deliver a specific subnetwork 
SDU will result in the timely disconnection of the 
subnetwork connection in both directions and that this 
failure will be reported to both systems;  and
f)Reporting of other subnetwork failures and degraded 
subnetwork conditions.
The required subnetwork guarantees for broadcast links are:
a)Multicast capability, i.e., the ability to address a subset 
of all connected systems with a single PDU;
b)The following events are low probability, which 
means that they occur sufficiently rarely so as not to 
impact performance, on the order of once per  thou
sand PDUs
1)Routeing PDU non-sequentiality, 
2)Routeing PDU loss due to detected corruption; and
3)Receiver overrun;
c)The following events are very low probability, 
which means performance will be impacted unless 
they are extremely rare, on the order of less than one 
event per four years
1)Delivery of NPDUs with undetected data corrup
tion; and
2)Non-transitive connectivity, i.e. where system A 
can receive transmissions from systems B and C, 
but system B cannot receive transmissions from 
system C. 

The following services are assumed to be not available 
from broadcast links:
a)Reporting of failures and degraded subnetwork condi
tions that result in NPDU loss, for instance receiver 
failure. The routeing functions are designed to account 
for these failures.
6.8 Functional Organisation of 
Subnetwork Independent 
Components
The Subnetwork Independent Functions are broken down 
into more specific functional components. These are de
scribed briefly in this sub-clause and in detail in clause 7. 
This International Standard uses a functional decomposition 
adapted from the model of routeing presented in clause 5.1 
of ISO/TR 9575. The decomposition is not identical to that 
in ISO/TR 9575, since that model is more general and not 
specifically oriented toward a detailed description of intra-
domain routeing functions such as supplied by this proto
col.

The functional decomposition is shown below in figure 2.
6.8.1 Routeing
The routeing processes are: 
-Decision Process 
-Update Process
NOTE  this comprises both the Information Collection 
and Information Distribution components identified in 
ISO/TR 9575.
-Forwarding Process 
-Receive Process
6.8.1.1 Decision Process
This process calculates routes to each destination in the do
main.  It is executed separately for level 1 and level 2 route
ing, and separately within each level for each of the route
ing metrics supported by the Intermediate system. It uses 
the Link State Database, which consists of information 

from the latest Link State PDUs from every other Interme
diate system in the area, to compute shortest paths from this 
IS to all other systems in the area  9in figure 2. The 
Link State Data Base is maintained by the Update Process.
Execution of the Decision Process results in the determina
tion of [circuit, neighbour] pairs (known as adjacencies), 
which are stored in the appropriate Forwarding Information 
base  10  and used by the Forwarding process as paths 
along which to forward NPDUs.
Several of the parameters in the routeing data base that the 
Decision Process uses are determined by the implementa
tion. These include:
-maximum number of Intermediate and End systems 
within the IS's area;
-maximum number of Intermediate and End system 
neighbours of the IS, etc., 
so that databases can be sized appropriately. Also parame
ters such as 
-routeing metrics for each circuit; and 
-timers 
can be adjusted for enhanced performance. The complete 
list of System Management set-able parameters is listed in 
clause 11. 
6.8.1.2 Update Process 
This process constructs, receives and propagates Link State 
PDUs. Each Link State PDU contains information about the 
identity and routeing metric values of the  adjacencies of 
the IS that originated the Link State PDU. 
The Update Process receives Link State and Sequence 
Numbers PDUs from the Receive Process  4in figure 
2. It places new routeing information in the routeing infor
mation base 6 and propagates routeing information to 
other Intermediate systems  7and 8 .
General characteristics of the Update Process are: 
-Link State PDUs are generated  as a result of topologi
cal changes, and also periodically. They may also be 
generated indirectly as a result of System Manage
ment actions (such as changing one of the routeing 
metrics for a circuit). 
-Level 1 Link State PDUs are propagated to all Inter
mediate systems within an area, but are not propa
gated out of an area. 
-Level 2 Link State PDUs are propagated to all Level 2 
Intermediate systems in the domain.
-Link State PDUs are not propagated outside of a do
main. 

-The update process, through a set of System Manage
ment parameters, enforces an upper bound on the 
amount of routeing traffic overhead it generates.
6.8.1.3 Forwarding Process
This process supplies and manages the buffers necessary to 
support NPDU relaying to all destinations. 
It receives, via the Receive Process, ISO 8473 PDUs to be 
forwarded  5 in figure 2.
It performs a lookup in the appropriate33The appropriate Forwarding
Database is selected by choosing a routeing metric based on fields in
the QoS Maintenance option of ISO 8473.
 Forwarding Data
base  11  to determine the possible output adjacencies 
to use for forwarding to a given destination, chooses one 
adjacency  12, generates error indications to ISO 8473 
 14 , and  signals ISO 9542 to issue Redirect PDUs  
13.
6.8.1.4 Receive Process
The Receive Process obtains its inputs from the following 
sources
-received PDUs with the NPID of Intra-Domain route
ing  2 in figure 2, 
-routeing information derived by the ESIS protocol 
from the receipt of ISO 9542 PDUs  1;  and
-ISO 8473 data PDUs handed to the routeing function 
by the ISO 8473 protocol machine  3. 
It then performs the appropriate actions, which may involve 
passing the PDU to some other function (e.g. to the For
warding Process for forwarding  5).
7 Subnetwork Independent 
Functions
This clause describes the algorithms and associated data
bases used by the routeing functions. The managed objects 
and attributes defined for System Management purposes are 
described in clause 11.
The following processes and data bases are used internally 
by the subnetwork independent functions. Following each 
process or data base title, in parentheses, is the type of sys
tems which must keep the database. The system types are 
L2 (level 2 Intermediate system), and L1 (level 1 Inter
mediate system). Note that a level 2 Intermediate system is 
also a level 1 Intermediate system in its home area, so it 
must keep level 1 databases as well as level 2 databases. 

Processes:
-Decision Process (L2, L1) 
-Update Process (L2, L1) 
-Forwarding Process (L2, L1) 
-Receive Process (L2, L1)
Databases:
-Level 1 Link State data base (L2, L1) 
-Level 2 Link State data base (L2) 
-Adjacency Database (L2, L1) 
-Circuit Database (L2, L1) 
-Level 1 Shortest Paths Database (L2, L1) 
-Level 2 Shortest Paths Database (L2) 
-Level 1 Forwarding Databases  one per routeing 
metric  (L2, L1) 
-Level 2 Forwarding Database  one per routeing 
metric  (L2)
7.1 Addresses 
The NSAP addresses and NETs of systems are variable 
length quantities that conform to the requirements of ISO 
8348/Add.2. The corresponding NPAI contained in ISO 
8473 PDUs and in this protocol's PDUs (such as LSPs and 
IIHs) must use the preferred binary encoding; the underly
ing syntax for this information may be either abstract binary 
syntax or abstract decimal syntax. Any of the AFIs and 
their corresponding DSP syntax may be used with this pro
tocol.
7.1.1 NPAI Of Systems Within A Routeing 
Domain 
Figure 3 illustrates the structure of an encoded NSAP ad
dress or NET. 

The structure of the NPAI will be interpreted in the follow
ing way by the protocol described in this international stan
dard: 
Area Address 	
address of one area within a routeing domain  a 
variable length quantity consisting of the entire high-
order part of the NPAI, excluding the ID and SEL 
fields, defined below. 
ID	System identifier  a variable length field from 1 to 
8 octets (inclusive). Each routeing domain employ
ing this protocol shall select a single size for the ID 
field and all Intermediate systems in the routeing do
main shall use this length for the system IDs of all 
systems in the routeing domain. 
	The set of ID lengths supported by an implementa
tion is an implementation choice, provided that at 
least one value in the permitted range can be ac
cepted. The routeing domain administrator must en
sure that all ISs included in a routeing domain are 
able to use the ID length chosen for that domain. 
SEL	NSAP Selector  a 1-octet field which acts as a se
lector for the entity which is to receive the PDU(this 
may be a Transport entity or the Intermediate system 
Network entity itself). It is the least significant (last) 
octet of the NPAI.
7.1.2 Deployment of Systems 
For correct operation of the routeing protocol defined in 
this international standard, systems deployed in a routeing 
domain must meet the following requirements:
a)For all systems:
1)Each system in an area must have a unique sys
temID: that is, no two systems (IS or ES) in an 
area can use the same ID value. 
2)Each area address must be unique within the global 
OSIE: that is, a given area address can be associ
ated with only one area. 
3)All systems having a given value of area address 
must be located in the same area. 

b)Additional Requirements for Intermediate systems: 
1)Each Level 2 Intermediate system within a route
ing domain must have a unique value for its ID 
field: that is, no two level 2 ISs in a routeing do
main can have the same value in their ID fields. 
c)Additional Requirements for End systems: 
1)No two End systems in an area may have ad
dresses that match in all but the SEL fields. 
d)An End system can be attached to a level 1 IS only if 
its area address matches one of the entries in the adja
cent IS's manual

Area

Addresses parameter.
It is the responsibility of the routeing domain's administra
tive authority to enforce the requirements of 7.1.2. The pro
tocol defined in this international standard assumes that 
these requirements are met, but has no means to verify 
compliance with them.
7.1.3 Manual area addresses 
The use of several synonymous area addresses by an IS is 
accommodated through the use of the management parame
ter manual

Area

Addresses. This parameter is set locally 
for each level 1 IS by system management; it contains a list 
of all synonymous area addresses associated with the IS, in
cluding the IS's area address as contained in its own NET. 
Each level 1 IS distributes its manual

Area

Addresses in 
its Level 1 LSP's Area Addresses field, thus allowing 
level 2 ISs to create a composite list of all area addresses 
supported within a given area. Level 2 ISs in turn advertise 
the composite list throughout the level 2 subdomain by in
cluding it in their Level 2 LSP's Area Addresses field, 
thus distributing information on all the area addresses asso
ciated with the entire routeing domain. The procedures for 
establishing an adjacency between two level 1 ISs require 
that there be at least one area address in common between 
their two manual

Area

Addresses lists, and the proce
dures for establishing an adjacency between a level 1 Is and 
an End system require that the End system's area address 
must match an entry in the IS's manual

Area

Addresses 
list. Therefore, it is the responsibility of System Manage
ment to ensure that each area address associated with an IS 
is included: in particular, system management must ensure 
that the area addresses of all ESs and Level 1 ISs adjacent 
to a given level 1 IS are included in that IS's manual

Area

Addresses list.
If the area address field for the destination address of an 
8473 PDU  or for the next entry in its source routeing 
field, when present  is not listed in the parameter area

Addresses of a level 1 IS receiving the PDU, then the 
destination system does not reside in the IS's area. Such 
PDUs will be routed by level-2 routeing.
7.1.4 Encoding of Level 2 Addresses
When a full NSAP address is encoded according to the pre
ferred binary encoding specified in ISO 8348/Add.2, the 

IDI is padded with leading digits (if necessary) to obtain the 
maximum IDP length specified for that AFI.
A Level 2 address prefix consists of a leading sub-string of 
a full NSAP address, such that it matches a set of full 
NSAP addresses that have the same leading sub-string. 
However this truncation and matching is performed on the 
NSAP represented by the abstract syntax of the NSAP ad
dress, not on the encoded (and hence padded) form.11An example of
prefix matching may be found in annex B, clause B.1.

Level 2 address prefixes are encoded in LSPs in the same 
way as full NSAP addresses, except when the end of the 
prefix falls within the IDP. In this case the prefix is directly 
encoded as the string of semi-octets with no padding. 
7.1.5 Comparison of Addresses
Unless otherwise stated, numerical comparison of addresses 
shall be performed on the encoded form of the address, by 
padding the shorter address with trailing zeros to the length 
of the longer address, and then performing a numerical 
comparison.
The addresses to which this precedure applies include 
NSAP addresses, Network Entity Titles, and SNPA ad
dresses.
7.2 The Decision Process
This process uses the database of Link State information to 
calculate the forwarding database(s), from which the for
warding process can know the proper next hop for each 
NPDU. The Level 1 Link State Database is used for calcu
lating the Level 1 Forwarding Database(s), and the Level 2 
Link State Database is used for calculating the Level 2 For
warding Database(s).
7.2.1 Input and output
INPUT
-Link State Database  This database is a set of infor
mation from the latest Link State PDUs from all 
known Intermediate systems (within this area, for 
Level 1, or within the level 2 subdomain, for Level 2). 
This database is received from the Update Process.
-Notification of an Event  This is a signal from the 
Update Process that a change to a link has occurred 
somewhere in the domain.
 OUTPUT
-Level 1 Forwarding Databases  one per routeing 
metric
-(Level 2 Intermediate systems only) Level 2 Forward
ing Databases   one per routeing metric
-(Level 2 Intermediate systems only) The Level 1 De
cision Process informs the Level 2 Update Process of 
the ID of the Level 2 Intermediate system within the 
area with lowest ID reachable with real level 1 links 

(as opposed to a virtual link consisting of a path 
through the level 2 subdomain) 
-(Level 2 Intermediate systems only) If this Intermedi
ate system is the Partition Designated Level 2 Inter
mediate system in this partition, the Level 2 Decision 
Process informs the Level 1 Update Process of the 
values of the default routeing metric to and ID of the 
partition designated level 2 Intermediate system in 
each other partition of this area. 
7.2.2 Routeing metrics
There are four routeing metrics defined, corresponding to 
the four possible orthogonal qualities of service defined by 
the QoS Maintenance field of ISO 8473. Each circuit ema
nating from an Intermediate system shall be assigned a 
value for one or more of these metrics by System manage
ment. The four metrics are as follows:
a)Default metric: This is a metric understood by every 
Intermediate system in the domain. Each circuit shall 
have a positive integral value assigned for this metric. 
The value may be associated with any objective func
tion of the circuit, but by convention is intended to 
measure the capacity of the circuit for handling traffic, 
for example, its throughput in bits-per-second.  Higher 
values indicate a lower capacity.
b)Delay metric:  This metric measures the transit delay 
of the associated circuit. It is an optional metric, which 
if assigned to a circuit shall have a positive integral 
value. Higher values indicate a longer transit delay.
c)Expense metric: This metric measures the monetary 
cost of utilising the associated circuit. It is an optional 
metric, which if assigned to a circuit shall have a posi
tive integral value22The path computation algorithm utilised in this
International Standard requires that all circuits be assigned a
positive value for a metric. Therefore, it is 
not possible to represent a free circuit by a zero value of the expense
metric. By convention, the value 1 is used to indicate a free circuit.
. Higher values indicate a larger 
monetary expense.
d)Error metric: This metric measures the residual error 
probability of the associated circuit. It is an optional 
metric, which if assigned to a circuit shall have a non-
zero value. Higher values indicate a larger probability 
of undetected errors on the circuit.
NOTE - The decision process combines metric values by 
simple addition.  It is important, therefore, that the values of 
the metrics be chosen accordingly.
Every Intermediate system shall be capable of calculating 
routes based on the default metric. Support of any or all of 
the other metrics is optional. If an Intermediate system sup
ports the calculation of routes based on a metric, its update 
process may report the metric value in the LSPs for the as
sociated circuit; otherwise, the IS shall not report the met
ric.
When calculating paths for one of the optional routeing 
metrics, the decision process only utilises LSPs with a 
value reported for the corresponding metric. If no value is 

associated with a metric for any of the IS's circuits the sys
tem shall not calculate routes based on that metric.
NOTE - A consequence of the above is that a system reach
able via the default metric may not be reachable by another 
metric.
See 7.4.2 for a description of how the forwarding process 
selects one of these metrics based on the contents of the 
ISO 8473 QoS Maintenance option.
Each of the four metrics described above may be of two 
types: an  Internal metric or an External metric. Internal 
metrics are used to describe links/routes to destinations in
ternal to the routeing domain. External metrics are used to 
describe links/routes to destinations outside of the routeing 
domain. These two types of metrics are not directly compa
rable, except the internal routes are always preferred over 
external routes. In other words an internal route will always 
be selected even if an external route with lower total cost 
exists.
7.2.3 Broadcast Subnetworks
Instead of treating a broadcast subnetwork as a fully con
nected topology, the broadcast subnetwork is treated as a 
pseudonode, with links to each attached system. Attached 
systems shall only report their link to the pseudonode. The 
designated Intermediate system, on behalf of the 
pseudonode, shall construct Link State PDUs reporting the 
links to all the systems on the broadcast subnetwork with a 
zero value for each supported routeing metric33They are set to zero
metric values since they have already been assigned  metrics by the
link to the pseudonode. Assigning a non-zero value in the 
pseudonode LSP would have the effect of doubling the actual value.
.
The pseudonode shall be identified by the sourceID of the 
Designated Intermediate system, followed by a non-zero 
pseudonodeID assigned by the Designated Intermediate 
system. The pseudonodeID is locally unique to the Desig
nated Intermediate system.
Designated Intermediate systems are determined separately 
for level 1 and level 2. They are known as the LAN Level 1 
Designated IS and the LAN Level 2 Designated IS respec
tively. See 8.4.4.
An Intermediate system may resign as Designated Interme
diate System on a broadcast circuit either because it (or it's 
SNPA on the broadcast subnetwork) is being shut down or 
because some other Intermediate system of higher priority 
has taken over that function. When an Intermediate system 
resigns as Designated Intermediate System, it shall initiate a 
network wide purge of its pseudonode Link State PDU(s) 
by setting their Remaining Lifetime to zero and performing 
the actions described in 7.3.16.4. A LAN Level 1 Desig
nated Intermediate System purges Level 1 Link State PDUs 
and a LAN Level 2 Designated Intermediate System purges 
Level 2 Link State PDUs.  An Intermediate system which 
has resigned as both Level 1 and Level 2 Designated Inter
mediate System shall purge both sets of LSPs.

When an Intermediate system declares itself as designated 
Intermediate system and it is in possession of a Link State 
PDU of the same level issued by the previous Designated 
Intermediate System for that circuit (if any), it shall initiate 
a network wide purge of that (or those) Link State PDU(s) 
as above.
7.2.4 Links
Two Intermediate systems are not considered neighbours 
unless each reports the other as directly reachable over one 
of their SNPAs. On a Connection-oriented subnetwork 
(either point-to-point or general topology), the two Interme
diate systems in question shall ascertain their neighbour re
lationship when a connection is established and hello PDUs 
exchanged. A malfunctioning IS might, however, report an
other IS to be a neighbour when in fact it is not. To detect 
this class of failure the decision process checks that each 
link reported as up in a LSP is so reported by both Inter
mediate systems. If an Intermediate system considers a link 
down it shall not mention the link in its Link State PDUs.
On broadcast subnetworks, this class of failure shall be de
tected by the designated IS, which has the responsibility to 
ascertain the set of Intermediate systems that can all com
municate on the subnetwork. The designated IS shall in
clude these Intermediate systems (and no others) in the 
Link State PDU it generates for the pseudonode represent
ing the broadcast subnetwork.
7.2.5 Multiple LSPs for the same system
The Update process is capable of dividing a single logical 
LSP into a number of separate PDUs for the purpose of 
conserving link bandwidth and processing (see 7.3.4).  The 
Decision Process, on the other hand, shall regard the LSP 
with LSP Number zero in a special way. If the LSP with 
LSP Number zero and remaining lifetime > 0, is not present 
for a particular system then the Decision Process shall not 
process any LSPs with non-zero LSP Number which may 
be stored for that system. 
The following information shall be taken only from the LSP 
with LSP Number zero. Any values which may be present 
in other LSPs for that system shall be disregarded by the 
Decision Process.
a)The setting of the LSP Database Overload bit. 
b)The value of the IS Type field. 
c)The Area Addresses option.
7.2.6 Routeing Algorithm Overview
The routeing algorithm used by the Decision Process is a 
shortest path first (SPF) algorithm. Instances of the algo
rithm are run independently and concurrently by all Inter
mediate systems in a routeing domain. Intra-Domain route
ing of a PDU occurs on a hop-by-hop basis: that is, the al
gorithm determines only the next hop, not the complete 
path, that a data PDU will take to reach its destination. To 
guarantee correct and consistent route computation by 
every Intermediate system in a routeing domain, this Inter
national Standard depends on the following properties:

a)All Intermediate systems in the routeing domain con
verge to using identical topology information; and
b)Each Intermediate system in the routeing domain gen
erates the same set of routes from the same input to
pology and set of metrics.
The first property is necessary in order to prevent inconsis
tent, potentially looping paths. The second property is nec
essary to meet the goal of determinism stated in 6.6.
A system executes the SPF algorithm to find a set of legal 
paths to a destination system in the routeing domain. The 
set may consist of:
a)a single path of minimum metric sum: these are 
termed minimum cost paths;
b)a set of paths of equal minimum metric sum: these are 
termed equal minimum cost paths; or
c)a set of paths which will get a PDU closer to its desti
nation than the local system: these are called down
stream paths.
Paths which do not meet the above conditions are illegal 
and shall not be used.
The Decision Process, in determining its paths, also ascer
tains the identity of the adjacency which lies on the first 
hop to the destination on each path. These adjacencies are 
used to form the Forwarding Database,  which the forward
ing process uses for relaying PDUs.
Separate route calculations are made for each pairing of a 
level in the routeing hierarchy (i.e. L1 and L2) with a sup
ported routeing metric. Since there are four routeing metrics 
and two levels some systems may execute multiple in
stances of the SPF algorithm. For example,
-if an IS is a L2 Intermediate system which supports all 
four metrics and computes minimum cost paths for all 
metrics, it would execute the SPF calculation eight 
times.
-if an IS is a L1 Intermediate system which supports all 
four metrics, and additionally computes downstream 
paths, it would execute the algorithm  4 W (number of 
neighbours + 1) times.
Any implementation of an SPF algorithm meeting both the 
static and dynamic conformance requirements of clause 12 
of this International Standard may be used. Recommended 
implementations are described in detail in Annex C.
7.2.7 Removal of Excess Paths
When there are more than max

i

mum

Path

Splits legal 
paths to a destination, this set shall be pruned until only 
max

i

mum

Path

Splits remain. The Intermediate system 
shall discriminate based upon:
NOTE - The precise precedence among the paths is speci
fied in order to meet the goal of determinism defined in 6.6.

-adjacency type: Paths associated with End system or 
level 2 reachable address prefix adjacencies are re
tained in preference to other adjacencies
-metric sum: Paths having a lesser metric sum are re
tained in preference to paths having a greater metric 
sum. By metric sum is understood the sum of the 
metrics along the path to the destination.
-neighbour ID: where two or more paths are associ
ated with adjacencies of the same type, an adjacency 
with a lower neighbour ID is retained in preference to 
an adjacency with a higher neighbour id.
-circuit ID: where two or more paths are associated 
with adjacencies of the same type, and same neigh
bour ID, an adjacency with a lower circuit ID is re
tained in preference to an adjacency with a higher cir
cuit ID, where circuit ID is the value of:
7ptPtCircuitID for non-broadcast circuits, 
7l1CircuitID for broadcast circuits when running 
the Level 1 Decision Process, and 
7l2CircuitID for broadcast circuits when running 
the Level 2 Decision Process. 
-lANAddress: where two or more adjacencies are of 
the same type, same neighbour ID, and same circuit 
ID (e.g. a system with multiple LAN adapters on the 
same circuit) an adjacency with a lower lANAddress 
is retained in preference to an adjacency with a higher 
lANAddress.
7.2.8 Robustness Checks
7.2.8.1 Computing Routes through Overloaded 
Intermediate systems
The Decision Process shall not utilise a link to an Interme
diate system neighbour from an IS whose LSPs have the 
LSP Database Overload indication set. Such paths may in
troduce loops since the overloaded IS does not have a com
plete routeing information base. The Decision Process shall, 
however utilise the link to reach End system neighbours 
since these paths are guaranteed to be non-looping.
7.2.8.2 Two-way connectivity check
The Decision Process shall not utilise a link between two 
Intermediate Systems unless both ISs report the link. 
NOTE - the check is not applicable to links to an End Sys
tem.
Reporting the link indicates that it has a defined value for at 
least the default routeing metric. It is permissible for two 
endpoints to report different defined values of the same 
metric for the same link. In this case, routes may be asym
metric.

7.2.9 Construction of a Forwarding Database
The information that is needed in the forwarding database 
for routeing metric k is the set of adjacencies for each sys
tem N.
7.2.9.1 Identification of Nearest Level 2 IS by a 
Level 1 IS
Level 1 Intermediate systems need one additional piece of 
information per routeing metric: the next hop to the nearest 
level 2 Intermediate system according to that routeing met
ric. A level 1 IS shall ascertain the set, R, of attached 
level 2 Intermediate system(s) for metric k such that the to
tal cost to R for metric k is minimal. 
If there are more adjacencies in this set than max

i

mum

Path

Splits, then the IS shall remove excess adjacencies as 
described in 7.2.7.
7.2.9.2 Setting the Attached Flag in Level 2 
Intermediate Systems
If a level 2 Intermediate system discovers, after computing 
the level 2 routes for metric k, that it cannot reach any other 
areas using that metric, it shall:
-set AttachedFlag for metric k to False;
-regenerate its Level 1 LSP with LSP number zero; and
-compute the nearest level 2 Intermediate system for 
metric k for insertion in the appropriate forwarding 
database, according to the algorithm described in 
7.2.9.1 for level 1 Intermediate systems.
NOTE - AttachedFlag for each metric k is examined by the 
Update Process, so that it will report the value in the ATT 
field of its Link State PDUs. 
If a level 2 Intermediate system discovers, after computing 
the level 2 routes for metric k, that it can reach at least one 
other area using that metric, it shall
-set AttachedFlag for metric k to True;
-regenerate its Level 1 LSP with LSP number zero; and 
-set the level 1 forwarding database entry for metric k 
which corresponds to nearest level 2 Intermediate 
system to Self.
7.2.10 Information for Repairing Partitioned 
Areas
An area may become partitioned as a result of failure of one 
or more links in the area. However, if each of the partitions 
has a connection to the level 2 subdomain, it is possible to 
repair the partition via the level 2 subdomain, provided that 
the level 2 subdomain itself is not partitioned. This is illus
trated in Figure 4.
All the systems A  I, R and P are in the same area n. 
When the link between D and E is broken, the area be

comes partitioned. Within each of the partitions the Parti
tion Designated Level 2 Intermediate system is selected 
from among the level 2 Intermediate systems in that parti
tion. In the case of partition 1 this is P, and in the case of 
partition 2 this is R. The level 1 repair path is then estab
lished between between these two level 2 Intermediate sys
tems. Note that the repaired link is now between P and R, 
not between D and E.
The Partition Designated Level 2 Intermediate Systems re
pair the partition by forwarding NPDUs destined for other 
partitions of the area through the level 2 subdomain. They 
do this by acting in their capacity as Level 1 Intermediate 
Systems and advertising in their Level 1 LSPs adjacencies 
to each Partition Designated Level 2 Intermediate System 
in the area. This adjacency is known as a Virtual Adja
cency or Virtual Link. Thus other Level 1 Intermediate 
Systems in a partition calculate paths to the other partitions 
through the Partition Designated Level 2 Intermediate Sys
tem. A Partition Designated Level 2 Intermediate System 
forwards the Level 1 NPDUs through the level 2 subdomain 
by encapsulating them in 8473 Data NPDUs with its Virtual 
Network Entity Title as the source NSAP and the adja
cent Partition Designated Level 2 Intermediate System's 
Virtual Network Entity Title as the destination NSAP. The 
following sub-clauses describe this in more detail.
7.2.10.1 Partition Detection and Virtual Level 1 
Link Creation
Partitions of a Level 1 area are detected by the Level 2 In
termediate System(s) operating within the area.  In order to 
participate in the partition repair process, these Level 2 In
termediate systems must also act as Level 1 Intermediate 
systems in the area. A partition of a given area exists when
ever two or more Level 2 ISs located in that area are re
ported in the L2 LSPs as being a Partition Designated 
Level 2 IS. Conversely, when only one Level 2 IS in an 
area is reported as being the Partition Designated Level 2 

IS, then that area is not partitioned.  Partition repair is ac
complished by the Partition Designated Level 2 IS.  The 
election of the Partition Designated Level 2 IS as described 
in the next subsection must be done before the detection 
and repair process can begin.
In order to repair a partition of a Level 1 area, the Partition 
designated Level 2 IS creates a Virtual Network Entity to 
represent the partition.  The Network Entity Title for this 
virtual network entity shall be constructed from the first 
listed area address from its Level 2 Link State PDU, and the 
ID of the Partition Designated Level 2 IS.  The IS shall also 
construct a virtual link (represented by a new Virtual Adja
cency managed object) to each Partition Designated Level 2 
IS in the area, with the NET of the partition recorded in the 
Identifier attribute.  The virtual links are the repair paths for 
the partition.  They are reported by the Partition Designated 
Level 2 IS into the entire Level 1 area by adding the ID of 
each adjacent Partition Designated Level 2 IS to the In
termediate System Neighbours field of its Level 1 Link 
State PDU.  The Virtual Flag shall be set True for these 
Intermediate System neighbours.  The metric value for this 
virtual link shall be the default metric value d(N) obtained 
from this system's Level 2 PATHS database, where N is the 
adjacent Partition Designated Level 2 IS via the Level 2 
subdomain.
An Intermediate System which operates as the Partition 
Designated Level 2 Intermediate System shall perform the 
following steps after completing the Level 2 shortest path 
computation in order to detect partitions in the Level 1 area 
and create repair paths: 
a)Examine Level 2 Link State PDUs of all Level 2 Inter
mediate systems. Search area

Addresses for any ad
dress that matches any of the addresses in partition

Area

Addresses. If a match is found, and the Parti
tion Designated Level 2 Intermediate system's ID 
does not equal this system's ID, then inform the level 
1 update process at this system of the identity of the 

Partition Designated Level 2 Intermediate system, to
gether with the path cost for the default routeing met
ric to that Intermediate system.
b)Continue examining Level 2 LSPs until all Partition 
Designated Level 2 Intermediate systems in other par
titions of this area are found, and inform the Level 1 
Update Process of all of the other Partition Designated 
Level 2 Intermediate systems in other partitions of this 
area, so that
1)Level 1 Link State PDUs can be propagated to all 
other Partition designated level 2 Intermediate sys
tems for this area (via the level 2 subdomain). 
2)All the Partition Designated Level 2 Intermediate 
systems for other partitions of this area can be re
ported as adjacencies in this system's Level 1 Link 
State PDUs.
If a partition has healed, the IS shall destroy the associated 
virtual network entity and virtual link by deleting the Vir
tual Adjacency.  The Partition Designated Level 2 IS de
tects a healed partition when another Partition Designated 
Level 2 IS listed as a virtual link in its Level 1 Link State 
PDU was not found after running the partition detection and 
virtual link creation algorithm described above.
If such a Virtual Adjacency is created or destroyed, the IS 
shall generate a partitionVirtualLinkChange notification.
7.2.10.2 Election of Partition Designated Level 2 
Intermediate System
 The Partition Designated Level 2 IS is a Level 2 IS which:
-reports itself as attached by the default metric in its 
LSPs;
-reports itself as implementing the partition repair op
tion;
-operates as a Level 1 IS in the area;
-is reachable via Level 1 routeing without traversing 
any virtual links; and
-has the lowest ID 
The election of the Partition Designated Level 2 IS is per
formed by running the decision process algorithm after the 
Level 1 decision process has finished, and before the 
Level 2 decision process to determine Level 2 paths is exe
cuted.  
In order to guarantee that the correct Partition Designated 
Level 2 IS is elected, the decision process is run using only 
the Level 1 LSPs for the area, and by examining only the 
Intermediate System Neighbours whose Virtual Flag is 
FALSE.  The results of this decision process is a set of all 
the Level 1 Intermediate Systems in the area that can be 
reached via Level 1, non-virtual link routeing.  From this 
set, the Partition Designated Level 2 IS is selected by 
choosing the IS for which
-IS Type (as reported in the Level 1 LSP) is Level 2 
Intermediate System;

-ATT  indicates attached by the default metric;
-P indicates support for the partition repair option;  and
-ID is the lowest among the subset of attached Level 2 
Intermediate Systems. 
7.2.10.3 Computation of Partition area addresses
A Level 2 Intermediate System shall compute the set of 
partition

Area

Addresses, which is the union of all 
manual

area

Addresses as reported in the Level 1 Link 
State PDUs of all Level 2 Intermediate systems reachable in 
the partition by the traversal of non-virtual links.  If more 
than max

i

mum

Area

Addresses are present, the Interme
diate system shall retain only those areas with numerically 
lowest area address (as described in 7.1.5). If one of the lo
cal system's manual

Area

Addresses is so rejected the 
notification manualAddressDroppedFromArea shall be 
generated.
7.2.10.4 Encapsulation of NPDUs Across the 
Virtual Link
All NPDUs sent over virtual links shall be encapsulated as 
ISO 8473 Data NPDUs.  The encapsulating Data NPDU 
shall contain the Virtual Network Entity Title of the Parti
tion Designated Level 2 IS that is forwarding the NPDU 
over the virtual link in the Source Address field, and the 
Virtual NET of the adjacent Partition Designated Level 2 
IS in the Destination Address field.  The SEL field in 
both NSAPs shall contain the IS-IS routeing selector 
value.  The QoS Maintenance field of the outer PDU shall 
be set to indicate forwarding via the default routeing metric 
(see table 1 on page 32).  
For Data and  Error Report NPDUs the Segmentation 
Permitted and Error Report flags and the Lifetime field  
of the outer NPDU shall be copied from the inner NPDU.  
When the inner NPDU is decapsulated, its Lifetime field  
shall be set to the value of the Lifetime field in the outer 
NPDU.
For LSPs and SNPs the Segmentation Permitted flag 
shall be set to True and the Error Report flag shall be set 
to False.  The Lifetime field shall be set to 255.  When an 
inner LSP is decapsulated, its remaining lifetime shall be 
decremented by half the difference between 255 and the 
value of the Lifetime field in the outer NPDU.
Data NPDUs shall not be fragmented before encapsulation, 
unless the total length of the Data NPDU (including header) 
exceeds 65535 octets.  In that case, the original Data NPDU 
shall first be fragmented, then encapsulated.  In all cases, 
the encapsulated Data NPDU may need to be fragmented 
by ISO 8473 before transmission in which case it must be 
reassembled and decapsulated by the destination Partition 
Designated Level 2 IS.  The encapsulation is further de
scribed as part of the forwarding process in 7.4.3.2.  The 
decapsulation is described as part of the Receive process in 
7.4.4.
7.2.11 Computation of area addresses
A Level 1 or Level 2 Intermediate System shall compute 
the values of area

Addresses (the set of area addresses 

for this Level 1 area), by forming the union of the sets of 
manual

area

Addresses reported in the Area Addresses 
field of all Level 1 LSPs with LSP number zero in the local 
Intermediate system's link state database.
NOTE - This includes all source systems, whether currently 
reachable or not. It also includes the local Intermediate sys
tem's own Level 1 LSP with LSP number zero.  
NOTE - There is no requirement for this set to be updated 
immediately on each change to the database contents. It is 
permitted to defer the computation until the next running of 
the Decision Process.  
If more than max

i

mum

Area

Addresses are present, the 
Intermediate system shall retain only those areas with nu
merically lowest area address (as described in 7.1.5). If one 
of the local system's manual

area

Addresses is rejected 
the notification manual

Address

Dropped

From

Area shall 
be generated.
7.2.12 Order of Preference of Routes
If an Intermediate system takes part in level 1 routeing, and 
determines (by looking at the area address) that a given des
tination is reachable within its area, then that destination 
will be reached exclusively by use of level 1 routeing. In 
particular:
a)Level 1 routeing is always based on internal metrics.
b)Amongst routes in the area, routes on which the re
quested QoS (if any) is supported are always preferred 
to routes on which the requested QoS is not supported.
c)Amongst routes in the area of the same QoS, the short
est routes are preferred. For determination of the 
shortest path, if a route with specific QoS support is 
available, then the specified QoS metric is used, other
wise the default metric is used. 
d)Amongst routes of equal cost, load splitting may be 
performed.
If an Intermediate system takes part in level 1 routeing, 
does not take part in level 2 routeing, and determines (by 
looking at the area address) that a given destination is not 
reachable within its area, and at least one attached level 2 
IS is reachable in the area, then that destination will be 
reached by routeing to a level 2 Intermediate system as fol
lows:
a)Level 1 routeing is always based on internal metrics.
b)Amongst routes in the area to attached level 2 ISs, 
routes on which the requested QoS (if any) is sup
ported are always preferred to routes on which the re
quested QoS is not supported.
c)Amongst routes in the area of the same QoS to at
tached level 2 ISs, the shortest route is preferred. For 
determination of the shortest path, if a route on which 
the specified QoS is available, then the specified QoS 
metric is used, otherwise the default metric is used.

d)Amongst routes of equal cost, load splitting may be 
performed.
If an Intermediate system takes part in level 2 routeing and 
is attached, and the IS determines (by looking at the area 
address) that a given destination is not reachable within its 
area, then that destination will be reached as follows:
a)Routes on which the requested QoS (if any) is sup
ported are always preferred to routes on which the re
quested QoS is not supported.
b)Amongst routes of the same QoS, routes are priori
tised as follows:
1)Highest precedence: routes matching the area ad
dress of any area in the routeing domain
2)Medium precedence: Routes matching a reachable 
address prefix with an internal metric. For destina
tions matching multiple reachable address prefix 
entries all with internal metrics, the longest prefix 
shall be preferred.
3)Lowest precedence: Routes matching a reachable 
address prefix with an external metric. For destina
tions matching multiple reachable address prefix 
entries all with external metrics, the longest prefix 
shall be preferred.
c)For routes with equal precedence as specified above, 
the shortest path shall be preferred. For determination 
of the shortest path, a route supporting the specified 
QoS is used if available; otherwise a route using the 
default metric shall be used. Amongst routes of equal 
cost, load splitting may be performed.
7.3 The Update Process
The Update Process is responsible for generating and 
propagating Link State information reliably throughout the 
routeing domain.
The Link State information is used by the Decision Process 
to calculate routes.
7.3.1 Input and Output
INPUT
-Adjacency Database  maintained by the Subnetwork 
Dependent Functions 
-Reachable Address managed objects - maintained by 
System Management
-Notification of Adjacency Database Change  notifi
cation by the Subnetwork Dependent Functions that 
an adjacency has come up, gone down, or changed 
cost. (Circuit up, Circuit down, Adjacency Up, Adja
cency Down, and Cost change events) 
-AttachedFlag  (level 2 Intermediate systems only), 
a flag computed by the Level 2 Decision Process indi
cating whether this system can reach (via level 2 
routeing) other areas 

-Link State PDUs  The Receive Process passes Link 
State PDUs to the Update Process, along with an indi
cation of which adjacency it was received on. 
-Sequence Numbers PDUs  The Receive Process 
passes Sequence Numbers PDUs to the Update Proc
ess, along with an indication of which adjacency it 
was received on. 
-Other Partitions  The Level 2 Decision Process 
makes available (to the Level 1 Update Process on a 
Level 2 Intermediate system) a list of aPartition Desig
nated Level 2 Intermediate system, Level 2 default 
metric valueq pairs, for other partitions of this area.
 OUTPUT
-Link State Database
-Signal to the Decision Process of an event, which is 
either the receipt of a Link State PDU with different 
information from the stored one, or the purging of a 
Link State PDU from the database. The reception of a 
Link State PDU which has a different sequence num
ber or Remaining Lifetime from one already stored in 
the database, but has an identical variable length por
tion, shall not cause such an event. 
NOTE - An implementation may compare the checksum of 
the stored Link State PDU, modified according to the 
change in sequence number, with the checksum of the re
ceived Link State PDU. If they differ, it may assume that the 
variable length portions are different and an event signalled 
to the Decision Process. However, if the checksums are the 
same, an octet for octet comparison must be made in order 
to determine whether or not to signal the event.
7.3.2 Generation of Local Link State 
Information
The Update Process is responsible for constructing a set of 
Link State PDUs. The purpose of these Link State PDUs is 
to inform all the other Intermediate systems (in the area, in 
the case of Level 1, or in the Level 2 subdomain, in the case 
of Level 2), of the state of the links between the Intermedi
ate system that generated the PDUs and its neighbours. 
The Update Process in an Intermediate system shall gener
ate one or more new Link State PDUs under the following 
circumstances:
a)upon timer expiration;
b)when notified by the Subnetwork Dependent Func
tions of an Adjacency Database Change;
c)when a change to some Network Management charac
teristic would cause the information in the LSP to 
change (for example, a change in manual

area

Addresses).
7.3.3 Use of Manual Routeing Information
Manual routeing information is routeing information en
tered by system management. It may be specified in two 
forms.

a)Manual Adjacencies 
b)Reachable Addresses 
These are described in the following sub-clauses.
7.3.3.1 Manual Adjacencies
An End system adjacency may be created by System Man
agement. Such an adjacency is termed a manual End sys
tem adjacency. In order to create a manual End system ad
jacency, system managements shall specify:
a)the (set of) system IDs reachable over that adjacency; 
and 
b)the corresponding SNPA Address.
 These adjacencies shall appear as adjacencies with type 
Manual, neighbourSystemType End system and  
state Up. Such adjacencies provide input to the Update 
Process in a similar way to adjacencies created through the 
operation of ISO 9542. When the state changes to Up the 
adjacency information is included in the Intermediate Sys
tem's own Level 1 LSPs. 
NOTE - Manual End system adjacencies shall not be in
cluded in a Level 1 LSPs issued on behalf of a pseudonode, 
since that would presuppose that all Intermediate systems on 
a broadcast subnetwork had the same set of manual adjacen
cies as defined for this circuit.
Metrics assigned to Manual adjacencies must be Internal 
metrics.
7.3.3.2 Reachable Addresses
A Level 2 Intermediate system may have a number of 
Reachable Address managed objects created by System 
management. When a Reachable Address is in state On 
and its parent Circuit is also in state On, the name and 
each of its defined routeing metrics shall be included in 
Level 2 LSPs generated by this system. 
Metrics assigned to Reachable Address managed objects 
may be either Internal or External.
A reachable address is considered to be active when all 
the following conditions are true:
a)The parent circuit is in state On;
b)the Reachable Address is in state On; and
c)the parent circuit is of type broadcast or is in data link 
state Running.
Whenever a reachable address changes from being inac
tive to active a signal shall be generated to the Update 
process to cause it to include the Address Prefix of the 
reachable address in the Level 2 LSPs generated by that 
system as described in 7.3.9.
Whenever a reachable address changes from being active 
to inactive, a signal shall be generated to the Update 

process to cause it to cease including the Address Prefix of 
the reachable address in the Level 2 LSPs.
7.3.4 Multiple LSPs
Because a Link State PDU is limited in size to Receive

LSP

Buffer

Size, it may not be possible to include infor
mation about all of a system's neighbours in a single LSP. 
In such cases, a system may use multiple LSPs to convey 
this information. Each LSP in the set carries the same 
sourceID field (see clause 9), but sets its own LSP Num
ber field individually. Each of the several LSPs is handled 
independently by the Update Process, thus allowing distri
bution of topology updates to be pipelined. However, the 
Decision Process recognises that they all pertain to a com
mon originating system because they all use the same 
sourceID.
NOTE - Even if the amount of information is small enough 
to fit in a single LSP, a system may optionally choose to use 
several LSPs to convey it; use of a single LSP in this situ
ation is not mandatory.
NOTE - In order to minimise the transmission of redundant 
information, it is advisable for an IS to group Reachable  
Address Prefix information by the circuit with which it is as
sociated. Doing so will ensure that the minimum  number of 
LSP fragments need be transmitted if a circuit to another 
routeing domain changes state.
The maximum sized Level 1 or Level 2 LSP which may be 
generated by a system is controlled by the values of the 
management parameters originating

L1

LSP

Buf

fer

Size or 
ori

ginat

ing

L2

LSP

Buffer

Size respectively. 
NOTE - These parameters should be set consistently by sys
tem management. If this is not done, some adjacencies will 
fail to initialise.
The IS shall treat the LSP with LSP Number zero in a spe
cial way, as follows:
a)The following fields are meaningful to the decision 
process only when they are present in the LSP with 
LSP Number zero:
1)The setting of the LSP Database Overload bit. 
2)The value of the IS Type field. 
3)The Area Addresses option. (This is only present 
in the LSP with LSP Number zero, see below).
b)When the values of any of the above items are 
changed, an Intermediate System shall re-issue the 
LSP with LSP Number zero, to inform other Interme
diate Systems of the change. Other LSPs need not be 
reissued.
Once a particular adjacency has been assigned to a particu
lar LSP Number, it is desirable that it not be moved to an
other LSP Number. This is because moving an adjacency 
from one LSP to another can cause temporary loss of 

connectivity to that system. This can occur if the new ver
sion of the LSP which originally contained information 
about the adjacency (which now does not contain that infor
mation) is propagated before the new version of the other 
LSP (which now contains the information about the adja
cency). In order to minimise the impact of this, the follow
ing restrictions are placed on the assignment of information 
to LSPs.
a)The Area Addresses option field shall occur only in 
the LSP with LSP Number  zero.
b)Intermediate System Neighbours options shall occur 
after the Area Addresses option and before any End 
System (or in the case of Level 2, Prefix) Neigh
bours options.
c)End System (or Prefix) Neighbour options (if any) 
shall occur after any Area Address or Intermediate 
System Neighbour options.
NOTE  In this context, after means at a higher octet 
number from the start of the same LSP or in an LSP with 
a higher LSP Number. 
NOTE  An implementation is recommended to ensure 
that the number of LSPs generated for a particular system 
is within approximately 10% of the optimal number 
which would be required if all LSPs were densely packed 
with neighbour options. Where possible this should be 
accomplished by re-using space in LSPs with a lower 
LSP Number for new adjacencies. If it is necessary to 
move an adjacency from one LSP to another, the 
SRMflags (see 7.3.15) for the two new LSPs shall be 
set as an atomic action.44If the two SRMflags are not set atomically, a
race condition will exist in which one of the two LSPs may be
propagated quickly, while the other waits for 
an entire propagation cycle. If this occurs, adjacencies will be
falsely eliminated from the topology and routes may become unstable for
period of time 
potentially as large as maximumLSPGeneratonInterval.

When some event requires changing the LSP information 
for a system, the system shall reissue that (or those) LSPs 
which would have different contents. It is not required to 
reissue the unchanged LSPs. Thus a single End system ad
jacency change only requires the reissuing of the LSP con
taining the End System Neighbours option referring to 
that adjacency. The parameters max

imum

LSP

Gen

er

a

tion

Int

er

val and minimumLSPGenerationInterval shall 
apply to each LSP individually. 
7.3.5 Periodic LSP Generation
The Update Process shall periodically re-generate and 
propagate on every circuit with an IS adjacency of the ap
propriate level (by setting SRMflag on each circuit), all the 
LSPs (Level 1 and/or Level 2) for the local system and any 
pseudonodes for which it is responsible. The Intermediate 
system shall re-generate each LSP at intervals of at most 
max

i

mum

LSP

Gen

era

tion

Interval seconds, with jitter 
applied as described in 10.1. 
These LSPs may all be generated on expiration of a single 
timer or alternatively separate timers may be kept for each 
LSP Number and the individual LSP generated on expira
tion of this timer. 

7.3.6 Event Driven LSP Generation
In addition to the periodic generation of LSPs, an Interme
diate system shall generate an LSP when an event occurs 
which would cause the information content to change. The 
following events may cause such a change.
-an Adjacency or Circuit Up/Down event
- a change in Circuit metric
-a change in Reachable Address metric
-a change in manual

Area

Addresses
-a change in systemID
-a change in Designated  Intermediate System status
-a change in the waiting status
When such an event occurs the IS shall re-generate changed 
LSP(s) with a new sequence number. If the event necessi
tated the generation of an LSP which had not previously 
been generated (for example, an adjacency Up event for 
an adjacency which could not be accommodated in an exist
ing LSP), the sequence number shall be set to one. The IS 
shall then propagate the LSP(s) on every circuit by setting 
SRMflag for each circuit. The timer maximum

LSP

Gen

er

ation

Interval shall not be reset.
There is a hold-down timer (min

i

mum

LSP

Generation

Interval) on the generation of each individual LSP.
7.3.7 Generation of Level 1 LSPs 
(non-pseudonode)
The Level 1 Link State PDU not generated on behalf of a 
pseudonode contains the following information in its vari
able length fields.
-In the Area Addresses option the set of manual

Area

Addresses for this Intermediate System.
-In the Intermediate System Neighbours option  
the set of Intermediate system IDs of neighbouring In
termediate systems formed from:
7The set of neighbourSystemIDs with an ap
pended zero octet (indicating non-pseudonode) 
from adjacencies in the state Up, on circuits of 
type Point-Point, In or Out, with
xneighbourSystemType L1 Intermediate 
System 
xneighbourSystemType L2 Intermediate 
System and adjacencyUsage Level 2 or 
Level1 and 2.
The metrics shall be set to the values of Level 1 
metrick of the circuit for each supported routeing 
metric.
7The set of l1CircuitIDs for all circuits of type 
Broadcast (i.e. the neighbouring pseudonode 
IDs) .

The metrics shall be set to the values of Level 1 
metrick of the circuit for each supported routeing 
metric.
7The set of IDs with an appended zero octet derived 
from the Network Entity Titles of all Virtual Adja
cencies of this IS. (Note that the Virtual Flag is set 
when encoding these entries in the LSP  see 
7.2.10.)
The default metric shall be set to the total cost to 
the virtual NET for the default routeing metric. 
The remaining metrics shall be set to the value in
dicating unsupported.
-In the End System Neighbours option  the set of 
IDs of neighbouring End systems formed from:
7The systemID of the Intermediate System itself, 
with a value of zero for all supported metrics.
7The set of endSystemIDs from all adjacencies 
with type Auto-configured, in state Up, on 
circuits of type Point-to-Point, In or Out, 
with neighbourSystemType End system. 
The metrics shall be set to the values of Level 1 
metrick of the circuit for each supported routeing 
metric.
7The set of endSystemIDs from all adjacencies 
with type Manual in state Up, on all circuits. 
The metrics shall be set to the values of Level 1 
metrick of the circuit for each supported routeing 
metric.
-In the Authentication Information field  if the 
system's areaTransmitPassword is non-null, in
clude the Authentication Information field contain
ing an Authentication Type  of Password, and the 
value of the areaTransmitPassword.
7.3.8 Generation of Level 1 Pseudonode LSPs
An IS shall generate a  Level 1 pseudonode Link State PDU 
for each circuit for which this Intermediate System is the 
Level 1 LAN Designated Intermediate System. The LSP 
shall specify the following information in its variable length 
fields. In all cases a value of zero shall be used for all sup
ported routeing metrics
-The Area Addresses option is not present.
Note - This information is not required since the set of 
area addresses for the node issuing the pseudonode 
LSP will already have been made available via its own 
non-pseudonode LSP.
-In the Intermediate System Neighbours option  
the set of Intermediate System IDs of neighbouring In
termediate Systems on the circuit for which this 
pseudonode LSP is being generated formed from:
7The Designated Intermediate System's own sys
temID with an appended zero octet (indicating 
non-pseudonode). 

7The set of neighbourSystemIDs with an ap
pended zero octet (indicating non-pseudonode) 
from adjacencies on this circuit in the state Up, 
with  
xneighbourSystemType L1 Intermediate 
System
xL2 Intermediate System and adjacency
Usage Level 1.
-In the End System Neighbours option  the set of 
IDs of neighbouring End systems formed from:
7The set of endSystemIDs from all adjacencies 
with type Auto-configured, in state Up, on 
the circuit for which this pseudonode is being gen
erated, with neighbourSystemType End sys
tem. 
-In the Authentication Information field  if the 
system's areaTransmitPassword is non-null, in
clude the Authentication Information field contain
ing an Authentication Type  of Password, and the 
value of the areaTransmitPassword.
7.3.9 Generation of Level 2 LSPs 
(non-pseudonode)
The Level 2 Link State PDU not generated on behalf of a 
pseudonode contains the following information in its vari
able length fields:
-In the Area Addresses option  the set of area

Addresses for this Intermediate system computed as 
described in 7.2.11.
-In the Partition Designated Level 2 IS option  the 
ID of the Partition Designated Level 2 Intermediate 
System for the partition.
-In the Intermediate System Neighbours option  
the set of Intermediate system IDs of neighbouring In
termediate systems formed from:
7The set of neighbourSystemIDs with an ap
pended zero octet (indicating non-pseudonode) 
from adjacencies in the state Up, on circuits of 
type Point-to-Point, In or Out, with neigh
bourSystemType L2 Intermediate System.
7The set of l2CircuitIDs for all circuits of type 
Broadcast. (i.e. the neighbouring pseudonode 
IDs)
The metric and metric type shall be set to the val
ues of Level 2 metrick of the circuit for each sup
ported routeing metric.
-In the Prefix Neighbours option  the set of vari
able length prefixes formed from: 
7The set of names of all Reachable Address man
aged objects in state On, on all circuits in state 
On.

The metrics shall be set to the values of Level 2  
metrick for the reachable address.
-In the Authentication Information field  if the 
system's domainTransmitPassword is non-null, 
include the Authentication Information field con
taining an Authentication Type  of Password, and 
the value of the domainTransmitPassword.
7.3.10 Generation of Level 2 Pseudonode LSPs
A Level 2 pseudonode Link State PDU is generated for 
each circuit for which this Intermediate System is the 
Level 2 LAN Designated Intermediate System and contains 
the following information in its variable length fields. In all 
cases a value of zero shall be used for all supported route
ing metrics.
-The Area Addresses option is not present.
Note - This information is not required since the set of 
area addresses for the node issuing the pseudonode 
LSP will already have been made available via its own 
non-pseudonode LSP.
-In the Intermediate System Neighbours option  
the set of Intermediate System IDs of neighbouring In
termediate Systems on the circuit for which this 
pseudonode LSP is being generated formed from: 
7The Designated Intermediate System's own sys
temID with an appended zero octet (indicating 
non-pseudonode). 
7The set of neighbourSystemIDs with an ap
pended zero octet (indicating non-pseudonode) 
from adjacencies on this circuit in the state Up 
with neighbourSystemType L2 Intermediate 
System.
-The Prefix Neighbours option is not present.
-In the Authentication Information field  if the 
system's domainTransmitPassword is non-null, 
include the Authentication Information field con
taining an Authentication Type  of Password, and 
the value of the domainTransmitPassword.
7.3.11 Generation of the Checksum
This International Standard makes use of the checksum 
function defined in ISO 8473.
The source IS shall compute the LSP Checksum when the 
LSP is generated. The checksum shall never be modified by 
any other system. The checksum allows the detection of 
memory corruptions and thus prevents both the use of in
correct routeing information and its further propagation by 
the Update Process. 
The checksum shall be computed over all fields in the LSP 
which appear after the Remaining Lifetime field. This 
field (and those appearing before it) are excluded so that the 
LSP may be aged by systems without requiring re-
computation. 

As an additional precaution against hardware failure, when 
the source computes the Checksum, it shall start with the 
two checksum variables (C0 and C1) initialised to what 
they would be after computing for the systemID portion 
(i.e. the first 6 octets) of its Source ID. (This value is com
puted and stored when the Network entity is enabled and 
whenever systemID changes.) The IS shall then resume 
Checksum computation on the contents of the PDU after 
the first ID Length octets of the Source ID field.
NOTE - All Checksum calculations on the LSP are per
formed treating the Source ID field as the first octet. This 
procedure prevents the source from accidentally sending out 
Link State PDUs with some other system's ID as source. 
7.3.12 Initiating Transmission
The IS shall store the generated Link State PDU in the Link 
State Database, overwriting any previous Link State PDU 
with the same LSP Number generated by this system. The 
IS shall then set all SRMflags for that Link State PDU, in
dicating it is to be propagated on all circuits with Intermedi
ate System adjacencies. 
An Intermediate system shall ensure (by reserving re
sources, or otherwise) that it will always be able to store 
and internalise its own non-pseudonode zeroth LSP. In the 
event that it is not capable of storing and internalising one 
of its own LSPs it shall enter the overloaded state as de
scribed in 7.3.19.1.
NOTE - It is recommended that an Intermediate system en
sure (by reserving resources, or otherwise) that it will al
ways be able to store and internalise all its own (zero and 
non-zero, pseudonode and non-pseudonode) LSPs.
7.3.13 Preservation of order
When an existing Link State PDU is re-transmitted (with 
the same or a different sequence number), but with the 
same information content (i.e. the variable length part) as a 
result of there having been no changes in the local topology 
databases, the order of the information in the variable 
length part shall be the same as that in the previously trans
mitted LSP.
NOTE - If a sequence of changes result in the state of the 
database returning to some previous value, there is no re
quirement to preserve the ordering. It is only required when 
there have been no changes whatever. This allows the re
ceiver to detect that there has been no change in the infor
mation content by performing an octet for octet comparison 
of the variable length part, and hence not re-run the decision 
process. 
7.3.14 Propagation of LSPs
The update process is responsible for propagating Link 
State PDUs throughout the domain (or in the case of 
Level 1, throughout the area).
The basic mechanism is flooding, in which each Intermedi
ate system propagates to all its neighbour Intermediate sys
tems except that neighbour from which it received the 
PDU. Duplicates are detected and dropped.

Link state PDUs are received from the Receive Process. 
The maximum size control PDU (Link State PDU or Se
quence Numbers PDU) which a system expects to receive 
shall be Receive

LSP

Buffer

Size octets. (i.e. the Update 
process must provide buffers of at least this size for the re
ception, storage and forwarding of received Link State 
PDUs and Sequence Numbers PDUs.)  If a control PDU 
larger than this size is received, it shall be treated as if it 
had an invalid checksum (i.e. ignored by the Update Proc
ess and a corruptedLSPReceived notification generated).
Upon receipt of a Link State PDU the Update Process shall 
perform the following functions:
a)Level 2 Link State PDUs shall be propagated on cir
cuits which have at least one Level 2 adjacency. 
b)Level 1 Link State PDUs shall be propagated on cir
cuits which have at least one Level 1 adjacency or at 
least one Level 2 adjacency not marked Level 2 
only. 
c)When propagating a Level 1 Link State PDU on a 
broadcast subnetwork, the IS shall transmit to the 
multi-destination subnetwork address AllL1IS. 
d)When propagating a Level 2 Link State PDU on a 
broadcast subnetwork, the IS shall transmit to the 
multi-destination subnetwork address AllL2IS.
NOTE  When propagating a Link State PDU on a 
general topology subnetwork the Data Link Address 
is unambiguous (because Link State PDUs are not 
propagated across Dynamically Assigned circuits). 
e)An Intermediate system receiving a Link State PDU 
with an incorrect LSP Checksum or with an invalid 
PDU syntax shall 
1)log a circuit notification, corruptedLSPRe
ceived, 
2)overwrite the Checksum and Remaining Lifetime 
with 0, and 
3)treat the Link State PDU as though its Remaining 
Lifetime had expired (see 7.3.16.4.)
f)A Intermediate system receiving a Link State PDU 
which is new (as identified in 7.3.16) shall
1)store the Link State PDU into Link State database, 
and 
2)mark it as needing to be propagated upon all cir
cuits except that upon which it was received. 
g)When a Intermediate system receives a Link State 
PDU from source S, which it considers older than the 
one stored in the database for S, it shall set the 
SRMflag for S's Link State PDU associated with the 
circuit from which the older Link State PDU was re
ceived. This indicates that the stored Link State PDU 
needs to be sent on the link from which the older one 
was received.

h)When a system receives a Link State PDU which is 
the same (not newer or older) as the one stored, the In
termediate system shall
1)acknowledge it if necessary, as described in 7.3.17, 
and 
2)clear the SRMflag for that circuit for that Link 
State PDU. 
i)A Link State PDU received with a zero checksum 
shall be treated as if the Remaining Lifetime were 0. 
The age, if not 0, shall be overwritten with 0.
The Update Process scans the Link State Database for Link 
State PDUs with SRMflags set. When one is found, pro
vided the timestamp lastSent indicates that it was propa
gated no more recently than min

i

mum

LSP

Trans

mis

sion

Int

er

val, the IS shall 
a)transmit it on all circuits with SRMflags set, and 
b)update lastSent.
7.3.15 Manipulation of SRM and SSN Flags
For each Link State PDU, and for each circuit over which 
routeing messages are to be exchanged (i.e. not on DA cir
cuits), there are two flags:
Send Routeing Message (SRMflag)  if set, indicates that 
Link State PDU should be transmitted on that cir
cuit.  On broadcast circuits SRMflag is cleared as 
soon as the LSP has been transmitted, but on non-
broadcast circuits SRMflag is only cleared on recep
tion of a Link State PDU or Sequence Numbers 
PDU as described below.
	SRMflag shall never be set for an LSP with se
quence number zero, nor on a circuit whose exter
nalDomain attribute is True (See 7.3.15.2).
Send Sequence Numbers (SSNflag)  if set, indicates that 
information about that Link State PDU should be in
cluded in a Partial Sequence Numbers PDU trans
mitted on that circuit.  When the Sequence Numbers 
PDU has been transmitted SSNflag is cleared.  Note 
that the Partial Sequence Numbers PDU serves as an 
acknowledgement that a Link State PDU was re
ceived.
	SSNflag shall never be set on a circuit whose ex
ternalDomain attribute is True.
7.3.15.1 Action on Receipt of a Link State PDU
 When a Link State PDU is received on a circuit C, the IS 
shall perform the following functions
a)Perform the following PDU acceptance tests:
1)If the LSP was received over a circuit whose ex
ternalDomain attribute is True, the IS shall dis
card the PDU.
2)If the ID Length field of the PDU is not equal to 
the value of the IS's routeingDomainIDLength, 

the PDU shall be discarded and an iDField
LengthMismatch notification generated.
3)If this is a level 1 LSP, and the set of areaRe
ceivePasswords is non-null, then perform the 
following tests:
i)If the PDU does not contain the Authentica
tion Information field then the PDU shall be 
discarded and an authenticationFailure no
tification generated.
ii)If the PDU contains the Authentication In
formation field, but the Authentication 
Type is not equal to Password, then the 
PDU shall be accepted unless the IS imple
ments the authenticatiion procedure indicated 
by the Authentication Type. In this case 
whether the IS accepts or ignores the PDU is 
outside the scope of this International Stan
dard.
iii)Otherwise, the IS shall compare the password 
in the received PDU with the passwords in the 
set of areaReceivePasswords, augmented 
by the value of the areaTransmitPassword. 
If the value in the PDU matches any of these 
passwords, the IS shall accept the PDU for 
further processing. If the value in the PDU 
does not match any of the above values, then 
the IS shall ignore the PDU and generate an 
authenticationFailure notification.
4)If this is a level 2 LSP, and the set of domainRe
ceivePasswords is non-null, then perform the 
following tests:
i)If the PDU does not contain the Authentica
tion Information field then the PDU shall be 
discarded and an authenticationFailure no
tification generated.
ii)If the PDU contains the Authentication In
formation field, but the Authentication 
Type is not equal to Password, then the 
PDU shall be accepted unless the IS imple
ments the authenticatiion procedure indicated 
by the Authentication Type. In this case 
whether the IS accepts or ignores the PDU is 
outside the scope of this International Stan
dard.
iii)Otherwise, the IS shall compare the password 
in the received PDU with the passwords in the 
set of domainReceivePasswords, aug
mented by the value of the domainTransmit
Password. If the value in the PDU matches 
any of these passwords, the IS shall accept the 
PDU for further processing. If the value in the 
PDU does not match any of the above values, 
then the IS shall ignore the PDU and generate 
an authenticationFailure notification.
b)If the LSP has zero Remaining Lifetime, perform the 
actions described in 7.3.16.4.
c)If the source S of the LSP is an IS or pseudonode for 
which all but the last octet are equal to the systemID 

of the receiving Intermediate System, and the receiv
ing Intermediate System does not have that LSP in its 
database, or has that LSP, but no longer considers it to 
be in the set of LSPs generated by this system (e.g. it 
was generated by a previous incarnation of the sys
tem), then initiate a network wide purge of that LSP as 
described in 7.3.16.4.
d)If the source S of the LSP is a system (pseudonode or 
otherwise) for which the first ID Length octets are 
equal to the systemID of the receiving Intermediate 
system, and the receiving Intermediate system has an 
LSP in the set of currently generated LSPs from that 
source in its database (i.e. it is an LSP generated by 
this Intermediate system), perform the actions de
scribed in 7.3.16.1. 
e)Otherwise, (the source S is some other system),
1)If the LSP is newer than the one in the database, or 
if an LSP from that source does not yet exist in the 
database: 
i)Store the new LSP in the database, overwriting 
the existing database LSP for that source (if 
any) with the received LSP. 
ii)Set SRMflag for that LSP for all circuits 
other than C. 
iii)Clear SRMflag for C. 
iv)If C is a non-broadcast circuit, set SSNflag 
for that LSP for C. 
v)Clear SSNflag for that LSP for the circuits 
other than C.
2)If the LSP is equal to the one in the database (same 
Sequence Number, Remaining Lifetimes both zero 
or both non-zero, same checksums):
i)Clear SRMflag for C. 
ii)If C is a non-broadcast circuit, set SSNflag 
for that LSP for C.
3)If the LSP is older than the one in the database:
i)Set SRMflag for C. 
ii)Clear SSNflag for C.
When storing a new LSP, the Intermediate system shall first 
ensure that it has sufficient memory resources to both store 
the LSP and generate whatever internal data structures will 
be required to process the LSP by the Update Process.  If 
these resources are not available the LSP shall be ignored. 
It shall neither be stored nor acknowledged. When an LSP 
is ignored for this reason the IS shall enter the Waiting 
State. (See 7.3.19).
When attempting to store a new version of an existing LSP 
(with the same LSPID), which has a length less than or 
equal to that of the existing LSP, the existing LSP shall be 
removed from the routeing information base and the new 
LSP stored as a single atomic action. This ensures that such 
an LSP (which may be carrying the LSP Database Overload 
indication from an overloaded IS) will never be ignored as 
a result of a lack of memory resources.

7.3.15.2 Action on Receipt of a Sequence Numbers 
PDU
When a Sequence Numbers PDU (Complete or Partial, see 
7.3.17) is received on circuit C the IS shall perform the fol
lowing functions:
a)Perform the following PDU acceptance tests:
1)If the SNP was received over a circuit whose ex
ternalDomain attribute is True, the IS shall dis
card the PDU.
2)If the ID Length field of the PDU is not equal to 
the value of the IS's routeingDomainIDLength, 
the PDU shall be discarded and an iDField

Length

Mismatch notification generated.
3)If this is a level 1 SNP and the set of areaRe
ceivePasswords is non-null, then perform the 
following tests:
i)If the PDU does not contain the Authentica
tion Information field then the PDU shall be 
discarded and an authenticationFailure no
tification generated.
ii)If the PDU contains the Authentication In
formation field, but the Authentication 
Type is not equal to Password, then the 
PDU shall be accepted unless the IS imple
ments the authenticatiion procedure indicated 
by the Authentication Type. In this case 
whether the IS accepts or ignores the PDU is 
outside the scope of this International Stan
dard.
iii)Otherwise, the IS shall compare the password 
in the received PDU with the passwords in the 
set of areaReceivePasswords, augmented 
by the value of the areaTransmitPassword. 
If the value in the PDU matches any of these 
passwords, the IS shall accept the PDU for 
further processing. If the value in the PDU 
does not match any of the above values, then 
the IS shall ignore the PDU and generate an 
authenticationFailure notification.
4)If this is a level 2 SNP, and the set of domainRe
ceivePasswords is non-null, then perform the 
following tests:
i)If the PDU does not contain the Authentica
tion Information field then the PDU shall be 
discarded and an authenticationFailure no
tification generated.
ii)If the PDU contains the Authentication In
formation field, but the Authentication 
Type is not equal to Password, then the 
PDU shall be accepted unless the IS imple
ments the authenticatiion procedure indicated 
by the Authentication Type. In this case 
whether the IS accepts or ignores the PDU is 
outside the scope of this International Stan
dard.

iii)Otherwise, the IS shall compare the password 
in the received PDU with the passwords in the 
set of domainReceivePasswords, aug
mented by the value of the domainTransmit
Password. If the value in the PDU matches 
any of these passwords, the IS shall accept the 
PDU for further processing. If the value in the 
PDU does not match any of the above values, 
then the IS shall ignore the PDU and generate 
an authenticationFailure notification.
b)For each LSP reported in the Sequence Numbers 
PDU:
1)If the reported value equals the database value and 
C is a non-broadcast circuit, Clear SRMflag for C 
for that LSP. 
2)If the reported value is older than the database 
value, Clear SSNflag, and Set SRMflag.  
3)If the reported value is newer than the database 
value, Set SSNflag, and if C is a non-broadcast 
circuit Clear SRMflag. 
4)If no database entry exists for the LSP, and the re
ported Remaining Lifetime, Checksum and Se
quence Number fields of the LSP are all non-
zero, create an entry with sequence number 0 (see 
7.3.16.1), and set SSNflag for that entry and cir
cuit C.  Under no circumstances shall SRMflag be 
set for such an LSP with zero sequence number. 
NOTE - This is because possessing a zero sequence 
number LSP is semantically equivalent to having no 
information about that LSP.  If such LSPs were 
propagated by setting SRMflag it would result in an 
unnecessary consumption of both bandwidth and 
memory resources.
c)If the Sequence Numbers PDU is a Complete Se
quence Numbers PDU, Set SRMflags for C for all 
LSPs in the database (except those with zero sequence 
number or zero remaining lifetime) with LSPIDs 
within the range specified for the CSNP by the Start 
LSPID and End LSPID fields, which were not men
tioned in the Complete Sequence Numbers PDU (i.e. 
LSPs this system has, which the neighbour does not 
claim to have).
7.3.15.3 Action on expiration of Complete SNP 
Interval
The IS shall perform the following actions every 
CompleteSNPInterval seconds for circuit C:
a)If C is a broadcast circuit, then
1)If this Intermediate system is a Level 1 Designated 
Intermediate System on circuit C, transmit a com
plete set of Level 1 Complete Sequence Numbers 
PDUs on circuit C. Ignore the setting of SSNflag 
on Level 1 Link State PDUs.
If the value of the IS's areaTransmitPassword 
is non-null, then the IS shall include the Authenti
cation Information field in the transmitted 

CSNP, indicating an Authentication Type of 
Password and containing the areaTransmit
Password as the authentication value.
2)If this Intermediate system is a Level 2 Designated 
Intermediate System on circuit C, transmit a com
plete set of Level 2 Complete Sequence Numbers 
PDUs on circuit C. Ignore the setting of SSNflag 
on Level 2 Link State PDUs.
If the value of the IS's domainTransmitPass
word is non-null, then the IS shall include the 
Authentication Information field in the trans
mitted CSNP, indicating an Authentication Type 
of Password and containing the domainTrans
mitPassword as the authentication value.
A complete set of CSNPs is a set whose startLSPID 
and endLSPID ranges cover the complete possible 
range of LSPIDs. (i.e. there is no possible LSPID 
value which does not appear within the range of one 
of the CSNPs in the set).  Where more than one CSNP 
is transmitted on a broadcast circuit, they shall be 
separated by an interval of at least min

i

mum

Broad

cast

LSP

TransmissionInterval seconds.  
NOTE  An IS is permitted to transmit a small number 
of CSNPs (no more than 10) with a shorter separation in
terval, (or even back to back), provided that no more 
than 1000/minimum

Broad

cast

LSP

Trans

mis

sion

Int

er

val CSNPs are transmitted in any one second period. 
b)Otherwise (C is a point to point circuit, including non-
DA DED circuits and virtual links), do nothing. 
CSNPs are only transmitted on point to point circuits 
at initialisation.
7.3.15.4 Action on expiration of Partial SNP 
Interval
The maximum sized Level 1 or Level 2 PSNP which may 
be generated by a system is controlled by the values of 
originating

L1

LSP

Buf

fer

Size or  originating

L2

LSP

Buffer

Size respectively. An Intermediate system shall per
form the following actions every partialSNPInterval sec
onds for circuit C with jitter applied as described in 10.1:
a)If C is a broadcast circuit, then
1)If this Intermediate system is a Level 1 Intermedi
ate System or a Level 2 Intermediate System with 
manual

L2

Only

Mode False, but is not a 
Level 1 Designated Intermediate System on circuit 
C, transmit a Level 1 Partial Sequence Numbers 
PDU on circuit C, containing entries for as many 
Level 1 Link State PDUs with SSNflag set as will 
fit in the PDU, and then clear SSNflag for these 
entries. To avoid the possibility of starvation, the 
scan of the LSP database for those with SSNflag 
set shall commence with the next LSP which was 
not included in the previous scan. If there were no 
Level 1 Link State PDUs with SSNflag set, do 
not transmit a Level 1 Partial Sequence Numbers 
PDU. 

If the value of the IS's areaTransmitPassword 
is non-null, then the IS shall include the Authenti
cation Information field in the transmitted 
PSNP, indicating an Authentication Type of 
Password and containing the areaTransmit
Password as the authentication value.
2)If this Intermediate system is a Level 2 Intermedi
ate System, but is not a Level 2 Designated Inter
mediate System on circuit C, transmit a Level 2 
Partial Sequence Numbers PDU on circuit C, con
taining entries for as many Level 2 Link State 
PDUs with SSNflag set as will fit in the PDU, 
and then clear SSNflag for these entries. To avoid 
the possibility of starvation, the scan of the LSP 
database for those with SSNflag set shall com
mence with the next LSP which was not included 
in the previous scan. If there were no Level 2 Link 
State PDUs with SSNflag set, do not transmit a 
Level 2 Partial Sequence Numbers PDU. 
If the value of the IS's domainTransmitPass
word is non-null, then the IS shall include the 
Authentication Information field in the trans
mitted PSNP, indicating an Authentication Type 
of Password and containing the domainTrans
mitPassword as the authentication value.
b)Otherwise (C is a point to point circuit, including non-
DA DED circuits and virtual links)
1)If this system is a Level 1 Intermediate system, 
transmit a Level 1 Partial Sequence Numbers PDU 
on circuit C, containing entries for as many Level 
1 Link State PDUs with SSNflag set as will fit in 
the PDU, and then clear SSNflag for these en
tries. To avoid the possibility of starvation, the 
scan of the LSP database for those with SSNflag 
set shall commence with the next LSP which was 
not included in the previous scan. If there were no 
Level 1 Link State PDUs with SSNflag set, do 
not transmit a Partial Sequence Numbers PDU.
If the value of the IS's areaTransmitPassword 
is non-null, then the IS shall include the Authenti
cation Information field in the transmitted 
PSNP, indicating an Authentication Type of 
Password and containing the areaTransmit
Password as the authentication value.
2)If this system is a Level 2 Intermediate system, 
transmit a Level 2 Partial Sequence Numbers PDU 
on circuit C, containing entries for as many Level 
2 Link State PDUs with SSNflag set as will fit in 
the PDU, and then clear SSNflag for these en
tries. To avoid the possibility of starvation, the 
scan of the LSP database for those with SSNflag 
set shall commence with the next LSP which was 
not included in the previous scan. If there were no 
Level 2 Link State PDUs with SSNflag set, do 
not transmit a Partial Sequence Numbers PDU.
If the value of the IS's domainTransmitPass
word is non-null, then the IS shall include the 
Authentication Information field in the trans
mitted PSNP, indicating an Authentication Type 

of Password and containing the domainTrans
mitPassword as the authentication value.
7.3.15.5 Action on expiration of Minimum LSP 
Transmission Interval
An IS shall perform the following actions every min

i

mum

LSP

Trans

mis

sion

Int

er

val seconds with jitter applied as 
described in 10.1. 
a)For all Point to Point circuits C transmit all LSPs that 
have SRMflag set on circuit C, but do not clear the 
SRMflag. The SRMflag will subsequently be 
cleared by receipt of a Complete or Partial Sequence 
Numbers PDU. 
The interval between two consecutive transmissions of the 
same LSP shall be at least min

i

mum

LSP

Trans

mis

sion

Int

er

val. Clearly, this can only be achieved precisely by keep
ing a separate timer for each LSP. This would be an unwar
ranted overhead. Any technique which ensures the interval 
will be between min

i

mum

LSP

Trans

mis

sion

Int

er

val and 
2 * min

i

mum

LSP

Trans

mis

sion

Int

er

val is acceptable.   
7.3.15.6 Controlling the Rate of Transmission on 
Broadcast Circuits
The attribute min

i

mum

Broad

cast

LSP

Trans

mis

sion

Inter

val indicates the minimum interval between PDU arri
vals which can be processed by the slowest Intermediate 
System on the LAN.
Setting SRMflags on an LSP for a broadcast circuit does 
not cause the LSP to be transmitted immediately. Instead 
the Intermediate system shall scan the LSP database every 
min

i

mum

Broad

cast

LSP

Trans

mis

sion

Int

er

val (with 
jitter applied as described in 10.1), and from the set of LSPs 
which have SRMflags set for this circuit, one LSP shall be 
chosen at random. This LSP shall be multicast on the cir
cuit, and SRMflags cleared.
NOTE - In practice it would be very inefficient to scan the 
whole database at this rate, particularly when only a few 
LSPs had SRMflags set.  Implementations may require ad
ditional data structures in order to reduce this overhead.
NOTE - An IS is permitted to transmit a small number of 
LSPs (no more than 10) with a shorter separation interval, 
(or even back to back), provided that no more than 
1000/min

i

mum

Broad

cast

LSP

Trans

mis

sion

Int

er

val LSPs 
are transmitted in any one second period. 
In addition, the presence of any LSPs which have been re
ceived on a particular circuit and are queued awaiting proc
essing shall inhibit transmission of LSPs on that circuit. 
However, LSPs may be transmitted at a minimum rate of 
one per second even in the presence of such a queue.
7.3.16 Determining the Latest Information
The Update Process is responsible for determining, given a 
received link state PDU, whether that received PDU repre
sents new, old, or duplicate information with respect to 
what is stored in the database.

It is also responsible for generating the information upon 
which this determination is based, for assigning a sequence 
number to its own Link State PDUs upon generation, and 
for correctly adjusting the Remaining Lifetime field upon 
broadcast of a link state PDU generated originally by any 
system in the domain.
7.3.16.1 Sequence Numbers
The sequence number is a 4 octet unsigned value. Sequence 
numbers shall increase from zero to (SequenceModulus 
- 1). When a system initialises, it shall start with sequence 
number 1 for its own Link State PDUs.55It starts with 1 rather than 0
so that the value 0 can be reserved to be guaranteed to be less than
the sequence number of any actually generated Link State 
PDU. This is a useful property for Sequence Numbers PDUs.

The sequence numbers the Intermediate system generates 
for its Link State PDUs with different values for LSP num
ber are independent. The algorithm for choosing the num
bers is the same, but operationally the numbers will not be 
synchronised. 
If an Intermediate system R somewhere in the domain has 
information that the current sequence number for source S 
is greater than that held by S, R will return to S a Link State 
PDU for S with R's value for the sequence number. When S 
receives this LSP it shall change its sequence number to be 
the next number greater than the new one received, and 
shall generate a link state PDU.
If an Intermediate system needs to increment its sequence 
number, but the sequence number is already equal to 
SequenceModulus  1, the notification attempt

To

Ex

ceed

Maximum

Se

quence

Num

ber shall be generated and 
the Routeing Module shall be disabled for a period of at 
least MaxAge + ZeroAgeLifetime, in order to be sure 
that any versions of this LSP with the high sequence num
ber have expired. When it is re-enabled the IS shall start 
again with sequence number 1.
7.3.16.2 LSP Confusion
It is possible for an LSP generated by a system in a previ
ous incarnation to be alive in the domain and have the same 
sequence number as the current LSP.
To ensure database consistency among the Intermediate 
Systems, it is essential to distinguish two such PDUs. This 
is done efficiently by comparing the checksum on a re
ceived LSP with the one stored in memory.
If the sequence numbers match, but the checksums do not 
and the LSP is not in the current set of LSPs generated by 
the local system, then the system that notices the mismatch 
shall treat the LSP as if its Remaining Lifetime had expired. 
It shall store one of the copies of the LSP, with zero written 
as the Remaining Lifetime, and flood the LSP.
If the LSP is in the current set of LSPs generated by the lo
cal system then the IS shall change the LSP's sequence 
number to be the next number greater than that of the re
ceived LSP and regenerate the LSP.

7.3.16.3 Remaining Lifetime field
When the source generates a link state PDU, it shall set the 
Remaining Lifetime to MaxAge. 
When a system holds the information for some time before 
successfully transmitting it to a neighbour, that system shall 
decrement the Remaining Lifetime field according to the 
holding time. Before transmitting a link state PDU to a 
neighbour, a system shall decrement the Remaining Life
time in the PDU being transmitted by at least 1, or more 
than 1  if the transit time to that neighbour is estimated to 
be greater than one second. When the Remaining Lifetime 
field reaches 0, the system shall purge that Link State PDU 
from its database. In order to keep the Intermediate Sys
tems' databases synchronised, the purging of an LSP due to 
Remaining Lifetime expiration is synchronised by flooding 
an expired LSP. See 7.3.16.4.
If the RemainingLifetime of the received LSP is zero it 
shall be processed as described in 7.3.16.4. If the Remain
ing Lifetime of the received LSP is non-zero, but there is an 
LSP in the database with the same sequence number and 
zero Remaining Lifetime, the LSP in the database shall be 
considered most recent. Otherwise, the PDU with the larger 
sequence number shall be considered the most recent.
If the value of Remaining Lifetime is greater than  
MaxAge, the LSP shall be processed as if there were a 
checksum error.
7.3.16.4  LSP Expiration Synchronisation 
When the Remaining Lifetime on an LSP in memory be
comes zero, the IS shall
a)set all SRMflags for that LSP, and 
b)retain only the LSP header.
c)record the time at which the Remaining Lifetime for 
this LSP became zero. When ZeroAgeLifetime has 
elapsed since the LSP Remaining Lifetime became 
zero, the LSP header shall be purged from the data
base. 
NOTE - A check of the checksum of a zero Remaining Life
time LSP succeeds even though the data portion is not pre
sent
When a purge of an LSP with non-zero Remaining Lifetime 
is initiated, the header shall be retained for MaxAge.
If an LSP from source S with zero Remaining Lifetime is 
received on circuit C :
a)If no LSP from S is in memory, then the IS shall
1)send an acknowledgement of the LSP on circuit C, 
but 
2)shall not retain the LSP after the acknowledgement 
has been sent. 

b)If an LSP from S is in the database, then
1)If the received LSP is newer than the one in the da
tabase (i.e. received LSP has higher sequence 
number, or same sequence number and database 
LSP has non-zero Remaining Lifetime) the IS 
shall:
i)overwrite the database LSP with the received 
LSP, and note the time at which the zero Re
maining Lifetime LSP was received, so that 
after ZeroAgeLifetime has elapsed, that LSP 
can be purged from the database,  
ii)set SRMflag for that LSP for all circuits other 
than C,
iii)clear SRMflag for C,
iv)if C is a non-broadcast circuit, set SSNflag 
for that LSP for C, and 
v)clear SSNflag for that LSP for the circuits 
other than C.
2)If the received LSP is equal to the one in the data
base (i.e. same Sequence Number, Remaining 
Lifetimes both zero) the IS shall:
i)clear SRMflag for C, and
ii)if C is a non-broadcast circuit, set SSNflag 
for that LSP for C.
3)If the received LSP is older than the one in the da
tabase (i.e. received LSP has lower sequence num
ber) the IS shall:
i)set SRMflag for C, and
ii)clear SSNflag for C.
c)If this system (or pseudonode) is S and there is an un-
expired LSP from S (i.e. its own LSP) in memory, 
then the IS:
1)shall not overwrite with the received LSP, but 
2)shall change the sequence number of the un-
expired LSP from S as described in 7.3.16.1, 
3)generate a new LSP; and 
4)set SRMflag on all circuits.
7.3.17 Making the Update Reliable
The update process is responsible for making sure the latest 
link state PDUs reach every reachable Intermediate System 
in the domain.
On point-to-point links the Intermediate system shall send 
an explicit acknowledgement encoded as a Partial Sequence 
Numbers PDU (PSNP) containing the following informa
tion: 
a)source's ID 
b)PDU type (Level 1 or 2) 
c)sequence number 

d)Remaining Lifetime 
e)checksum
This shall be done for all received link state PDUs which 
are newer than the one in the database, or duplicates of the 
one in the database. Link state PDUs which are older than 
that stored in the database are answered instead by a newer 
link state PDU, as specified in 7.3.14 above. 
On broadcast links, instead of explicit acknowledgements 
for each link state PDU by each Intermediate system, a spe
cial PDU known as a Complete Sequence Numbers PDU 
(CSNP), shall be multicast periodically by the Designated 
Intermediate System. The PDU shall contain a list of all 
LSPs in the database, together with enough information so 
that Intermediate systems receiving the CSNP can compare 
with their LSP database to determine whether they and the 
CSNP transmitter have synchronised LSP databases.  The 
maximum sized Level 1 or Level 2 Sequence Numbers 
PDU which may be generated by a system is controlled by 
the values of originating

L1

LSP

Buf

fer

Size or originat
ingL2LSPBufferSize respectively. In practice, the infor
mation required to be transmitted in a single CSNP may be 
greater than will fit in a single PDU. Therefore each CSNP 
carries an inclusive range of LSPIDs to which it refers. The 
complete set of information shall be conveyed by transmit
ting a series of individual CSNPs, each referring to a subset 
of the complete range. The ranges of the complete set of 
CSNPs shall be contiguous (though not necessarily trans
mitted in order) and shall cover the entire range of possible 
LSPIDs.
The  LAN Level 1 Designated Intermediate System shall 
periodically multicast complete sets of Level 1 CSNPs to 
the multi-destination address AllL1ISs. The LAN Level 2 
Designated Intermediate System shall periodically multicast 
complete sets of Level 2 CSNPs to the multi-destination ad
dress AllL2ISs.
Absence of an LSPID from a Complete Sequence Numbers 
PDU whose range includes that LSPID indicates total lack 
of information about that LSPID.
If an Intermediate system, upon receipt of a Complete Se
quence Numbers PDU, detects that the transmitter was out 
of date, the receiver shall multicast the missing information.
NOTE - Receipt of a link state PDU on a link is the same as 
successfully transmitting the Link State PDU on that link, so 
once the first Intermediate system responds, no others will, 
unless they have already transmitted replies.
If an Intermediate system detects that the transmitter had 
more up to date information, the receiving Intermediate sys
tem shall multicast a Partial Sequence Numbers PDU 
(PSNP), containing information about LSPs for which it has 
older information. This serves as an implicit request for the 
missing information. Although the PSNP is multicast, only 
the Designated Intermediate System of the appropriate level 
shall respond to the PSNP. 
NOTE - This is equivalent to the PSNP being transmitted di
rectly to the Designated Intermediate System, in that it 
avoids each Intermediate System unnecessarily sending the 
same LSP(s) in response. However, it has the advantage of 
preserving the property that all routeing messages can be re

ceived on the multi-destination addresses, and hence by a 
LAN adapter dedicated to the multi-destination address.
When a non-broadcast circuit (re)starts, the IS shall:
a)set SRMflag for that circuit on all LSPs, and 
b)send a Complete set of Complete Sequence Numbers 
PDUs on that circuit.
7.3.18 Validation of Databases
An Intermediate System shall not continue to operate for an 
extended period with corrupted routeing information. The 
IS shall therefore operate in a fail-stop manner. If a failure 
is detected, the Intermediate system Network entity shall be 
disabled until the failure is corrected. In the absence of an 
implementation-specific method for ensuring this, the IS 
shall perform the following checks at least every max

i

mum

LSPGenerationInterval seconds:
a)On expiration of this timer the IS shall re-check the 
checksum of every LSP in the LSP database (except 
those with a Remaining Lifetime of zero) in order to 
detect corruption of the LSP while in memory. If the 
checksum of any LSP is incorrect, the notification 
corruptedLSPDetected shall be logged, and as a 
minimum the entire Link State Database shall be de
leted and action taken to cause it to be re-acquired. 
One way to achieve this is to disable and re-enable the 
IS Network entity. 
NOTE  On point to point links, this requires at least 
that a CSNP be transmitted.
b)On completion of these checks the decision process 
shall be notified of an event (even if any newly gener
ated LSPs have identical contents to the previous 
ones). This causes the decision process to be run and 
the forwarding databases re-computed, thus protecting 
against possible corruption of the forwarding data
bases in memory, which would not otherwise be de
tected in a stable topology. 
c)The IS shall reset the timer for a period of 
maximumLSPGenerationInterval with jitter ap
plied as described in 10.1.
7.3.19 LSP Database Overload
As a result of network mis-configuration, or certain transi
tory conditions, it is possible that there may be insufficient 
memory resources available to store a received Link State 
PDU. When this occurs, an IS needs to take certain steps to 
ensure that if its LSP database becomes inconsistent with 
the other ISs', that these ISs do not rely on forwarding 
paths through the overloaded IS.
7.3.19.1 Entering the Waiting State
When an LSP cannot be stored, the LSP shall be ignored 
and Waiting State shall be entered. A timer shall be started 
for waitingTime seconds, and the Intermediate System 
shall generate and flood its own LSP with zero LSP number 
with the LSP Database Overload Bit set. This prevents 

this Intermediate system from being considered as a for
warding path by other Intermediate Systems. 
It is possible that although there are sufficient resources to 
store an LSP and permit the operation of the Update Proc
ess on that LSP, the Decision Process may subsequently re
quire further resources in order to complete. If these re
sources are not available, the Intermediate system shall then 
(i.e. during the attempt to run the Decision Process) enter 
Waiting State until such time as they are available and 
waitingTime seconds have elapsed since the last LSP was 
ignored by the Update Process. 
An implementation shall partition the available memory re
sources between the Level 1 and Level 2 databases. An 
overload condition can therefore exist independently for 
Level 1 or Level 2 (or both). The status attributes l1State 
and l2State indicate the condition for the Level 1 and 
Level 2 databases respectively. On entering Level 1 Wait
ing State the IS shall generate the lSP

L1

Data

base

Over

load notification, and on entering Level 2 Waiting State 
the IS shall generate the lSP

L2

Data

base

Over

load notifi
cation. 
7.3.19.2 Actions in Level 1 Waiting State
While in Level 1 waiting state
a)If a Link State PDU cannot be stored, the IS shall ig
nore it and restart the timer for waitingTime seconds.
b)The IS shall continue to run the Decision and For
warding processes as normal.
c)When the waitingTime timer expires, the IS shall:
1)Generate an lSP

L1

Data

base

Over

load  (recov
ered) notification. 
2)Clear the LSP Database Overload bit in its own 
Level 1 LSP with zero LSP number and re-issue it.  
3)Set the l1State to On.
4)Resume normal operation.
7.3.19.3 Actions in Level 2 Waiting State
While in Level 2 waiting state
a)If a Link State PDU cannot be stored, the IS shall ig
nore it and restart the timer for waitingTime seconds. 
b)The IS shall continue to run the Decision and For
warding processes as normal. 
c)When the waitingTime timer expires, the IS shall:
1)Generate an lSP

L2

Data

base

Over

load  (recov
ered) notification. 
2)Clear the LSP Database Overload bit in its own 
Level 2 LSP with zero LSP number and re-issue it.  
3)Set the l2State to On.
4)Resume normal operation.

7.3.20 Use of the Link State Database
The only portion of the database relevant to the  Decision 
Process is the data portion of the Link State PDUs.
The Update Process additionally uses the fields Sequence 
Number, Remaining Lifetime, and variable SRMflag. 
The Remaining Lifetimes in the stored link state PDUs can 
either be periodically decremented, or converted upon re
ceipt into an internal timestamp, and converted back into a 
Remaining Lifetime upon transmission.
7.3.20.1 Synchronisation with the Decision Process
Since the Update Process and the Decision Process share 
the Link State Database, care must be taken that the Update 
Process does not modify the Link State Database while the 
Decision Process is running.
There are two approaches to this. In one approach, the De
cision Process signals when it is running. During this time, 
the Update Process queues incoming Link State PDUs, and 
does not write them into the Link State Database. If more 
Link State PDUs arrive than can fit into the queue allotted 
while the Decision Process is running, the Update Process 
drops them and does not acknowledge them.
Another approach is to have two copies of the Link State 
Database  one in which the Decision Process is comput
ing, and the other in which the Update Process initially cop
ies over the first database, and in which all new Link State 
PDUs are written. Additionally, depending on the hashing 
scheme, it is likely that a second copy of the address hash 
table will be required, so that the Update Process can do a 
rehash occasionally for efficiency.
When the Decision Process is ready to run again, it locks 
the new copy of the Link State Database, leaving the Up
date Process to copy over the information into the first area, 
and write new updates while the Decision Process runs 
again.
The advantage of the first approach is that it takes less 
memory. The advantage of the second approach is that Link 
State PDUs will never need to be dropped.
NOTE - If the decision process is implemented according to 
the specification in C.2, a finer level of parallelism is possi
ble, as described below.

Arrival of a Link State PDU for a system before that system 
has been put into TENT is permitted. The new Link State 
PDU is used when that system is eventually put into TENT. 
Similarly, arrival of a new Link State PDU for a system af
ter that system has been put into PATHS is permitted. That 
system has already been completely processed. The arrival 
of the new Link State PDU is noted and the decision process 
re-executed when the current execution has completed. An 
in-progress execution of the decision process shall not be 
abandoned, since this could prevent the decision process 
from ever completing.

Arrival of a Link State PDU for a system between that sys
tem being put on TENT and being transferred to PATHS 
shall be treated as equivalent to one of the previous two 
cases (for example, by buffering, or taking some corrective 
action).

7.3.20.2 Use of Buffers and Link Bandwidth
Implementations shall have a buffer management strategy 
that does not prevent other clients of the buffering service 
from acquiring buffers due to excessive use by the Update 
Process. They shall also ensure that the Update Process 
does not consume all the available bandwidth of links. In 
particular no type of traffic should experience starvation for 
longer than its acceptable latency. Acceptable latencies are 
approximately as follows:
-Hello traffic  Hello timer W 0.5 
-Data Traffic  10 seconds.
NOTE - The first of these requirements can be met by re
stricting the Update process to the use of a single buffer on 
each circuit for transmission. This may also cause the sec
ond requirement to be met, depending on the processor 
speed.

7.3.21 Parameters
MaxAge  This is the amount of time that may elapse 
since the estimated origination of the stored Link 
State PDU by the source before the LSP is consid
ered expired. The expired LSP can be deleted from 
the database after a further ZeroAgeLifetime has 
expired.  MaxAge shall be larger than maximum

LSP

Generation

Interval, so that a system is not 
purged merely because of lack of events for report
ing Link State PDUs.
	MaxAge is an architectural constant equal to 20 
minutes.
ZeroAgeLifetime - This is the minimum amount of time 
for which the header of an expired LSP shall be re
tained after it has been flooded with zero Remaining 
Lifetime.  A very safe value for this would be 
2 W MaxAge.  However all that is required is that 
the header be retained until the zero Remaining Life
time LSP has been safely propagated to all the 
neighbours.
	ZeroAgeLifetime is an architectural constant with 
a value of 1 minute.
maximumLSPGenerationInterval  This is the maxi
mum amount of time allowed to elapse between gen
eration of Link State PDUs by a source. It shall be 
less than MaxAge.
	Setting this parameter too fast adds overhead to the 
algorithms (a lot of Link State PDUs). Setting this 
parameter too slow (and not violating constraints) 
causes the algorithm to wait a long time to recover 
in the unlikely event that incorrect Link State infor
mation exists somewhere in the domain about the 
system.
	A reasonable setting is 15 minutes. 

minimumLSPGenerationInterval  This is the minimum 
time interval between generation of Link State 
PDUs.  A source Intermediate system shall wait at 
least this long before re-generating one of its own 
Link State PDUs.
	Setting this too large causes a delay in reporting new 
information. Setting this too small allows too much 
overhead.
	A reasonable setting is 30 seconds.
min

i

mum

LSP

Trans

mis

sion

Int

er

val  This is the amount 
of time an Intermediate system shall wait before fur
ther propagating another Link State PDU from the 
same source system.
	Setting this too large causes a delay in propagation 
of routeing information and stabilisation of the 
routeing algorithm. Setting this too small allows the 
possibility that the routeing algorithm, under low 
probability circumstances, will use too many re
sources (CPU and bandwidth).
	Setting min

i

mum

LSP

Trans

mis

sion

Int

er

val greater 
than minimumLSPGenerationInterval makes no 
sense, because the source would be allowed to gen
erate LSPs more quickly than they'd be allowed to 
be broadcast. Setting min

i

mum

LSP

Trans

mis

sion

Int

er

val smaller than min

i

mum

LSP

Generation

Inter

val is desirable to recover from lost LSPs.
	A reasonable value is 5 seconds.
CompleteSNPInterval  This is the amount of time be
tween periodic transmissions of a complete set of 
Sequence Number PDUs by the Designated Interme
diate system on a broadcast link. Setting this too low 
slows down the convergence of the routeing algo
rithm when Link State PDUs are lost due to the 
datagram environment of the Data Link layer on the 
broadcast link.
	Setting this too high results in extra control traffic 
overhead.
	A reasonable value is 10 seconds. 

7.4 The Forwarding Process
The forwarding process is responsible both for transmitting 
NPDUs originated by this system, and for forwarding 
NPDUs originated by other systems
7.4.1 Input and Output
INPUT
-NPDUs from the ISO 8473 protocol machine 
-PDUs from Update Process 
-PDUs from Receive Process 
-Forwarding Databases (Level 1 and 2)  one for each 
routeing metric
OUTPUT
-PDUs to Data Link Layer
7.4.2 Routeing Metric Selection
The Forwarding process selects a forwarding database for 
each NPDU to be relayed based on:
-the level at which the forwarding is to occur: level 1 
or level 2; and
-a mapping of the ISO 8473 QoS Maintenance field 
onto one of the Intermediate system's supported route
ing metrics. 
The former selection is made by examining the Destination 
Address field of the NPDU.
The latter selection is made as follows:
a)If the QoS Maintenance field is not present in the 
NPDU, then the IS shall select the forwarding data
base calculated for the default metric.
b)If the QoS Maintenance field is present, the IS shall 
examine bits 7 and 8 of the parameter value octet. If 
these two bits specify any combination other than 1 
1 (meaning globally unique QoS), then the IS shall 
select the forwarding database calculated for the de
fault metric, otherwise
c)The IS shall select a forwarding database by mapping 
the values of bits 3, 2 and 1 of the parameter value as 
shown below in table 1 and shall proceed as follows:
1)If the IS does not support the selected routeing 
metric, the IS shall forward based upon the default 
metric;
2)If the forwarding database for one of the optional 
routeing metrics is selected and the database either 
does not contain an entry for the Destination Ad
dress in the NPDU being relayed, or contains an 
entry indicating that the destination is unreachable 
using that metric, then the IS shall attempt to for
ward based upon the default metric;

3)Otherwise, forward based on the selected optional 
metric.
Table 1 - QoS Maintenance bits to routeing 
metric mappingsSelected Routeing Metric
bit 3
bit 2
bit 1
expense metric
0
0
0
default metric
0
0
1
expense metric
0
1
0
delay  metric
1
0
0
error metric
0
1
1
delay metric
1
0
1
error metric
1
1
1
default metric
1
1
0

7.4.3 Forwarding Decision
7.4.3.1 Basic Operation
Let DEST = the Network Layer destination address of the 
PDU to be forwarded, or the next entry in the source route
ing field, if present. It consists of sub-fields Area Address, 
ID, and SEL. 
NOTE - The SEL field in the destination address is not ex
amined by Intermediate Systems. It is used by End Systems 
to select the proper Transport entity to which to deliver NS
DUs. 
This system's (the one examining this PDU for proper for
warding decision) address consists of sub-fields area ad
dress  and ID.
a)If the local system type is a level 1 Intermediate sys
tem, or the local system type is a level 2 Intermediate 
system and AttachedFlagk = False, then:
1)If the Area Address in the PDU to be forwarded 
matches any one of the area addresses of this IS, 
then consult the level 1 forwarding database to de
termine the adjacency which is the next hop on the 
path to the NPDU's destination. Forward the 
NPDU on this adjacency.
2)Otherwise, consult the level 1 forwarding database 
to determine the adjacency which is the next hop 
on the path to the nearest level 2 is in the area, and 
forward the NPDU on this adjacency.
b)If the local system type is Level 2, and Attached
Flagk = True then:
1)If the Area Address in the PDU to be forwarded 
matches any one of the area addresses of this IS, 

then consult the level 1 forwarding database to de
termine the adjacency which is the next hop on the 
path to the NPDU's destination. Forward the 
NPDU on this adjacency.
2)Otherwise, consult the level 2 forwarding database 
to determine the adjacency which is the next hop 
on the path to the destination area, and forward the 
NPDU on this adjacency.
7.4.3.2 Encapsulation for Partition Repair
If this Intermediate system is the Partition Designated 
Level 2 IS for this partition, and the PDU is being for
warded onto the special adjacency to a Partition Designated 
Level 2 Intermediate system in a different partition of this 
area, encapsulate the complete PDU as the data field of a 
data NPDU (i.e., with an additional layer of header), mak
ing this system the Source address and the other Partition 
Designated Level 2 Intermediate system (obtained from the 
identifier attribute of the Virtual Adjacency managed ob
ject) the Destination Address field in the outer PDU 
header. Set the QoS Maintenance field of the outer PDU 
to indicate forwarding via the default routeing metric (see 
table 1). Then forward the encapsulated PDU onto an adja
cency ADJ, obtained by calling the Forward procedure, de
scribed below. 
7.4.3.3 The Procedure Forward
This procedure chooses, from a Level 1 forwarding data
base  if level is level1, or from a Level 2 forwarding da
tabase  if level is level2, an adjacency on which to for
ward NPDUs for destination dest. A pointer to the adja
cency is returned in adj, and the procedure returns the value 
True. A destination of 0 at level 1 selects the adjacency 
for the nearest level 2 IS computed as described in 7.2.9.1.
If there are multiple possible adjacencies, as a result of mul
tiple minimum cost paths, then one of those adjacencies 
shall be chosen. An implementation may chose the adja
cency at random, or may use the possible adjacencies in 
round robin fashion.
If there is no entry in the selected forwarding database for 
the address dest, and the NPDU originated from the a local 
Transport entity  and the system has one or more Intermedi
ate System adjacencies, then one of those is chosen at ran
dom (or in round robin fashion) and the procedure returns 
the value True. Otherwise the procedure returns the value 
False.66This is done so that a system in the overloaded state will
still be able to originate or forward NPDUs. If a system with a partial
routeing information base 
were prohibited from attempting to forward to an unknown destination,
system management would be unable to either communicate with this system, or 
route through it, for the purpose of diagnosing and/or correcting the
underlying fault.

NOTE -  Since the local adjacency database is pre-loaded 
into the decision process, there will always be an entry in 
the forwarding database for destinations to which an adja
cency exists.
NOTE - The PDU to be forwarded may require fragmenta
tion, depending on which circuit it is to be forwarded over. 
Generating Redirect PDUs

In addition to forwarding an NPDU, the IS shall inform the 
local ISO 9542 protocol machine to generate a Redirect 
PDU if the PDU is being forwarded onto the same circuit 
from which it came, and if the source SNPA address of the 
NPDU indicates that the NPDU was received from an End 
System. 
7.4.4 The Receive Process
The Receive Process is passed information from any of the 
following sources.
-received PDUs with the NLPID of Intra-Domain 
routeing, 
-configuration information from the ISO 9542 protocol 
machine, 
-ISO 8473 data PDUs handed to the routeing function 
by the ISO 8473 protocol machine.
When an area is partitioned, a level 2 path is used as a 
level 1 link to repair the partitioned area. When this occurs, 
all PDUs (between the neighbours which must utilise a 
multi-hop path for communication) shall be encapsulated in 
a data NPDU, addressed to the Intra-Domain routeing se
lector. Control traffic (LSPs, Sequence Numbers PDUs) 
shall also be encapsulated, as well as data NPDUs that are 
to be passed between the neighbours.
NOTE - It is not necessary to transmit encapsulated IIH 
PDUs over a virtual link, since virtual adjacencies are estab
lished and monitored by the operation of the Decision Proc
ess and not the Subnetwork Dependent functions 
The Receive Process shall perform the following functions:
-If it is a data NPDU, addressed to this system with 
SEL = Intra-Domain routeing, then 
7decapsulate the NPDU (remove the outer NPDU 
header).
7If the decapsulated PDU is a data NPDU, move 
the congestion indications to the decapsulated 
NPDU, and pass it to the ISO 8473 protocol ma
chine.
7Otherwise, if the decapsulated PDU is not an ISO 
8473 PDU, perform the following steps on the de
capsulated PDU:
-If it is a Link State PDU, pass it to the Update Process 
-If it is a Sequence Numbers PDU, pass it to the Up
date Process 
-If it is an IIH PDU, pass it to the appropriate 
Subnetwork Dependent Function 
-If it is a data NPDU or Error Report for another desti
nation, pass it to the Forwarding Process
-Otherwise, ignore the PDU

7.5 Routeing Parameters
The routeing parameters setable by System Management 
are listed for each managed object in clause 11.
7.5.1 Architectural Constants
The architectural constants are described in Table 2.

Table 2 - Routeing architectural constantsName
Value
Description
MaxLinkMetric
63.
Maximum value of a routeing metric assign
able to a circuit
MaxPathMetric
1023.
Maximum total metric value for a complete 
path
AllL1ISs
01-80-C2-00-00-14
The multi-destination address All Level 1 In
termediate Systems
AllL2ISs
01-80-C2-00-00-15
The multi-destination address All Level 2 In
termediate Systems
AllIntermediateSystems
09-00-2B-00-00-05
The multi-destination address All Intermedi
ate  Systems used by ISO 9542
ISO-SAP
FE
The SAP for ISO Network Layer on 
ISO 8802-3 LANs
IntradomainRoute

ing-
PD
10000011
The Network Layer Protocol Discriminator 
assigned by ISO/TR 9577 for this Protocol
IntradomainRouteing
Selector
0.
The NSAP selector for the Intermediate Sys
tem Network entity
SequenceModulus
232
Size of the sequence number space used by 
the Update Process
ReceiveLSPBuffer

Size
1492.
The size of LSP which all Intermediate sys
tems must be capable of receiving.
MaxAge
1200.
Number of seconds before LSP considered ex
pired.
ZeroAgeLifetime
60.
Number of seconds that an LSP with zero Re
maining Lifetime shall be retained after 
propagating a purge.
AllEndSystems
09-00-2B-00-00-04
The multi-destination address All End Sys
tems used by ISO 9542
Max

i

mum

Area

Addresses
3.
The maximum number of area addresses 
which may exist for a single area.
HoldingMultiplier
3.
The number by which to multiply hello

Timer 
to obtain Holding Timer for ISH PDUs and 
for Point to Point IIH PDUs.
ISISHoldingMultiplier
10.
The number by which to multiply iSISHel
loTimer to obtain Holding Timer for Level 1 
and Level 2 LAN IIH PDUs.
Jitter
25.
The percentage of jitter which is applied to the 
generation of periodic PDUs.

8 Subnetwork Dependent 
Functions
The Subnetwork Dependent Functions mask the charac
teristics of the different kinds of Subnetworks from the 
Subnetwork Independent Routeing Functions. The only 
two types of circuits the Subnetwork Independent Functions 
recognise are broadcast and general topology.
The Subnetwork Dependent Functions include:
-The use of the ISO 8473 Subnetwork Dependent 
Convergence Functions (SNDCF) so that this proto
col may transmit and receive PDUs over the same 
subnetwork types, using the same techniques, as does 
ISO 8473.
-Co-ordination with the operation of the ESIS proto
col (ISO 9542) in order to determine the Network 
layer addresses (and on Broadcast subnetworks, the 
subnetwork points of attachment) and identities (End 
System or Intermediate System) of all adjacent neigh
bours. This information is held in the Adjacency data 
base. It is used to construct Link State PDUs.
-The exchange of IIH PDUs. While it is possible for an 
Intermediate System to identify that it has an Interme
diate System neighbour by the receipt of an ISO 9542 
ISH PDU, there is no provision within ISO 9542 to in
dicate whether the neighbour is a Level 1 or a Level 2 
Intermediate System. Specific PDUs (LAN Level 1, 
LAN Level 2 and Point to point IIH PDUs) are de
fined to convey this information.
8.1 Multi-destination Circuits on ISs at 
a Domain Boundary
Routeing information (e.g. Link State PDUs) is not ex
changed across a routeing domain boundary. All routeing 
information relating to a circuit connected to another route
ing domain is therefore entered via the Reachable Address 
managed objects. This information is disseminated to the 
rest of the routeing domain via Link State PDUs as de
scribed in 7.3.3.2. This has the effect of causing NPDUs 
destined for NSAPs which are included in the 
addressPrefixes of the Reachable Addresses to be re
layed to that Intermediate System at the domain boundary. 
On receipt of such an NPDU the Intermediate system shall 
forward it onto the appropriate circuit, based on its own 
Link State information. However in the case of multi-
destination subnetworks (such as an ISO 8208 subnetwork 
using Dynamic Assignment, a broadcast subnetwork, or a 
connectionless subnetwork) it is necessary to ascertain ad
ditional subnetwork dependent addressing information in 
order to forward the NPDU to a suitable SNPA. (This may 
be the target End system or an Intermediate system within 
the other domain.) 
In general the SNPA address to which an NPDU is to be 
forwarded can be derived from the destination NSAP of the 
NPDU. It may be possible to perform some algorithmic ma
nipulation of the NSAP address in order to derive the 
SNPA address. However there may be some NSAPs where 

this is not possible. In these cases it is necessary to have 
pre-configured information relating an address prefix to a 
particular SNPA address.
This is achieved by additional information contained in the 
Reachable Address managed object. The mappingType 
attribute may be specified as Manual, in which case a 
particular SNPA address or set of SNPA addresses is speci
fied in the SNPA Address characteristic.  Alternatively the 
name of an SNPA address extraction algorithm may be 
specified.
8.2 Point to Point Subnetworks
This clause describes the identification of neighbours on 
both point to point links and Static circuits.
The IS shall operate the ISO 9542 protocol, shall be able to 
receive ISO 9542 ISH PDUs from other ISs, and shall store 
the information so obtained in the adjacency database.
8.2.1 Receipt of ESH PDUs  Database of End 
Systems
An IS shall enter an End system into the adjacency database 
when an ESH PDU is received on a circuit. If an ESH PDU  
is received on the same circuit, but with a different NSAP 
address, the new address shall be added to the adjacency, 
with a separate timer. A single ESH PDU may contain more 
than one NSAP address. When a new data link address or 
NSAP address is added to the adjacency database, the IS 
shall generate an adjacencyStateChange (Up) notifica
tion on that adjacency.
The IS shall set a timer for the value of Holding Time in 
the received ESH PDU. If another ESH PDU is not re
ceived from the ES before that timer expires, the ES shall 
be purged from the database, provided that the Subnetwork 
Independent Functions associated with initialising the adja
cency have been completed. Otherwise the IS shall clear the 
adjacency as soon as those functions are completed.
When the adjacency is cleared, the Subnetwork Independ
ent Functions shall be informed of an adjacencyState
Change (Down) notification, and the adjacency can be re-
used after the Subnetwork Independent Functions associ
ated with bringing down the adjacency have been com
pleted.
8.2.2 Receiving ISH PDUs by an Intermediate 
System
On receipt of an ISH PDU by an Intermediate System, the 
IS shall create an adjacency (with state Initialising and 
neighbourSystemType Unknown), if one does not al
ready exist, and then perform the following actions:.
a)If the Adjacency state is Up  and the ID portion of 
the NET field in the ISH PDU does not match the 
neighbourID of the adjacency then the IS shall:
1)generate an adjacencyStateChange (Down) no
tification;
2)delete the adjacency; and

3)create a new adjacency with:
i)state set to Initialising, and
ii)neighbourSystemType set to Unknown.
4)perform the following actions..
b)If the Adjacency state is Initialising, and the 
neighbourSystemType status is Intermediate Sys
tem, the ISH PDU shall be ignored.
c)If the Adjacency state is Initialising and the neigh
bourSystemType status is not Intermediate Sys
tem, a point to point IIH PDU shall be transmitted as 
described in 8.2.3.
d)The neighbourSystemType status shall be set to In
termediate System indicating that the neighbour is an 
Intermediate system, but the type (L1 or L2) is, as yet, 
unknown.
8.2.3 Sending Point to Point IIH PDUs
An IS shall send Point-to-Point IIH PDUs on those Point-
to-Point circuits whose externalDomain attribute is set 
False. The IIH shall be constructed and transmitted as 
follows:
a)The Circuit Type field shall be set according to Ta
ble 3.
b)The Local Circuit ID field shall be set to a value as
signed by this Intermediate system when the circuit is 
created. This value shall be unique among all the cir
cuits of this Intermediate system.
c)The first Point to Point IIH PDU (i.e. that transmitted 
as a result of receiving an ISH PDU, rather than as a 
result of timer expiration) shall be padded (with trail
ing PAD options containing arbitrary valued octets) so 
that the SNSDU containing the IIH PDU has a length 
of at least maxsize - 1 octets77The minimum length of PAD which may be
added is 2 octets, since that is the size of the option header. Where
possible the PDU should be padded to 
maxsize, but if the PDU length is maxsize- 1 octets no padding is
possible (or required).
 where maxsize is the 
maximum of 
1)dataLinkBlocksize 
2)originating

L1

LSP

Buf

fer

Size   
3)originatingL2LSPBufferSize 
This is done to ensure that an adjacency will only be 
formed between systems which are capable of ex
changing PDUs of length up to maxsize octets. In the 
absence of this check, it would be possible for an adja
cency to exist with a lower maximum block size, with 

the result that some LSPs and SNPs (i.e. those longer 
than this maximum, but less than maxsize) would not 
be exchanged.
NOTE - It is necessary for the manager to ensure that the 
value of dataLinkBlocksize on a circuit which will be 
used to form an Intermediate system to Intermediate sys
tem adjacency is set to a value greater than or equal to the 
maximum of the LSPBufferSize characteristics listed 
above. If this is not done, the adjacency will fail to initial
ise. It is not possible to enforce this requirement, since it 
is not known until initialisation time whether or not the 
neighbour on the circuit will be an End system or an In
termediate system. An End system adjacency may oper
ate with a lower value for dataLinkBlocksize.
d)If the value of the circuitTransmitPassword for the 
circuit is non-null, then the IS shall include the 
Authentication Information field in the transmitted 
IIH PDU, indicating an Authentication Type of 
Password and containing the circuitTransmit
Password as the authentication value.
8.2.4 Receiving Point to Point IIH PDUs
8.2.4.1 PDU Acceptance Tests
On receipt of a Point-to-Point IIH PDU, perform the fol
lowing PDU acceptance tests:
a)If the IIH PDU was received over a circuit whose ex
ternalDomain attribute is set True, the IS shall dis
card the PDU.
b)If the ID Length field of the PDU is not equal to the 
value of the IS's routeingDomainIDLength, the 
PDU shall be discarded and an iDFieldLengthMis
match notification generated.
c)If the set of  circuitReceivePasswords for this cir
cuit is non-null, then perform the following tests:
1)If the PDU does not contain the Authentication 
Information field then the PDU shall be discarded 
and an authenticationFailure notification gener
ated.
2)If the PDU contains the Authentication Infor
mation field, but the Authentication Type is not 
equal to Password, then the PDU shall be ac
cepted unless the IS implements the authentica
tiion procedure indicated by the Authentication 

Type. In this case whether the IS accepts or ig
nores the PDU is outside the scope of this Interna
tional Standard.
3)Otherwise, the IS shall compare the password in 
the received PDU with the passwords in the set of 
circuitReceivePasswords for the circuit on 
which the PDU was received. If the value in the 
PDU matches any of these passwords, the IS shall 
accept the PDU for further processing. If the value 
in the PDU does not match any of the circuitRe
ceivePasswords, then the IS shall ignore the 
PDU and generate an authenticationFailure no
tification.
8.2.4.2 IIH PDU Processing
When a Point to Point IIH PDU is received by an Interme
diate system, the area addresses of the two Intermediate 
Systems shall be compared to ascertain the validity of the 
adjacency. If the two Intermediate systems have an area ad
dress in common, the adjacency is valid for all combina
tions of Intermediate system types (except where a Level 1 
Intermediate system is connected to a Level 2 Intermediate 
system with manualL2OnlyMode set True). However, 
if they have no area address in common, the adjacency is 
only valid if both Intermediate systems are Level 2, and the 
IS shall mark the adjacency as Level 2 Only. This is de
scribed in more detail below.
On receipt of a Point to Point IIH PDU, each of the area ad
dresses from the PDU shall be compared with the set of 
area addresses in the manual

Area

Addresses attribute.
a)If a match is detected between any pair the following 
actions are taken.
1)If the local system is of iSType L1

Inter

mediate

Sys

tem the IS shall perform the action indicated 
by Table 4.

2)If the local system is of iSType L2

Intermediate

System and the Circuit manualL2OnlyMode 
has the value False, the IS shall perform the ac
tion indicated by Table 5.
3)If the local system is of iSType L2

Intermediate

System and the Circuit manualL2OnlyMode 
has the value True, the IS shall perform the ac
tion indicated by Table 6.
b)If a no match is detected between any pair, the follow
ing actions shall be performed.
1)If the local system is of iSType L1

Inter

mediate

Sys

tem and the adjacency is not in state Up, 
the IS shall delete the adjacency (if any) and gen
erate an initialisationFailure (Area Mismatch) 
notification. 
2)If the local system is of iSType L1

Inter

mediate

Sys

tem and the adjacency is in state Up, the IS 
shall delete the adjacency  and generate an adja
cencyStateChange (Down  Area Mismatch) 
notification .
3)If the local system is of iSType L2

Intermediate

System the IS shall perform the action indicated 
by Table 7 (irrespective of the value of manu
alL2OnlyMode for this circuit).
c)If the action taken is Up, as detailed in the tables 
referenced above, the IS shall compare the Source ID 
field of the PDU with the local systemID. 
1)If the local Intermediate system has the higher 
Source ID, the IS shall set the Circuit CircuitID 
status to the concatenation of the local systemID 
and the Local Circuit ID (as sent in the Local Cir
cuit ID field of point to point IIH PDUs from this 
Intermediate System) of this circuit. 

2)If the remote Intermediate system has the higher 
Source ID, the IS shall set the Circuit CircuitID 
status to the concatenation of the remote system's 
Source ID (from the Source ID field of the PDU), 
and the remote system's Local Circuit ID (from the 
Local Circuit ID field of the PDU). 
3)If the two source IDs are the same (i.e. the system 
is initialising to itself), the local systemID is used.
NOTE  The circuitID status is not used to generate 
the Local Circuit ID to be sent in the Local Circuit 
ID field of IIH PDUs transmitted by this Intermedi
ate system. The Local Circuit ID value is assigned 
once, when the circuit is created and is not subse
quently changed.
d)If the action taken is Accept and the new value com
puted for the circuitID is different from that in the ex
isting adjacency, the IS shall
1)generate an adjacencyStateChange(Down) noti
fication, and 
2)delete the adjacency.
e)If the action taken is Up or Accept the IS shall
1)copy the Adjacency neighbourAreas entries 
from the PDU,
2)set the holdingTimer to the value of the Holding 
Time from the PDU, and

3)set the neighbourSystemID to the value of the 
Source ID from the PDU.
8.2.5 Monitoring Point-to-point Adjacencies
The IS shall keep a holding time (adjacency holding

Timer) for the point-to-point adjacency. The value of the 
holding

Timer shall be set to the Holding Time as reported 
in the Holding Timer field of the Pt-Pt IIH PDU. If a neigh
bour is not heard from in that time, the IS shall
a)purge it from the database; and 
b)generate an adjacencyStateChange (Down) notifi
cation.
8.3 ISO 8208 Subnetworks
8.3.1 Network Layer Protocols
The way in which the underlying service assumed by ISO 
8473 is provided for ISO 8208 subnetworks is described in 
clause 8 of ISO 8473. This defines a set of Subnetwork De
pendent Convergence Functions (SNDCFs) that relate the 
service provided by specific individual ISO-standard 
subnetworks to the abstract underlying service defined in 
clause 5.5 of ISO 8473. In particular 8.4.3 describes the 
Subnetwork Dependent Convergence Functions used with 
ISO 8208 Subnetworks.

8.3.2 SVC Establishment
8.3.2.1 Use of ISO 8473 Subnetwork Dependent 
Convergence Functions
SVCs shall be established according to the procedures de
fined in the ISO 8208 Subnetwork Dependent Convergence 
Functions of ISO 8473 (this may be on system management 
action or on arrival of data depending on the type of cir
cuit). The Call Request shall contain a Protocol Discrimina
tor specifying ISO 8473 in the first octet of Call Userdata.
In the case of a static circuit, an SVC shall be established 
only upon system management action. The IS shall use 
neighbourSNPAAddress as the called SNPA address.
In the case of a DA circuit, the call establishment proce
dures are initiated by the arrival of traffic for the circuit.
8.3.2.2 Dynamically Assigned Circuits
A dynamically assigned circuit has multiple adjacencies, 
and can therefore establish SVCs to multiple SNPAs. In 
general the SNPA address to which a call is to be estab
lished can be derived from the NSAP to which an NPDU is 
to be forwarded. In the case where all the NSAPs accessible 
over the ISO 8208 subnetwork have IDIs which are their 
SNPA addresses, the correct SNPA can be ascertained by 
extracting the IDI. However there may be some NSAPs, 
which it is required to reach over the ISO 8208 subnetwork, 
whose IDI does not correspond to the SNPA address of 
their point of attachment to the ISO 8208 subnetwork. The 
IDI may refer to some other SNPA address which is sub-
optimally connected to the target NSAP (or not even con
nected at all), or the IDP may not contain an X.121 address 
at all (e.g. ISO DCC scheme). In these cases the IS shall 
have pre-configured information relating an IDP (or address 
prefix) to a particular SNPA address to call.
This is achieved, as described in 8.1, by additional informa
tion contained in the Reachable Address managed object. 
The address extraction algorithm may be specified to ex
tract the IDI portion where the IDI is the required X.121 ad
dress. An example of a set of Reachable Addresses is 
shown in Table 8.
Table 8 - Example of address prefixesAddress Prefix	

39 	
37 aaaaa	
37	
*	
37 D	
SNPA Address
123X
B
Y
Extract X.121 SNPA address
R, S, T

This is interpreted as follows: 
a)For the ISO DCC prefix 39 123, call the SNPA ad
dress X.

b)For the X.121 IDI address prefix 37 aaaaa, don't 
call aaaaa, but call B instead.
c)For all IDPs based on SNPAs with DNIC D (i.e. with 
address prefix 37 D), call the address Y (which 
would probably be a gateway to a subnetwork with 
DNIC D).
d)For any other X.121 IDI (i.e. address prefix 37)  call 
the SNPA whose address is used as the IDI.
e)Anything else (* in table 8)  call one of the SNPA 
addresses R, S or T. These would typically be the 
SNPA addresses of Level 2 Intermediate Systems 
through which any other addresses could potentially 
be reached.
NOTE - If a DA circuit is defined with a reachable address 
prefix which includes the addresses reachable over a DCM 
or STATIC circuit, the cost(s) for the DA circuit must be 
greater than those of the STATIC circuit. If this is not the 
case, the DA circuit may be used to establish a call to the re
mote SNPA supporting the STATIC circuit, which would 
then (wrongly) assume it was the STATIC circuit.
8.3.2.3 Initiating Calls (Level 2 Intermediate 
Systems) 
When an NPDU is to be forwarded on a dynamically as
signed circuit, for destination NSAP address D, the IS shall:
a)Calculate D's subnetwork address, either as explicitly 
stated in the circuit database, or as extracted from the 
IDP.
1)If this system is an ES and there is an entry in the 
RedirectCache or ReversePathCache for D, use the 
subnetwork address in the cache entry. 
2)If this system is an ES or Level 2 Intermediate sys
tem, and the address matches one of the listed 
reachable address prefixes (including *, if pre
sent),  the subnetwork address is that specified ac
cording to the mappingType attribute (either 
Manual, indicating that the set of addresses in 
the sNPAAddresses attribute of that Reachable 
Address are to be used, or Algorithm, indicating 
that it is to be extracted from the IDP using the 
specified algorithm). If multiple SNPA addresses 
are specified, and there is already an adjacency up 
to one of those SNPA addresses, then choose that 
subnetwork address, otherwise choose the 
subnetwork address with the oldest timestamp  as 
described in 8.3.2.4.
3)If the address does not match one of the listed 
reachable address prefixes (and there is no * en
try), invoke the ISO 8473 Discard PDU function.
b)Scan the adjacencies for one already open to D's 
subnetwork address (i.e. reserveTimer has not yet 
expired). If one is found, transmit the NPDU on that 
adjacency.
c)If no adjacency has a call established to the required 
subnetwork address, but there is a free adjacency, at

tempt to establish the call using that subnetwork ad
dress.
d)If there is no free adjacency invoke the ISO 8473 Dis
card PDU function.
NOTE  Where possible, when an adjacency is reserved 
(when an SVC has been cleared as a result of the 
idleTimer expiring, but the reserveTimer has not yet ex
pired), resources within the subnetwork service provider 
should be reserved, in order to minimise the probability 
that the adjacency will not be able to initiate a call when 
required. 
8.3.2.4 Call Attempt Failures
The Reachable Address managed objects may contain a set 
of SNPA addresses, each of which has an associated time-
stamp. The time-stamps shall be initialised to infinitely 
old.
Some of the SNPAs in this set may be unreachable. If a call 
attempt fails to one of the SNPA addresses listed, the IS 
shall mark that entry in the list with the time of the latest 
failed attempt. When an SNPA address is to be chosen from 
the list, the IS shall choose the one with the oldest time-
stamp , unless the oldest time-stamp is more recent than 
recallTimer. If the oldest time-stamp is more recent than 
recallTimer, all SNPAs in the set shall be assumed tempo
rarily unreachable and no call attempt is made. The IS shall 
instead invoke the ISO 8473 Discard PDU function.
When attempting to establish a connection to a single spe
cific subnetwork address (not through one of a set of SNPA 
addresses), if a call attempt to a particular SNPA address, 
A, fails for any reason, the IS shall invoke the ISO 8473 
Discard PDU function. Additionally the adjacency on 
which the call attempt was placed shall be placed in 
Failed state, and the recall timer set. Until it expires, the 
IS shall not attempt call establishment for future NPDUs to 
be forwarded over subnetwork address A, but instead the IS 
shall invoke the ISO 8473 Discard PDU function.
When the recall timer expires, the IS shall free the adja
cency for calls to a different destination or retry attempts to 
subnetwork address A.
NOTE - If an implementation can store the knowledge of 
SNPA addresses that have failed along with the time since 
the attempt was made in a location other than the adjacency 
on which the call was attempted, then that adjacency can be 
used for other calls.
8.3.3 Reverse Path Forwarding on DA Circuits
Where a subdomain is attached to a Connection-oriented 
subnetwork by two or more SNPAs, the IDP for the ad
dresses within the subdomain may be chosen to be con
structed from the address of one of the points of attachment. 
(It need not be. The whole subdomain could be multi-
homed by using both SNPA addresses, or some other IDP 
could be chosen; e.g. ISO DCC.) Traffic to the subdomain 
from some other SNPA will cause a call to be established to 
the SNPA corresponding to the IDP of the addresses in the 
subdomain. Traffic from the subdomain may use either of 
the SNPAs depending on the routeing decisions made by 

the subdomain. This is illustrated in the diagram below (fig
ure 5). 
Figure 5 - B.xB.yC.zISO 8208 SubnetworkBACExample for reverse path 
forwarding
The subdomain is attached to the connection-oriented 
subnetwork via SNPAs A and B. The addresses on the 
subdomain are constructed using the SNPA address of B as 
the IDI. If traffic for C.z is sent from B.x, a call will be es
tablished from A to C. The reverse traffic from C.z to B.x 
will cause another call to be established from C to B. Thus 
two SVCs have been established where only one is re
quired.
This problem is prevented by the local system retaining a 
cache (known as the ReversePathCache) of NSAP ad
dresses from which traffic has been received over each ad
jacency. When it has traffic to forward over the connection-
oriented subnetwork, the IS shall it first check to see if the 
destination NSAP is in the cache of any of its adjacencies, 
and if so forwards the traffic over that adjacency. An NSAP 
shall only be added to the cache when the remote SNPA ad
dress of the adjacency over which it is received differs from 
the SNPA address to be called which would be generated 
by checking against the Circuit Reachable Addresses man
aged objects. If the cache is full, the IS shall overwrite the 
least recently used entry. The ReversePathCache, if imple
mented, shall have a size of at least one entry. The IS shall 
purge the cache when the adjacency is taken down (i.e. 
when the reserve timer expires).
8.3.4 Use of ISO 9542 on ISO 8208 
subnetworks 
STATIC and DA circuits are equivalent to point to point 
links, and as such permit the operation of ISO 9542 as de
scribed for point to point links in 8.2.
For DA circuits, it is impractical to use ISO 9542 to obtain 
configuration information, such as the location of Interme
diate systems, since this would require calls to be estab
lished to all possible SNPA addresses.
The IS shall not send ISO 9542 ISH PDUs on a DA circuit. 
The IS shall take no action on receipt of an ESH PDU or 
ISH PDU, and the circuit shall complete initialisation with
out waiting  for their arrival.
The IS shall not send Point to point IIH PDU on DA cir
cuits. The IS shall ignore receipt of a point-point IIH PDU. 
(This would only occur if a STATIC or DA circuit became 

erroneously connected to an SVC being used for a DA cir
cuit.) 
8.3.5 Interactions with the Update Process
A dynamically assigned circuit contains a list of <reachable 
address prefix, cost, SNPA address> tuples. Also, each dy
namically assigned circuit has a specified call establishment 
cost measured by call

Estab

lish

ment

Met

rick (where k in
dexes the four defined metrics). The call establishment cost 
is always an internal metric, and is therefore directly com
parable with the reachable address metric only if the reach
able address metric is also internal.
When the circuit is enabled, the Subnetwork Dependent 
functions in an Intermediate system shall report (to the Up
date Process) adjacency cost change events for all ad
dress prefixes in the circuit Reachable Address managed 
object, together with the Reachable address metrick + Del
tak increment. If reachable address metrick is internal, then 
Deltak = call

Estab

lish

ment

Met

rick. If reachable address 
metrick is external, then Deltak = 0.
This causes this information to be included in subsequently 
generated LSPs as described in 7.3.3.2.
Routeing PDUs (LSPs and Sequence number PDUs) shall 
not be sent on dynamically assigned circuits.
NOTE - In the following sub-clauses, it is assumed that the 
Reachable Addresses referenced are only those which have 
been enabled (i.e. that have state On), and whose parent 
circuit is also in state On.
8.3.5.1 Adjacency Creation
After an SVC to SNPA address D is successfully estab
lished and a new adjacency created for it (whether it was in
itiated by the local or the remote system), if call

Estab

lish

ment

Met

rickIncrement is greater than 0, the IS shall scan 
the circuit Reachable Address managed objects for all 
addressPrefixes listed with D as (one of) the sNPAAd
dress(es).
For Reachable Addresses with mappingType Algo
rithm, the IS shall construct an implied address prefix88i.e. some
address prefix which matches the addressPrefix of the Reachable
Address, and which would generate the SNPA Address D when the extrac
tion algorithm is applied

from the actual remote SNPA address D and the address ex
traction algorithm. The IS shall generate an Adjacency cost 
change event for each such address prefix (both actual and 
implied) with the Reachable Address metrick (without the 
added call

Estab

lish

ment

Met

rickIncrement). This causes 
information that those address prefixes are reachable with 
the lower cost to be included in subsequently generated 
LSPs. The effect of this is to encourage the use of already 
established SVCs where possible.
8.3.5.2 Adjacency Deletion
When the adjacency with sNPAAddress D is freed (Re
serve Timer has expired, or the adjacency is deleted by Sys
tem Management action) then if call

Estab

lish

ment

Met

rickIncrement is greater than 0, the IS shall scan the Cir

cuit Reachable Address managed objects for all those with 
mappingType Manual and (one of) their sNPAAd
dresses  equal to D. The IS shall generate Adjacency 
cost change events to the Update Process for all such ad
dress prefixes with the Reachable Address metrick + Deltak 
increment (where Deltak is the same as defined above). For 
Reachable Addresses with mappingType X.121 for 
which it is possible to construct an implied address prefix 
as above, the IS shall generate an adjacencyState
Change notification for that implied prefix. 
A cost change event shall only be generated when the count 
of the number of subnetwork addresses which have an es
tablished SVC changes between 1 and 0.
8.3.5.3 Circuit Call Establishment Increment 
Change
On a dynamically assigned circuit, when system manage
ment changes the Circuit call

Estab

lish

ment

Met

rickIncrement for that circuit, the IS shall generate adja
cency cost change events for all address prefixes affected 
by the change (i.e. those for which calls are not currently 
established). 
The IS shall scan all the Reachable Address managed ob
jects of that Circuit. If the Reachable Address has 
mappingType X.121, the IS shall generate an adja
cency cost change event for that name with the Reach
able Address metrick + the new value of Deltak. If  (based 
on the new value of callEstab

lish

ment

Met

rickIncrement) 
the Reachable Address has mappingType Manual, the 
IS shall scan all the Adjacencies of the Circuit for an Adja
cency with sNPAAddress equal to (one of) the sN
PAAddresses of that Reachable Address. If no such adja
cency is found the IS shall generate an adjacency cost 
change event for that name with the Reachable Address 
metrick + the new value of Deltak (based on the new value 
of callEstlishmentMetrickIncrement).
8.3.5.4 Reachable Address Cost Change
When the metrick characteristic of a Reachable Address in 
state On is changed by system management, the IS shall 
generate cost change events to the Update Process to reflect 
this change. 
If the Reachable Address has mappingType Manual, 
the IS shall scan all the Adjacencies of the Circuit for an 
Adjacency with sNPAAddress equal to (one of) the sN
PAAddresses of that Reachable Address. If one or more 
such adjacencies are found, the IS shall generate an adja
cency cost change event for that name with the new 
Reachable Address metrick. If no such adjacency is found 
the IS shall generate an adjacency cost change event for 
that name with the new Reachable Address metrick.
If the Reachable Address has mappingType X.121, the 
IS shall generate an adjacency cost change event for that 
name with the new Reachable Address metrick + Deltak 
(based on the new value of call

Estab

lish

ment

Met

rick

Increment). In addition, for all Adjacencies of the Circuit 

with an sNPAAddress for which an implied address pre
fix can be generated for this Reachable Address, the IS 
shall generate an adjacency cost change event for that im
plied address prefix and the new Reachable Address met
rick.
8.3.5.5 Disabling a Reachable Address
When a Reachable Address managed object is disabled via 
management action, the IS shall generate an  Adjacency 
down event to the Update Process for the name of that 
Reachable Address and also for any implied prefixes asso
ciated with that Reachable Address.
8.3.5.6 Enabling a Reachable Address
When a Reachable Address is enabled via system manage
ment action, the IS shall generate Adjacency cost change 
events as described for Reachable Address cost change in 
8.3.5.4 above.
8.4 Broadcast Subnetworks
8.4.1 Broadcast Subnetwork IIH PDUs
All Intermediate systems on broadcast circuits (both 
Level 1 and Level 2) shall transmit LAN IIH PDUs as de
scribed in 8.4.3. Level 1 Intermediate systems shall transmit 
only Level 1 LAN IIH PDUs. Level 2 Intermediate Systems 
on circuits with manualL2OnlyMode set to the value 
True, shall transmit only Level 2 LAN IIH PDUs. 
Level 2 Intermediate systems on circuits with manu
alL2OnlyMode set to the value False, shall transmit 
both.
Level n LAN IIH PDUs contain the transmitting Intermedi
ate system's ID, holding timer, Level n Priority and 
manual

Area

Addresses, plus a list containing the lA
NAddresses of all the adjacencies of neighbourSystem
Type Ln Intermediate System (in state Initialising or 
Up) on this circuit.
LAN IIH PDUs shall be padded (with trailing PAD options 
containing arbitrary valued octets) so that the SNSDU con
taining the IIH PDU has a length of at least maxsize- 1 oc
tets99The minimum length of PAD which may be added is 2 octets, since
that is the size of the option header. Where possible the PDU should be padded to 
maxsize, but if the PDU length is maxsize- 1 octets no padding is
possible (or required).
 where maxsize for Level 1 IIH PDUs is the maximum 
of 
-dataLinkBlocksize 
-originating

L1

LSP

Buf

fer

Size
and for Level 2 IIH PDUs is the maximum of
-dataLinkBlocksize 
-originatingL2LSPBufferSize
This is done to ensure that an adjacency will only be 
formed between systems which are capable of exchanging 
PDUs of length up to maxsize octets. In the absence of this 

check, it would be possible for an adjacency to exist with a 
lower maximum block size, with the result that some LSPs 
and SNPs (i.e. those longer than this maximum, but less 
than maxsize) would not be exchanged.
NOTE - An example of a topology where this could occur is 
one where an extended LAN is constructed from LAN seg
ments with different maximum block sizes. If, as a result of 
mis-configuration or some dynamic reconfiguration, a path 
exists between two Intermediate systems on separate LAN 
segments having a large maximum block size, which in
volves transit of a LAN segment with a smaller maximum 
block size, loss of larger PDUs will occur if the Intermediate 
systems continue to use the larger maximum block size. It is 
better to refuse to bring up the adjacency in these circum
stances.
Level 1 Intermediate systems shall transmit Level 1 LAN 
IIH PDUs to the multi-destination address AllL1ISs, and 
also listen on that address. They shall also listen for ESH 
PDUs on the multi-destination address AllIntermediateSys
tems. The list of neighbour Intermediate systems shall con
tain only Level 1 Intermediate Systems within the same 
area. (i.e. Adjacencies of neighbourSystemType L1 In
termediate System.) 
Level 2 Only Intermediate systems (i.e. Level 2 Intermedi
ate systems which have the Circuit manualL2OnlyMode 
characteristic set to the value True) shall transmit Level 2 
LAN IIH PDUs to the multi-destination address AllL2ISs, 
and also listen on that address. The list of neighbour Inter
mediate systems shall contain only Level 2 Intermediate 
systems. (i.e. Adjacencies of neighbourSystemType L2 
Intermediate System.) 
Level 2 Intermediate systems (with manualL2OnlyMode 
False) shall perform both of the above actions. Separate 
Level 1 and Level 2 LAN IIH PDUs shall be sent to the 
multi-destination addresses AllL1ISs and AllL2ISs de
scribing the neighbour Intermediate systems for Level 1 
and Level 2 respectively. Separate adjacencies shall be cre
ated by the receipt of Level 1 and Level 2 LAN IIH PDUs.
8.4.1.1 IIH PDU Acceptance Tests
On receipt of a Broadcast IIH PDU, perform the following 
PDU acceptance tests:
a)If the IIH PDU was received over a circuit whose ex
ternalDomain attribute is True, the IS shall discard 
the PDU.
b)If the ID Length field of the PDU is not equal to the 
value of the IS's routeingDomainIDLength, the 
PDU shall be discarded and an iDFieldLengthMis
match notification generated.
c)If the set of circuitReceivePasswords for this cir
cuit is non-null, then perform the following tests:
1)If the PDU does not contain the Authentication 
Information field then the PDU shall be discarded 

and an authenticationFailure notification gener
ated.
2)If the PDU contains the Authentication Infor
mation field, but the Authentication Type is not 
equal to Password, then the PDU shall be ac
cepted unless the IS implements the authentica
tiion procedure indicated by the Authentication 
Type. In this case whether the IS accepts or ig
nores the PDU is outside the scope of this Interna
tional Standard.
3)Otherwise, the IS shall compare the password in 
the received PDU with the passwords in the set of 
circuitReceivePasswords for the circuit on 
which the PDU was received. If the value in the 
PDU matches any of these passwords, the IS shall 
accept the PDU for further processing. If the value 
in the PDU does not match any of the circuitRe
ceivePasswords, then the IS shall ignore the 
PDU and generate an authenticationFailure no
tification.
8.4.1.2 Receipt of Level 1 IIH PDUs
On receipt of a Level 1 LAN IIH PDU on the multi-
destination address AllL1ISs, the IS shall compare each of 
the area addresses, from the received IIH PDU with the set 
of area addresses in the manual

Area

Addresses charac
teristic. If a match is not found between any pair (i.e. the lo
cal and remote system have no area address in common), 
the IS shall reject the adjacency and generate an initialisa
tionFailure (area mismatch) notification. Otherwise (a 
match is found) the IS shall accept the adjacency and set the 
Adjacency neighbourSystemType to L1 Intermediate 
System.
8.4.1.3 Receipt of Level 2 IIH PDUs 
On receipt of a Level 2 LAN IIH PDU on the multi-
destination address AllL2ISs, the IS shall accept the adja
cency, and set the Adjacency neighbourSystemType to 
L2 Intermediate System.
8.4.1.4 Existing Adjacencies
When a Level n LAN IIH PDU (Level 1 or Level 2) is re
ceived from an Intermediate system for which there is al
ready an adjacency with
a)the Adjacency lANAddress equal to the MAC Source 
address of the PDU, and
b)the Adjacency neighbourSystemID equal to the 
Source ID field from the PDU and
c)the neighbourSystemType equal to Ln Intermedi
ate System, 
the IS shall update the holding timer, LAN Priority and 
neighbourAreas according to the values in the PDU.

8.4.1.5 New Adjacencies 
When 
a)a Level n LAN IIH PDU (Level 1 or Level 2) is re
ceived (from Intermediate system R), and 
b)there is no adjacency for which the Adjacency lANAd
dress is equal to the MAC Source address of the 
PDU; and
c)the Adjacency neighbourSystemID is equal to the 
Source ID field from the PDU, and 
d)neighbourSystemType is Ln Intermediate System, 
the IS shall create a new adjacency. However, if there is in
sufficient space in the adjacency database, to permit the 
creation of a new adjacency the IS shall instead perform the 
actions described in 8.4.2.
The IS shall 
a)set neighbourSystemType status to Ln Intermedi
ate System (where n is the level of the IIH PDU),  
b)set the holding timer, LAN Priority, neighbourID 
and neighbourAreas according to the values in the 
PDU., and 
c)set the lANAddress according to the MAC source ad
dress of the PDU.
The IS shall set the state of the adjacency to initialising, 
until it is known that the communication between this sys
tem and the source of the PDU (R) is two-way. However R 
shall be included in future Level n LAN IIH PDUs trans
mitted by this system.
When R reports this circuit's lANAddress in its Level n 
LAN IIH PDUs, the IS shall
a)set the adjacency's state to Up, and 
b)generate an adjacencyStateChange (Up) notifica
tion.
The IS shall keep a separate Holding Time (Adjacency 
holding

Timer) for each Ln Intermediate System adja
cency. The value of holding

Timer shall be set to the Hold
ing Time as reported in the Holding Timer field of the 
Level n LAN IIH PDUs. If a neighbour is not heard from in 
that time, the IS shall
a)purge it from the database; and 
b)generate an adjacencyStateChange (Down) notifi
cation.
If a Level n LAN IIH PDU is received from neighbour N, 
and this system's lANAddress is no longer in N's IIH 
PDU, the IS shall
a)set the adjacency's state to initialising, and 

b)generate an adjacencyStateChange (Down) notifi
cation.
8.4.2 Insufficient Space in Adjacency Database
If an IS needs to create a new Intermediate system adja
cency, but there is insufficient space in the adjacency data
base, the adjacency of neighbourSystemType Ln Inter
mediate System with lowest lANPriority (or if more than 
one adjacency has the lowest priority, the adjacency with 
the lowest lANAddress, from among those with the lowest 
priority) shall be purged from the database. If the new adja
cency would have the lowest priority, it shall be ignored, 
and a rejectedAdjacency notification generated.
If an old adjacency must be purged, the IS shall generate an 
adjacencyStateChange (Down) notification for that adja
cency. After the Subnetwork Independent Functions issue 
an adjacency down complete, the IS may create a new ad
jacency.
8.4.3 Transmission of LAN IIH PDUs
A Level 1 IS shall transmit a Level 1 LAN IIH PDU imme
diately when any circuit whose externalDomain attribute 
is False has been enabled.  A Level 2 Intermediate Sys
tem shall transmit a Level 2 LAN IIH PDU. A Level 2 In
termediate System shall also transmit a Level 1 LAN IIH 
PDU unless the circuit is marked as manualL2OnlyMode 
True.
The IS shall also transmit a  LAN IIH PDU when at least 1 
second has transpired since the last transmission of a LAN 
IIH PDU of the same type on this circuit by this system 
and:
a)iSIS

Hello

Timer seconds have elapsed1010Jitter is applied as described in 10.1.
 since the last 
periodic LAN IIH PDU transmission 
The Holding Time is set to ISISHoldingMultiplier W 
iSIS

Hello

Timer. For a Designated Intermediate Sys
tem the value of dRISIS

Hello

Timer1111 In this case jitter is not applied, since it would result in
intervals of less than one second.
 is used instead 
of iSISHelloTimer. The Holding Time for this PDU 
shall therefore be set to ISISHoldingMultiplier W 
dR

ISIS

Hello

Timer seconds. This permits failing 
Designated Intermediate Systems to be detected more 
rapidly, 
or
b)the contents of the next IIH PDU to be transmitted 
would differ from the contents of the previous IIH 
PDU transmitted by this system, or 
c)this system has determined that it is to become or re
sign as LAN Designated Intermediate System for that 
level. 
To minimise the possibility of the IIH PDU transmissions 
of all Intermediate systems on the LAN becoming synchro
nised, the Hello Time timer shall only be reset when a IIH 

PDU is transmitted as a result of timer expiration, or on be
coming or resigning as Designated Intermediate System.
Where an Intermediate system is transmitting both Level 1 
and Level 2 LAN IIH PDUs, it shall maintain a separate 
timer (separately jittered) for the transmission of the 
Level 1 and Level 2 IIH PDUs. This avoids correlation be
tween the Level 1 and Level 2 IIH PDUs and allows the re
ception buffer requirements to be minimised.
If the value of the circuitTransmitPassword for the cir
cuit is non-null, then the IS shall include the Authentica
tion Information field in the transmitted IIH PDU, indicat
ing an Authentication Type of Password and contain
ing the circuitTransmitPassword as the authentication 
value.
8.4.4 LAN Designated Intermediate Systems
A LAN Designated Intermediate System is the highest pri
ority Intermediate system in a particular set on the LAN, 
with numerically highest MAC source lANAddress break
ing ties. (See 7.1.5 for how to compare LAN addresses.) 
There are, in general, two LAN Designated Intermediate 
Systems on each LAN, namely the LAN Level 1 Desig
nated Intermediate System and the LAN Level 2 Desig
nated Intermediate System. They are elected as follows.
a)Level 1 Intermediate systems elect the LAN Level 1 
Designated Intermediate System.
b)Level 2 Only Intermediate systems (i.e. Level 2 Inter
mediate Systems which have the Circuit manual

L2

Only

Mode characteristic set to the value True) 
elect the LAN Level 2 Designated Intermediate Sys
tem.
c)Level 2 Intermediate systems (with manu
alL2OnlyMode False) elect both the LAN Level 1 
and LAN Level 2 Designated Intermediate Systems.
The set of Intermediate systems to be considered includes 
the local Intermediate system, together with all Intermedi
ate systems of the appropriate type as follows.
a)For the LAN Level 1 Designated Intermediate System, 
it is the set of Intermediate systems from which LAN 
Level 1 IIH PDUs are received and to which Level 1 
adjacencies exist in state Up. When the local sys
tem either becomes or resigns as LAN Level 1 Desig
nated Intermediate System, the IS shall generate a lan
Level1

Designated

Inter

mediate

Sys

tem

Change 
notification. In addition, when it becomes LAN 
Level 1 Designated Intermediate System, it shall per
form the following actions.
1)Generate and transmit Level 1 pseudonode LSPs 
using the existing End system configuration.

2)Purge the Level 1 pseudonode LSPs issued by the 
previous LAN Level 1 Designated Intermediate 
System (if any) as described in 7.2.3.
3)Solicit the new End system configuration as de
scribed in 8.4.5.
b)For the LAN Level 2 Designated Intermediate System, 
it is the set of Intermediate systems from which LAN 
Level 2 IIH PDUs are received and to which Level 2 
adjacencies exist in state Up. When the local sys
tem either becomes or resigns as LAN Level 2 Desig
nated Intermediate System, the IS shall generate a lan
Level2

Designated

Inter

mediate

System

Change 
notification. In addition, when it becomes LAN 
Level 2 Designated Intermediate System, it shall per
form the following actions.
1)Generate and transmit a Level 2 pseudonode LSP. 
2)Purge the Level 2 pseudonode LSPs issued by the 
previous LAN Level 2 Designated Intermediate 
System (if any) as described in 7.2.3.
When an Intermediate system resigns as LAN Level 1 or 
Level 2 Designated Intermediate System it shall perform 
the actions on Link State PDUs described in 7.2.3.
When the broadcast circuit is enabled on an Intermediate 
system the IS shall perform the following actions. 
a)Commence sending IIH PDUs with the LAN ID field 
set to the concatenation of its own systemID and its 
locally assigned one octet Local Circuit ID. 
b)Solicit the End system configuration as described in 
8.4.5. 
c)Start listening for ISO 9542 ISH PDUs and ESH 
PDUs and acquire adjacencies as appropriate. Do not 
run the Designated Intermediate System election proc
ess. 
d)After waiting iSIS

Hello

Timer * 2 seconds, run the 
Level 1 and or the Level 2 Designated Intermediate 
System election process depending on the Intermedi
ate system type. This shall be run subsequently when
ever an IIH PDU is received or transmitted as de
scribed in 8.4.3. (For these purposes, the transmission 
of the system's own IIH PDU is equivalent to receiv
ing it). If there has been no change to the information 
on which the election is performed since the last time 
it was run, the previous result can be assumed. The 
relevant information is
1)the set of Intermediate system adjacency states, 
2)the set of Intermediate System priorities (including 
this system's) and 
3)the existence (or otherwise) of at least one Up  
End system (not including Manual Adjacencies) or 
Intermediate system adjacency on the circuit.
An Intermediate system shall not declare itself to be a LAN 
Designated Intermediate system of any type until it has at 
least one Up End system (not including Manual Adjacen
cies) or Intermediate system adjacency on the circuit. (This 

prevents an Intermediate System which has a defective re
ceiver and is incapable of receiving any PDUs from errone
ously electing itself LAN Designated Intermediate System.) 
The LAN ID field in the LAN IIH PDUs transmitted by this 
system shall be set to the value of the LAN ID field reported 
in the LAN IIH PDU (for the appropriate level) received 
from the system which this system considers to be the Des
ignated Intermediate System. This value shall also be 
passed to the Update Process, as the pseudonode ID, to en
able Link State PDUs to be issued for this system claiming 
connectivity to the pseudonode.
If this system, as a result of the Designated Intermediate 
System election process, considers itself to be designated 
Intermediate System, the LAN ID field shall be set to the 
concatenation of this system's own system ID and the lo
cally assigned one octet Local Circuit ID.
8.4.5 Soliciting the ES configuration
When there is a change in the topology or configuration of 
the LAN (for example the partitioning of a LAN into two 
segments by the failure of a repeater or bridge), it is desir
able for the (new) Designated Intermediate System to ac
quire the new End system configuration of the LAN as 
quickly as possible in order that it may generate Link State 
PDUs which accurately reflect the actual configuration. 
This is achieved as follows.
When the circuit is enabled, or the Intermediate system de
tects a change in the set of Intermediate systems on the 
LAN, or a change in the Designated Intermediate System 
ID, the IS shall initiate a poll of the ES configuration by 
performing the following actions.
a)Delay a random interval between 0 and iSIS

Hello

Timer seconds. (This is to avoid synchronisation with 
other Intermediate systems which have detected the 
change.) 
b)If (and only if) an Intermediate System had been re
moved from the set of Intermediate systems on the 
LAN, reset the entryRemainingTime field in the 
endSystemIDs adjacency database record of all adja
cencies on this circuit to the value 
(iSIS

Hello

Timer + pollESHelloRate) W 
HoldingMultiplier
or the existing value whichever is the lower. (This 
causes any End systems which are no longer present 
on the LAN to be rapidly timed out, but not before 
they have a chance to respond to the poll.)
c)Transmit HoldingMultiplier ISH PDUs for each NET 
possessed by the Intermediate system with a Sug
gested ES Configuration Timer value of poll

ES

Hello

Rate at an interval between them of iSIS

Hello

Timer seconds and a holding time of hello

Timer * 
HoldingMultiplier.
d)Resume sending ISH PDUs at intervals of hello

Timer seconds with a Suggested ES Configuration 
Timer value of defaultESHelloTimer.

8.4.6 Receipt of ESH PDUs  Database of End 
Systems
An IS shall enter an End system into the adjacency database 
when an ESH PDU is received from a new data link ad
dress. If an ESH PDU is received with the same data link 
address as a current adjacency, but with a different NSAP 
address, the new address shall be added to the adjacency, 
with a separate timer. A single ESH PDU may contain more 
than one NSAP address. When a new data link address or 
NSAP address is added to the adjacency database, the IS 
shall generate an adjacencyStateChange (Up) notifica
tion on that adjacency.
The IS shall set a timer for the value of the Holding Time 
field in the received ESH PDU. If another ESH PDU is not 
received from the ES before that timer expires, the ES shall 
be purged from the database, provided that the Subnetwork 
Independent Functions associated with initialising the adja
cency have been completed. Otherwise the IS shall clear the 
adjacency as soon as those functions are completed.
When the adjacency is cleared, the Subnetwork Independ
ent Functions shall be informed of an adjacencyState
Change (Down) notification, and the adjacency can be re-
used after the Subnetwork Independent Functions associ
ated with bringing down the adjacency have been com
pleted.
9 Structure and Encoding of PDUs
This clause describes the PDU formats of the Intra-Domain 
Routeing protocol.
9.1 General encoding Rules
Octets in a PDU are numbered starting from 1, in increasing 
order. Bits in a octet are numbered from 1 to 8, where bit 1 
is the least significant bit and is pictured on the right. When 
consecutive octets are used to represent a number, the lower 
octet number has the most significant value.
Fields marked Reserved (or simply R) are transmitted as 
zero, and ignored on receipt, unless otherwise noted.
Values are given in decimal. All numeric fields are un
signed integers, unless otherwise noted.
9.2 Encoding of Network Layer 
Addresses
Network Layer addresses (NSAP addresses, NETs, area ad
dresses and Address Prefixes) are encoded in PDUs accord
ing to the preferred binary encoding specified in 
ISO 8348/Add.2; the entire address, taken as a whole is rep
resented explicitly as a string of binary octets. This string is 
conveyed in its entirety in the address fields of the PDUs. 
The rules governing the generation of the preferred binary 
encoding are described in ISO 8348/Add.2. The address so 
generated is encoded with the most significant octet (i.e. the 
AFI) of the address being the first octet transmitted, and the 
more significant semi-octet of each pair of semi-octets in 

the address is encoded in the more significant semi-octet of 
each octet (i.e. in the high order 4 bits). Thus the address 
/371234 is encoded as
Figure 6 - 111No. of Octets3
7
1
2
3
4
Address encoding example
9.3 Encoding of SNPA Addresses
SNPA addresses (e.g. lANAddress) shall be encoded ac
cording to the rules specified for the particular type of 
subnetwork being used. In the case of an ISO 8802 
subnetwork, the SNPA address is the MAC address defined 
in ISO 10039, which is encoded according to the binary 
representation of MAC addresses specified in ISO 10039.
9.4 PDU Types
The types of PDUs are: 
-Level 1 LAN IS to IS Hello PDU 
-Level 2 LAN IS to IS Hello PDU 
-Point-to-Point IS to IS Hello PDU 
-Level 1 Link State PDU
-Level 2 Link State PDU
-Level 1 Complete Sequence Numbers PDU
-Level 2 Complete Sequence Numbers PDU
-Level 1 Partial Sequence Numbers PDU
-Level 2 Partial Sequence Numbers PDU
These are described in the following subclauses.

9.5 Level 1 LAN IS to IS Hello PDU
This PDU is multicast by Intermediate systems on broad
cast circuits to the multi-destination address AllL1ISs.  
The purpose of this PDU is for Intermediate systems on 
broadcast circuits to discover the identity of other Level 1 
Intermediate systems on that circuit. Trailing Pad options 
are inserted to make PDU Length equal to at least maxsize 
- 1 where maxsize is the maximum of 
-dataLinkBlocksize
-originating

L1

LSP

Buf

fer

Size
(see 8.4.1). 11No. of Octets1111111ID Length2ID Length +
121VARIABLEIntradomain Routeing 
Protocol Discriminator
Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type
R
R
R
Version
ECO
User ECO
Reserved/Circuit Type
Source ID
Holding Time
LAN ID
PDU Length
Priority
R
VARIABLE LENGTH FIELDS

-Intradomain Routeing Protocol Discriminator  
architectural constant
-Length Indicator  Length of the fixed header in oc
tets
-Version/Protocol ID Extension  1 
-ID Length  Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This 
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating 
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field 
length
7The value 255, whhich means a null ID field (i.e. 
zero length)

All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5)  15. Note bits 6, 7 and 
8 are Reserved, which means they are transmitted as 0 
and ignored on receipt.
-Version  1
-ECO  transmitted as zero, ignored on receipt
-User ECO  transmitted as zero, ignored on receipt
-Reserved/Circuit Type  Most significant 6 bits re
served (Transmitted as zero, ignored on receipt). Low 
order bits (bits 1 and 2) indicate: 
70  reserved value (if specified the entire PDU 
shall be ignored) 
71  Level 1 only 
72  Level 2 only (sender is Level 2 Intermediate 
system with manualL2OnlyMode set True for 
this circuit, and will use this link only for Level 2 
traffic) 
73  both Level 1 and Level 2 (sender is Level 2 In
termediate system, and will use this link both for 
Level 1 and Level 2 traffic)
NOTE  In a LAN Level 1 IIH PDU the Circuit 
Type shall be either 1 or 3.
-Source ID  the system ID of transmitting Intermedi
ate system 
-Holding Time  Holding Timer to be used for this In
termediate system 
-PDU Length  Entire length of this PDU, in octets, 
including header
-Reserved/Priority  Bit 8 reserved (Transmitted as 
zero, ignored on receipt). Bits 1 through 7  priority 
for being LAN Level 1 Designated Intermediate Sys
tem. Higher number has higher priority for being LAN 
Level 1 Designated Intermediate System. Unsigned 
integer.
-LAN ID  a field composed the system ID (18 octets) 
of the LAN Level 1 Designated Intermediate System, 
plus a low order octet assigned by LAN Level 1 Des
ignated Intermediate System. Copied from LAN 
Level 1 Designated Intermediate System's IIH PDU.
-VARIABLE LENGTH FIELDS  fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE

Any codes in a received PDU that are not recognised 
shall be ignored.

Currently defined codes are: 
7Area Addresses  the set of manual

Area

Addresses of this Intermediate System.
xCODE  1 
xLENGTH  total length of the value field.
xVALUE 1Address Length1Address LengthNo. of OctetsAddress Length
Area Address
Address Length

Area Address

7Address Length  Length of Area Ad
dress in octets.
7Area Address  Area address.
7Intermediate System Neighbours  This option 
field can occur multiple times. The set of Interme
diate systems on this LAN to which adjacencies of 
neighbourSystemType L1 Intermediate Sys
tem exist in state Up or Initialising (i.e.  
those from which Level 1 IIH PDUs have been 
heard).
xCODE  6 
xLENGTH  total length of the value field.
xVALUE 66No. of OctetsLAN Address
LAN Address

7LAN Address  6 octet MAC Address of 
Intermediate System neighbour.
7Padding  This option may occur multiple times. 
It is used to pad the PDU to at least maxsize - 1.
xCODE  8.
xLENGTH  total length of the value field (may 
be zero).
xVALUE  LENGTH octets of arbitrary value.
7Authentication Information  information for 
performing authentication of the originator of the 
PDU.
xCODE  10.
xLENGTH  variable from 1254 octets

xVALUE 1VARIABLENo. of OctetsAuthentication Type

Authentication Value

7Authentication Type  a one octet iden
tifier for the type of authentication to be 
carried out. The following values are de
fined:
0  RESERVED
1  Cleartext Password
2254  RESERVED
255  Routeing Domain private 
authentication method
7Authentication Value  determined by 
the value of the authentication type. If 
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.

9.6 Level 2 LAN IS to IS Hello PDU
This PDU is multicast by Intermediate systems on broad
cast circuits to the multi-destination address AllL2ISs.  
The purpose of this PDU is for Intermediate systems on 
broadcast circuits to discover the identity of other Level 2 
Intermediate systems on that circuit. Trailing Pad options 
are inserted to make PDU Length equal to at least maxsize 
- 1 where
-dataLinkBlocksize
-originatingL2LSPBufferSize
(see 8.4.1). 11No. of Octets1111111ID Length2ID Length +
121VARIABLEIntradomain Routeing
Protocol Discriminator
Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type
R
R
R
Version
ECO
User ECO
Reserved/Circuit Type
Source ID
Holding Time
LAN ID
PDU Length
Priority
R
VARIABLE LENGTH FIELDS

-Intradomain Routeing Protocol Discriminator  ar
chitectural constant 
-Length Indicator  Length of fixed header in octets 
-Version/Protocol ID Extension  1 
-ID Length  Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This 
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating 
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field 
length
7The value 255, whhich means a null ID field (i.e. 
zero length)
All other values are illegal and shall not be used.

-PDU Type (bits 1 through 5)  16. Note bits 6, 7 and 
8 are Reserved, which means they are transmitted as 0 
and ignored on receipt.
-Version  1
-ECO  transmitted as zero, ignored on receipt
-User ECO  transmitted as zero, ignored on receipt
-Reserved/Circuit Type  Most significant 6 bits re
served (Transmitted as zero, ignored on receipt). Low 
order bits (bits 1 and 2) indicate:
70  reserved value (if specified the entire PDU 
shall be ignored) 
71  Level 1 only 
72  Level 2 only (sender is Level 2 Intermediate 
System with manualL2OnlyMode set True for 
this circuit, and will use this link only for Level 2 
traffic) 
73  both Level 1 and Level 2 (sender is Level 2 In
termediate System, and will use this link both for 
Level 1 and Level 2 traffic)
NOTE  In a LAN Level 2 IIH PDU the Circuit Type 
shall be either 2 or 3.
-Source ID  the system ID of transmitting Intermedi
ate System 
-Holding Time  Holding Timer to be used for this In
termediate System 
-PDU Length  Entire length of this PDU, in octets, 
including header 
-Reserved/Priority  Bit 8 reserved (Transmitted as 
zero, ignored on receipt). Bits 1 through 7  priority 
for being LAN Level 2 Designated Intermediate Sys
tem. Higher number has higher priority for being LAN 
Level 2 Designated Intermediate System. Unsigned 
integer.
-LAN ID  a field composed the system ID (18 octets) 
of the LAN Level 1 Designated Intermediate System, 
plus a low order octet assigned by LAN Level 1 Des
ignated Intermediate System. Copied from LAN 
Level 1 Designated Intermediate System's IIH PDU.
-VARIABLE LENGTH FIELDS  fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE

Any codes in a received PDU that are not recognised 
shall be ignored.
Currently defined codes are:

7Area addresses  the set of manual

Area

Addresses of this Intermediate system.
xCODE  1 
xLENGTH  total length of the value field.
xVALUE 1Address Length1Address LengthNo. of OctetsAddress Length
Area Address
Address Length

Area Address

7Address Length  Length of area address 
in octets.
7Area Address  Area address.
7Intermediate System Neighbours  This option 
can occur multiple times. The set of Intermediate 
systems on this LAN to which adjacencies of 
neighbourSystemType L2 Intermediate Sys
tem exist in state Up or Initialising (i.e.  
those from which Level 2 IIH PDUs have been 
heard).
xCODE  6 
xLENGTH  total length of the value field.
xVALUE 66No. of OctetsLAN Address
LAN Address

xLAN Address  6 octet MAC Address of In
termediate System neighbour
7Padding  This option may occur multiple times. 
It is used to pad the PDU to at least maxsize 1.
xCODE  8.
xLENGTH  total length of the value field (may 
be zero).
xVALUE  LENGTH octets of arbitrary value.
7Authentication Information  information for 
performing authentication of the originator of the 
PDU.
xCODE  10.
xLENGTH  variable from 1254 octets

xVALUE 1VARIABLENo. of OctetsAuthentication Type

Authentication Value

7Authentication Type  a one octet iden
tifier for the type of authentication to be 
carried out. The following values are de
fined:
0  RESERVED
1  Cleartext Password
2254  RESERVED
255  Routeing Domain private 
authentication method
7Authentication Value  determined by 
the value of the authentication type. If 
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.

9.7 Point-to-Point IS to IS Hello PDU
This PDU is transmitted by Intermediate systems on non-
broadcast circuits, after receiving an ISH PDU from the 
neighbour system. Its purpose is to determine whether the 
neighbour is a Level 1 or a Level 2 Intermediate System. 
Trailing pad options are inserted to make PDU Length 
equal to at least maxsize  1 where maxsize is the maxi
mum of 
-dataLinkBlocksize
-originating

L1

LSP

Buf

fer

Size 
-originatingL2LSPBufferSize
(see 8.2.3).11No. of Octets1111111ID Length212VARIABLEIntradomain Routeing
Protocol Discriminator
Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type
R
R
R
Version
ECO
User ECO
Reserved/Circuit Type
Source ID
Holding Time
Local Circuit ID
PDU Length
VARIABLE LENGTH FIELDS

-Intradomain Routeing Protocol Discriminator  
architectural constant 
-Length Indicator  Length of fixed header in octets 
-Version/Protocol ID Extension  1 
-ID Length  Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This 
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating 
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field 
length
7The value 255, whhich means a null ID field (i.e. 
zero length)
All other values are illegal and shall not be used.

-PDU Type  (bits 1 through 5)  17. Note bits 6, 7 
and 8 are Reserved, which means they are transmitted 
as 0 and ignored on receipt.
-Version  1
-ECO  transmitted as zero, ignored on receipt
-User ECO  transmitted as zero, ignored on receipt
-Reserved/Circuit Type  Most significant 6 bits re
served (Transmitted as zero, ignored on receipt). Low 
order bits (bits 1 and 2) indicate:
70  reserved value (if specified the entire PDU 
shall be ignored) 
71  Level 1 only
72  Level 2 only (sender is Level 2 Intermediate 
system with manualL2OnlyMode set True for 
this circuit, and will use this link only for Level 2 
traffic) 
73  both Level 1 and Level 2 (sender is Level 2 In
termediate system and will use this link both for 
Level 1 and Level 2 traffic)
-Source ID  the system ID of transmitting Intermedi
ate system 
-Holding Time  Holding Timer to be used for this In
termediate system 
-PDU Length  Entire length of this PDU, in octets, 
including header 
-Local Circuit ID  1 octet unique ID assigned to this 
circuit when it is created by this Intermediate system. 
The actual ID by which the circuit is known to both 
ends of the link is determined by the Intermediate sys
tem with the lower Source ID.
-VARIABLE LENGTH FIELDS  fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE

Any codes in a received PDU that are not recognised 
shall be ignored.
Currently defined codes are:
7Area addresses  the set of manual

Area

Addresses of this Intermediate system
xCODE  1 
xLENGTH  total length of the value field.

xVALUE 1Address Length1Address LengthNo. of OctetsAddress Length
Area Address
Address Length
Area Address

7Address Length  Length of area address 
in octets.
7Area Address  Area address.
7Padding  This option may occur multiple times. 
It is used to pad the PDU to at least maxsize 1.
xCODE  8.
xLENGTH  total length of the value field (may 
be zero).
xVALUE  LENGTH octets of arbitrary value.
7Authentication Information  information for 
performing authentication of the originator of the 
PDU.
xCODE  10.
xLENGTH  variable from 1254 octets
xVALUE 1VARIABLENo. of OctetsAuthentication Type

Authentication Value

7Authentication Type  a one octet iden
tifier for the type of authentication to be 
carried out. The following values are de
fined:
0  RESERVED
1  Cleartext Password
2254  RESERVED
255  Routeing Domain private 
authentication method
7Authentication Value  determined by 
the value of the authentication type. If 
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.

9.8 Level 1 Link State PDU
Level 1 Link State PDUs are generated by Level 1 and 
Level 2 Intermediate systems, and propagated throughout 
an area. The contents of the Level 1 Link State PDU indi
cates the state of the adjacencies to neighbour Intermediate 
Systems, or pseudonodes, and End systems of the Interme
diate system that originally generated the PDU.11No. of
Octets11111122ID Length + 214VARIABLE2Intradomain Routeing 
Protocol Discriminator
Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type
R
R
R
Version
ECO
User ECO
PDU Length
Remaining Lifetime
LSP ID
P
Sequence Number
VARIABLE LENGTH FIELDS
LSPDBOL
IS Type

Checksum
ATT

-Intradomain Routeing Protocol Discriminator  ar
chitectural constant
-Length Indicator  Length if fixed header in octets 
-Version/Protocol ID Extension  1
-ID Length  Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This 
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating 
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field 
length
7The value 255, whhich means a null ID field (i.e. 
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5)  18. Note bits 6, 7 and 
8 are Reserved, which means they are transmitted as 0 
and ignored on receipt.
-Version  1 
-ECO  transmitted as zero, ignored on receipt 

-User ECO  transmitted as zero, ignored on receipt 
-PDU Length  Entire Length of this PDU, in octets, 
including header 
-Remaining Lifetime  Number of seconds before 
LSP considered expired 
-LSP ID  the system ID of the source of the Link 
State PDU. It is structured as follows:ID Length1No. of Octets1Source ID
Pseudonode ID
LSP Number

-Sequence Number  sequence number of LSP 
-Checksum  Checksum of contents of LSP from 
Source ID to end. Checksum is computed as de
scribed in 7.3.11.
-P/ATT/LSPDBOL/IS Type
-P  Bit 8, indicates when set that the issuing Interme
diate System supports the Partition Repair optional 
function. 
7ATT - Bits 7-4 indicate, when set, that the issuing 
Intermediate System is `attached' to other areas 
using:
xBit 4 - the Default Metric
xBit 5 - the Delay Metric
xBit 6 - the Expense Metric
xBit 7 - the Error Metric. 
7LSPDBOL  Bit 3  A value of 0 indicates no 
LSP Database Overload, and a value of 1 indicates 
that the LSP Database is Overloaded. An LSP with 
this bit set will not be used by any decision proc
ess to calculate routes to another IS through the 
originating system.
7IS Type  Bits 1 and 2 indicate the type of Inter
mediate System  One of the following values: 
x0  Unused value
x1  ( i.e. bit 1 set) Level 1 Intermediate system
x2  Unused value 
x3  (i.e. bits 1 and 2 set) Level 2 Intermediate 
system.
-VARIABLE LENGTH FIELDS  fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE

Any codes in a received LSP that are not recognised 
are ignored and passed through unchanged.
Currently defined codes are:
7Area Addresses  the set of manual

Area

Addresses of this Intermediate system.  For 
LSPs not generated on behalf of the pseudonode 
this option shall always be present in the LSP with 
LSP number zero, and shall never be present in an 
LSP with non-zero LSP number. It shall appear 
before any Intermediate System Neighbours or 
End System Neighbours options. This option 
shall never be present in pseudonode LSPs. 
xCODE  1 
xLENGTH  total length of the value field.
xVALUE 1Address Length1Address LengthNo. of OctetsAddress Length
Area Address
Address Length
Area Address

7Address Length  Length of area address 
in octets.
7Area Address  Area address. 
7Intermediate System Neighbours  Intermedi
ate system and pseudonode neighbours.
This is permitted to appear multiple times, and in 
an LSP with any LSP number. However, all the 
Intermediate System Neighbours options 
shall precede the End System Neighbours op
tions. i.e. they shall appear before any End system 
Neighbour options in the same LSP and no End 
system Neighbour options shall appear in an LSP 
with lower LSP number.
xCODE  2.
xLENGTH  1. plus a multiple of 11.

xVALUE No. of Octets11ID Length + 11111ID Length + 1111Virtual Flag
Default Metric
Neighbour ID
Delay Metric
Expense Metric
Error Metric
I/E
0
I/E
S
I/E
S
I/E
S
Default Metric
Neighbour ID
Delay Metric
Expense Metric
Error Metric
I/E
0
I/E
S
I/E
S
I/E
S

7Virtual Flag is a Boolean. If equal to 1, this 
indicates the link is really a Level 2 path to 
repair an area partition. (Level 1 Intermedi
ate Systems would always report this octet 
as 0 to all neighbours). 
7Default Metric is the value of the default 
metric for the link to the listed neighbour. 
Bit 8  of this field is reserved. Bit 7 of this 
field (marked I/E) indicates the metric type, 
and shall contain the value 0, indicating 
an Internal metric.
7Delay Metric is the value of the delay met
ric for the link to the listed neighbour.  If 
this IS does not support this metric it shall 
set the bit S to 1 to indicate that the met
ric is unsupported. Bit 7 of this field 
(marked I/E) indicates the metric type, and 
shall contain the value 0, indicating an 
Internal metric.
7Expense Metric is the value of the ex
pense metric for the link to the listed neigh
bour.  If this IS does not support this metric 
it shall set the bit S to 1 to indicate that 
the metric is unsupported. Bit 7 of this field 
(marked I/E) indicates the metric type, and 
shall contain the value 0, indicating an 
Internal metric.
7Error Metric is the value of the error metric 
for the link to the listed neighbour.  If this 
IS does not support this metric it shall set 
the bit S to 1 to indicate that the metric is 
unsupported. Bit 7 of this field (marked 
I/E) indicates the metric type, and shall 
contain the value 0, indicating an Internal 
metric.
7Neighbour ID. For Intermediate System 
neighbours, the first ID Length octets are 
the neighbour's system ID, and the last oc
tet is 0. For pseudonode neighbours, the 
first ID Length octets is the LAN Level 1 

Designated Intermediate System's ID, and 
the last octet is a non-zero quantity defined 
by the LAN Level 1 Designated Intermedi
ate System. 
7End System Neighbours  End system neigh
bours 
This may appear multiple times, and in an LSP 
with any LSP number. See the description of the 
Intermediate System Neighbours option 
above for the relative ordering constraints. Only 
adjacencies with identical costs can appear in the 
same list.
xCODE  3.
xLENGTH  4. plus a multiple of 6.
xVALUE ID LengthNo. of Octets1ID Length111Neighbour ID
Default Metric
Neighbour ID
Delay Metric

Expense Metric

Error Metric
I/E

0

I/E

S

I/E

S

I/E
S

7Default Metric is the value of the default 
metric for the link to each of the listed 
neighbours. Bit 8 of this field is reserved. 
Bit 7 of this field (marked I/E) indicates the 
metric type, and shall contain the value 0, 
indicating an Internal metric.
7Delay Metric is the value of the delay met
ric for the link to each of the listed neigh
bours.  If this IS does not support this met
ric it shall set the bit S to 1 to indicate 
that the metric is unsupported. Bit 7 of this 
field (marked I/E) indicates the metric type, 
and shall contain the value 0, indicating 
an Internal metric.
7Expense Metric is the value of the ex
pense metric for the link to each of the 
listed neighbours.  If this IS does not sup
port this metric it shall set the bit S to 1 
to indicate that the metric is unsupported. 
Bit 7 of this field (marked I/E) indicates the 
metric type, and shall contain the value 0, 
indicating an Internal metric.
7Error Metric is the value of the error metric 
for the link to each of the listed neighbour.  
If this IS does not support this metric it 
shall set the bit S to 1 to indicate that the 
metric is unsupported. Bit 7 of this field 
(marked I/E) indicates the metric type, and 
shall contain the value 0, indicating an 
Internal metric.
7Neighbour ID  system ID of End system 
neighbour.

7Authentication Information  information for 
performing authentication of the originator of the 
PDU.
xCODE  10.
xLENGTH  variable from 1254 octets
xVALUE 1VARIABLENo. of OctetsAuthentication Type

Authentication Value

7Authentication Type  a one octet iden
tifier for the type of authentication to be 
carried out. The following values are de
fined:
0  RESERVED
1  Cleartext Password
2254  RESERVED
255  Routeing Domain private 
authentication method
7Authentication Value  determined by 
the value of the authentication type. If 
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.

9.9 Level 2 Link State PDU
Level 2 Link State PDUs are generated by Level 2 Interme
diate systems, and propagated throughout the level 2 do
main. The contents of the Level 2 Link State PDU indicates 
the state of the adjacencies to neighbour Level 2 Intermedi
ate Systems, or pseudonodes, and to reachable address pre
fixes of the Intermediate system that originally generated 
the PDU.11No. of Octets11111122ID Length + 214VARIABLE2Intradomain Routeing 
Protocol Discriminator

Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type

R
R
R
Version
ECO
User ECO
PDU Length
Remaining Lifetime
LSP ID
P
Sequence Number
VARIABLE LENGTH FIELDS
LSPDBOL
IS Type

Checksum
ATT

-Intradomain Routeing Protocol Discriminator  ar
chitectural constant 
-Length Indicator  Length of fixed header in octets
-Version/Protocol ID Extension  1
-ID Length  Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This 
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating 
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field 
length
7The value 255, whhich means a null ID field (i.e. 
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5)  20. Note bits 6, 7 and 
8 are Reserved, which means they are transmitted as 0 
and ignored on receipt.
-Version  1 
-ECO  transmitted as zero, ignored on receipt 

-User ECO  transmitted as zero, ignored on receipt 
-PDU Length  Entire Length of this PDU, in octets, 
including header.
-Remaining Lifetime  Number of seconds before 
LSP considered expired 
-LSP ID  the system ID of the source of the Link 
State PDU. It is structured as follows:ID Length1No. of Octets1Source ID
Pseudonode ID
LSP Number

-Sequence Number  sequence number of LSP 
-Checksum  Checksum of contents of LSP from 
Source ID to end. Checksum is computed as de
scribed in 7.3.11.
-P/ATT/LSPDBOL/IS Type
7P  Bit 8, indicates when set that the issuing Inter
mediate System supports the Partition Repair op
tional function.
7ATT - Bits 7-4 indicate, when set, that the issuing 
Intermediate System is `attached' to other areas 
using:
xBit 4 - the Default Metric
xBit 5 - the Delay Metric
xBit 6 - the Expense Metric
xBit 7 - the Error Metric.
7LSPDBOL  Bit 3  A value of 0 indicates no 
LSP Database Overload, and a value of 1 indicates 
that the LSP Database is Overloaded. An LSP with 
this bit set will not be used by any decision proc
ess to calculate routes to another IS through the 
originating system.
7IS Type  Bits 1 and 2 indicate the type of Inter
mediate System  One of the following values: 
x0  Unused value
x1  ( i.e. bit 1 set) Level 1 Intermediate system
x2  Unused value 
x3  (i.e. bits 1 and 2 set) Level 2 Intermediate 
system.
NOTE  In a Level 2 Link State PDU, IS Type 
shall be 3.

-VARIABLE LENGTH FIELDS  fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE

Any codes in a received LSP that are not recognised 
are ignored and passed through unchanged.
Currently defined codes are:
7Area Addresses  the set of partition

Area

Addresses of this Intermediate system.  For non-
pseudonode LSPs this option shall always be pre
sent in the LSP with LSP number zero, and shall 
never be present in an LSP with non-zero LSP 
number. It shall appear before any Intermediate 
System Neighbours or Prefix Neighbours op
tions. This option shall never be present in 
pseudonode LSPs. 
xCODE  1 
xLENGTH  total length of the value field.
xVALUE 1Address Length1Address LengthNo. of OctetsAddress Length
Area Address
Address Length

Area Address

7Address Length  Length of area address 
in octets.
7Area Address  Area address.
7Partition Designated Level 2 Intermediate 
System  ID of Designated Level 2 Intermediate 
System for the partition. For non-pseudonode 
LSPs issued by Intermediate Systems which sup
port the partition repair optional function this op
tion shall always be present in the LSP with LSP 
number zero, and shall never be present in an LSP 
with non-zero LSP number. It shall appear before 
any Intermediate System Neighbours or Prefix 
Neighbours options. This option shall never be 
present in pseudonode LSPs.
xCODE  4.
xLENGTH  6 
xVALUE  ID of Partition Designated Level 2 
Intermediate System for the partition.
7Intermediate System Neighbours  Intermedi
ate system and pseudonode neighbours.
This is permitted to appear multiple times, and in 
an LSP with any LSP number. However, all the 
Intermediate System Neighbours options 

shall precede the Prefix Neighbours options. 
i.e. they shall appear before any Prefix Neighbour 
options in the same LSP and no Prefix Neighbour 
options shall appear in an LSP with lower LSP 
number.
xCODE  2.
xLENGTH  1. plus a multiple of 11.
xVALUE No. of Octets11ID Length + 11111ID Length + 1111Virtual Flag
Default Metric
Neighbour ID
Delay Metric
Expense Metric
Error Metric
I/E
0
I/E
S
I/E
S
I/E
S
Default Metric
Neighbour ID
Delay Metric
Expense Metric
Error Metric
I/E
0
I/E
S
I/E
S
I/E
S

7Virtual Flag is a Boolean. If equal to 1, this 
indicates the link is really a Level 2 path to 
repair an area partition. (Level 1 Intermedi
ate Systems would always report this octet 
as 0 to all neighbours). 
7Default Metric is the value of the default 
metric for the link to the listed neighbour. 
Bit 8  of this field is reserved. Bit 7 of this 
field (marked I/E) indicates the metric type, 
and shall contain the value 0, indicating 
an Internal metric.
7Delay Metric is the value of the delay met
ric for the link to the listed neighbour.  If 
this IS does not support this metric it shall 
set bit S to 1 to indicate that the metric is 
unsupported. Bit 7 of this field (marked 
I/E) indicates the metric type, and shall 
contain the value 0, indicating an Internal 
metric.
7Expense Metric is the value of the ex
pense metric for the link to the listed neigh
bour.  If this IS does not support this metric 
it shall set bit S to 1 to indicate that the 
metric is unsupported. Bit 7 of this field 
(marked I/E) indicates the metric type, and 
shall contain the value 0, indicating an 
Internal metric.
7Error Metric is the value of the error metric 
for the link to the listed neighbour.  If this 
IS does not support this metric it shall set 
bit S to 1 to indicate that the metric is un
supported. Bit 7 of this field (marked I/E) 

indicates the metric type, and shall contain 
the value 0, indicating an Internal metric.
7Neighbour ID. For Intermediate System 
neighbours, the first ID Length octets are 
the neighbour's system ID, and the last oc
tet is 0. For pseudonode neighbours, the 
first ID Length octets is the LAN Level 1 
Designated Intermediate System's ID, and 
the last octet is a non-zero quantity defined 
by the LAN Level 1 Designated Intermedi
ate System. 
7Prefix Neighbours  reachable address prefix 
neighbours 
This may appear multiple times, and in an LSP 
with any LSP number. See the description of the 
Intermediate System Neighbours option 
above for the relative ordering constraints. Only 
adjacencies with identical costs can appear in the 
same list.
xCODE  5. 
xLENGTH  Total length of the VALUE field.
xVALUE 1iAddress Prefix Length /2y1No. of OctetsiAddress Prefix Length
/2y1111Address Prefix Length
Address Prefix
Address Prefix Length

Address Prefix
Default Metric

Delay Metric

Expense Metric

Error Metric

I/E

0

I/E

S

I/E

S

I/E

S

7Default Metric is the value of the default 
metric for the link to each of the listed 
neighbours. Bit 8 of this field is reserved. 
Bit 7 (marked I/E) indicates the metric 
type, and may be set to zero indicating an 
internal metric, or may be set to 1 indicat
ing an external metric.
7Delay Metric is the value of the delay met
ric for the link to each of the listed neigh
bours.  If this IS does not support this met
ric it shall set the bit S to 1 to indicate 
that the metric is unsupported. Bit 7 
(marked I/E) indicates the metric type, and 
may be set to zero indicating an internal 
metric, or may be set to 1 indicating an ex
ternal metric.
7Expense Metric is the value of the ex
pense metric for the link to each of the 
listed neighbours.  If this IS does not sup
port this metric it shall set the bit S to 1 
to indicate that the metric is unsupported. 

Bit 7 (marked I/E) indicates the metric 
type, and may be set to zero indicating an 
internal metric, or may be set to 1 indicat
ing an external metric.
7Error Metric is the value of the error metric 
for the link to each of the listed neighbour.  
If this IS does not support this metric it 
shall set the bit S to 1 to indicate that the 
metric is unsupported. Bit 7 (marked I/E) 
indicates the metric type, and may be set to 
zero indicating an internal metric, or may 
be set to 1 indicating an external metric.
7Address Prefix Length is the length in 
semi-octets of the following prefix. A 
length of zero indicates a prefix that 
matches all NSAPs.
7Address Prefix is a reachable address pre
fix encoded as described in 7.1.4. If the 
length in semi-octets is odd, the prefix is 
padded out to an integral number of octets 
with a trailing zero semi-octet.
Note that the area addresses listed in the Area Ad
dresses option of Level 2 Link State PDU with 
LSP number zero, are understood to be reachable 
address neighbours with cost 0. They are not listed 
separately in the Prefix Neighbours options.
7Authentication Information  information for 
performing authentication of the originator of the 
PDU.
xCODE  10.
xLENGTH  variable from 1254 octets
xVALUE 1VARIABLENo. of OctetsAuthentication Type

Authentication Value

7Authentication Type  a one octet iden
tifier for the type of authentication to be 
carried out. The following values are de
fined:
0  RESERVED
1  Cleartext Password
2254  RESERVED
255  Routeing Domain private 
authentication method
7Authentication Value  determined by 
the value of the authentication type. If 
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.

9.10 Level 1 Complete Sequence 
Numbers PDU11No. of Octets1111112ID Length + 1ID Length + 2ID Length +
2VARIABLEIntradomain Routeing 
Protocol Discriminator

Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type

R
R
R
Version
ECO
User ECO
PDU Length
Source ID
Start LSP ID
End LSP ID
VARIABLE LENGTH FIELDS

-Intradomain Routeing Protocol Discriminator  ar
chitectural constant 
-Length Indicator  Length of fixed header in octets
-Version/Protocol ID Extension  1 
-ID Length  Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This 
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating 
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field 
length
7The value 255, whhich means a null ID field (i.e. 
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5)  24. Note bits 6, 7 and 
8 are Reserved, which means they are transmitted as 0 
and ignored on receipt.
-Version  1 
-ECO  transmitted as zero, ignored on receipt 
-User ECO  transmitted as zero, ignored on receipt 
-PDU Length  Entire Length of this PDU, in octets, 
including header 
-Source ID  the system ID of Intermediate System 
(with zero Circuit ID) generating this Sequence Num
bers PDU.

-Start LSP ID  the system ID of first LSP in the 
range covered by this Complete Sequence Numbers 
PDU.
-End LSP ID  the system ID of last LSP in the range 
covered by this Complete Sequence Numbers PDU.
-VARIABLE LENGTH FIELDS  fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE

Any codes in a received CSNP that are not recognised 
are ignored.
Currently defined codes are:
7LSP Entries  This may appear multiple times. 
The option fields, if they appear more than once, 
shall appear sorted into ascending LSPID order.
xCODE  9
xLENGTH  total length of the value field.
xVALUE  a list of LSP entries of the form:4No. of Octets2ID Length +
2242ID Length + 22LSP Sequence Number
Checksum
Remaining Lifetime
LSP ID
LSP Sequence Number
Checksum
Remaining Lifetime
LSP ID

7Remaining Lifetime  Remaining Life
time of LSP.
7LSP ID  system ID of the LSP to which 
this entry refers.
7LSP Sequence Number  Sequence 
number of LSP.
7Checksum  Checksum reported in LSP.
The entries shall be sorted into ascending 
LSPID order (the LSP number octet of the 
LSPID is the least significant octet).
7Authentication Information  information for 
performing authentication of the originator of the 
PDU.
xCODE  10.
xLENGTH  variable from 1254 octets

xVALUE 1VARIABLENo. of OctetsAuthentication Type

Authentication Value

7Authentication Type  a one octet iden
tifier for the type of authentication to be 
carried out. The following values are de
fined:
0  RESERVED
1  Cleartext Password
2254  RESERVED
255  Routeing Domain private 
authentication method
7Authentication Value  determined by 
the value of the authentication type. If 
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.

9.11 Level 2 Complete Sequence 
Numbers PDU
11No. of Octets1111112ID Length + 1ID Length + 2ID Length +
2VARIABLEIntradomain Routeing 
Protocol Discriminator

Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type

R
R
R
Version
ECO
User ECO
PDU Length
Source ID
Start LSP ID
End LSP ID
VARIABLE LENGTH FIELDS

-Intradomain Routeing Protocol Discriminator  ar
chitectural constant
-Length Indicator  Length of fixed header in octets 
-Version/Protocol ID Extension  1
-ID Length  Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This 
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating 
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field 
length
7The value 255, whhich means a null ID field (i.e. 
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5)  25. Note bits 6, 7 and 
8 are Reserved, which means they are transmitted as 0 
and ignored on receipt.
-Version  1 
-ECO  transmitted as zero, ignored on receipt 
-User ECO  transmitted as zero, ignored on receipt 
-PDU Length  Entire Length of this PDU, in octets, 
including header 

-Source ID  the system ID of Intermediate System 
(with zero Circuit ID) generating this Sequence Num
bers PDU.
-Start LSP ID  the system ID of first LSP in the 
range covered by this Complete Sequence Numbers 
PDU.
-End LSP ID  the system ID of last LSP in the range 
covered by this Complete Sequence Numbers PDU.
-VARIABLE LENGTH FIELDS  fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE

Any codes in a received CSNP that are not recognised 
are ignored.
Currently defined codes are:
7LSP Entries  this may appear multiple times. 
The option fields, if they appear more than once, 
shall appear sorted into ascending LSPID order.
xCODE  9
xLENGTH  total length of the value field.
xVALUE  a list of LSP entries of the form:4No. of Octets2ID Length +
2242ID Length + 22LSP Sequence Number
Checksum
Remaining Lifetime
LSP ID
LSP Sequence Number
Checksum
Remaining Lifetime
LSP ID

7Remaining Lifetime  Remaining Life
time of LSP.
7LSP ID  the system ID of the LSP to 
which this entry refers.
7LSP Sequence Number  Sequence 
number of LSP.
7Checksum  Checksum reported in LSP.
The entries shall be sorted into ascending 
LSPID order (the LSP number octet of the 
LSPID is the least significant octet).
7Authentication Information  information for 
performing authentication of the originator of the 
PDU.

xCODE  10.
xLENGTH  variable from 1254 octets
xVALUE 1VARIABLENo. of OctetsAuthentication Type

Authentication Value

7Authentication Type  a one octet iden
tifier for the type of authentication to be 
carried out. The following values are de
fined:
0  RESERVED
1  Cleartext Password
2254  RESERVED
255  Routeing Domain private 
authentication method
7Authentication Value  determined by 
the value of the authentication type. If 
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.

9.12 Level 1 Partial Sequence Numbers 
PDU
11No. of Octets1111112ID Length + 1VARIABLEIntradomain Routeing 
Protocol Discriminator

Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type

R
R
R
Version
ECO
User ECO
PDU Length
Source ID
VARIABLE LENGTH FIELDS

-Intradomain Routeing Protocol Discriminator  ar
chitectural constant
-Length Indicator  Length of fixed header in octets
-Version/Protocol ID Extension  1
-ID Length  Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This 
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating 
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field 
length
7The value 255, whhich means a null ID field (i.e. 
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5)  26. Note bits 6, 7 and 
8 are Reserved, which means they are transmitted as 0 
and ignored on receipt.
-Version  1 
-ECO  transmitted as zero, ignored on receipt 
-User ECO  transmitted as zero, ignored on receipt 
-PDU Length  Entire Length of this PDU, in octets, 
including header 
-Source ID  the system ID of Intermediate system 
(with zero Circuit ID) generating this Sequence Num
bers PDU.

-VARIABLE LENGTH FIELDS  fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE

Any codes in a received PSNP that are not recognised 
are ignored.
Currently defined codes are:
7LSP Entries  this may appear multiple times. 
The option fields, if they appear more than once, 
shall appear sorted into ascending LSPID order.
xCODE  9
xLENGTH  total length of the value field.
xVALUE  a list of LSP entries of the form:4No. of Octets2ID Length +
2242ID Length + 22LSP Sequence Number
Checksum
Remaining Lifetime
LSP ID
LSP Sequence Number
Checksum
Remaining Lifetime
LSP ID

7Remaining Lifetime  Remaining Life
time of LSP.
7LSP ID  the system ID of the LSP to 
which this entry refers.
7LSP Sequence Number  Sequence 
number of LSP.
7Checksum  Checksum reported in LSP.
The entries shall be sorted into ascending 
LSPID order (the LSP number octet of the 
LSPID is the least significant octet).
7Authentication Information  information for 
performing authentication of the originator of the 
PDU.
xCODE  10.
xLENGTH  variable from 1254 octets

xVALUE 1VARIABLENo. of OctetsAuthentication Type

Authentication Value

7Authentication Type  a one octet iden
tifier for the type of authentication to be 
carried out. The following values are de
fined:
0  RESERVED
1  Cleartext Password
2254  RESERVED
255  Routeing Domain private 
authentication method
7Authentication Value  determined by 
the value of the authentication type. If 
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.

9.13 Level 2 Partial Sequence Numbers 
PDU
11No. of Octets1111112ID Length + 1VARIABLEIntradomain Routeing 
Protocol Discriminator

Length Indicator
Version/Protocol ID Extension
ID Length
PDU Type

R
R
R
Version
ECO
User ECO
PDU Length
Source ID
VARIABLE LENGTH FIELDS

-Intradomain Routeing Protocol Discriminator  ar
chitectural constant
-Length Indicator  Length of fixed header in octets 
-Version/Protocol ID Extension  1 
-ID Length  Length of the ID field of NSAP ad
dresses and NETs used in this routeing domain. This 
field shall take on one of the following values:
7An integer between 1 and 8, inclusive, indicating 
an ID field of the corresponding length
7The value zero, which indicates a 6 octet ID field 
length
7The value 255, whhich means a null ID field (i.e. 
zero length)
All other values are illegal and shall not be used.
-PDU Type (bits 1 through 5)  27. Note bits 6, 7 and 
8 are Reserved, which means they are transmitted as 0 
and ignored on receipt.
-Version  1 
-ECO  transmitted as zero, ignored on receipt 
-User ECO  transmitted as zero, ignored on receipt 
-PDU Length  Entire Length of this PDU, in octets, 
including header 
-Source ID  the system ID of Intermediate system 
(with zero Circuit ID) generating this Sequence Num
bers PDU.

-VARIABLE LENGTH FIELDS  fields of the form:11No. of OctetsLENGTHCODE
LENGTH
VALUE

Any codes in a received PSNP that are not recognised 
are ignored.
Currently defined codes are:
7LSP Entries  this may appear multiple times. 
The option fields, if they appear more than once, 
shall appear sorted into ascending LSPID order.
xCODE  9
xLENGTH  total length of the value field.
xVALUE  a list of LSP entries of the form:4No. of Octets2ID Length +
2242ID Length + 22LSP Sequence Number
Checksum
Remaining Lifetime
LSP ID
LSP Sequence Number
Checksum
Remaining Lifetime
LSP ID

7Remaining Lifetime  Remaining Life
time of LSP.
7LSP ID  the system ID of the LSP to 
which this entry refers.
7LSP Sequence Number  Sequence 
number of LSP.
7Checksum  Checksum reported in LSP.
The entries shall be sorted into ascending 
LSPID order (the LSP number octet of the 
LSPID is the least significant octet).
7Authentication Information  information for 
performing authentication of the originator of the 
PDU.
xCODE  10.
xLENGTH  variable from 1254 octets

xVALUE 1VARIABLENo. of OctetsAuthentication Type

Authentication Value

7Authentication Type  a one octet iden
tifier for the type of authentication to be 
carried out. The following values are de
fined:
0  RESERVED
1  Cleartext Password
2254  RESERVED
255  Routeing Domain private 
authentication method
7Authentication Value  determined by 
the value of the authentication type. If 
Cleartext Password as defined in this Inter
national Standard is used, then the authenti
cation value is an octet string.

10 System Environment
10.1 Generating Jitter on Timers
When PDUs are transmitted as a result of timer expiration, 
there is a danger that the timers of individual systems may 
become synchronised. The result of this is that the traffic 
distribution will contain peaks. Where there are a large 
number of synchronised systems, this can cause overload
ing of both the transmission medium and the systems re
ceiving the PDUs. In order to prevent this from occurring, 
all periodic timers, the expiration of which can cause the 
transmission of PDUs, shall have jitter introduced as de
fined in the following algorithm.
CONSTANT 
Jitter = 25; 	
(* The percentage jitter as defined in the architectural 
constant Jitter *)
Resolution = 100; 	
(* The timer resolution in milliseconds *)

PROCEDURE Random(max : Integer): Integer;
 (* This procedure delivers a Uniformly distributed 
random integer R such that 0 < R < max  *)

PROCEDURE 
DefineJitteredTimer(baseTimeValueInSeconds: Integer; 
expirationAction : Procedure);

VAR
baseTimeValue, maximumTimeModifier, waitTime : 
Integer;
nextexpiration : Time;

BEGIN
baseTimeValue := baseTimeValueInSeconds * 1000 / 
Resolution;
maximumTimeModifier := baseTimeValue * Jitter / 
100; (* Compute maximum possible jitter *)
WHILE running DO 
BEGIN 
(* First compute next expiration time *)
randomTimeModifier := 
Random(maximumTimeModifier);
waitTime := baseTimeValue - 
randomTimeModifier;
nextexpiration := CurrentTime + waitTime;
(* Then perform expiration Action *)
expirationAction; 
WaitUntil(nextexpiration);
END (* of Loop *)
END (* of DefineJitteredTimer *)
Thus the call DefineJitteredTimer(HelloTime, SendHel
loPDU); where HelloTime is 10 seconds, will cause the 
action SendHelloPDU to be performed at random inter
vals of between 7.5 and 10 seconds. The essential point of 
this algorithm is that the value of randomTimeModifier is 
randomised within the inner loop. Note that the new expira
tion time is set immediately on expiration of the last inter
val, rather than when the expiration action has been com
pleted. 

The time resolution shall be less than or equal to 100 milli
seconds. It is recommended to be less than or equal to 10 
milliseconds. The time resolution is the maximum interval 
that can elapse without there being any change in the value 
of the timer. The periodic transmission period shall be ran
dom or pseudo-random in the specified range, with uniform 
distribution across similar implementations.
10.2 Resolution of Timers
All timers specified in units of seconds shall have a resolu
tion of no less than 11 second.
All timers specified in units of milliseconds shall have a 
resolution of no less than 110 milliseconds
10.3 Requirements on the Operation of 
ISO 9542
This International Standard places certain requirements on 
the use of ISO 9542 by Intermediate systems which go be
yond those mandatory requirements stated in the 
conformance clause of ISO 9542. These requirements are:
a)The IS shall operate the Configuration Information 
functions on all types of subnetworks supported by the 
IS. This includes the reception of ESH PDUs, and the 
reception and transmission of ISH PDUs.
b)The IS shall enable the All Intermediate Systems 
multi-destination subnetwork address.

11 System Management 
11.1 General
The operation of the Intra-domain ISIS routeing functions 
may be monitored and controlled using System Manage
ment. This clause is the management specification for ISO 
10589 in the GDMO notation as defined in ISO 10165-4.
11.1.1 Naming Hierarchy
The containment hierarchy for ISO 10589 is illustrated be
low in figure
8NetworkVirtualAdjacencyAdjacencyDestinationSystemDestinationAreaCircuit
ReachableAddressEntityCLNS(ISO 10589 Package)(ISO 10589
Package)ManualAdjacencyLevel 2 OnlyFigure 8 - Containment and Naming Hierarchy

. 
11.1.2 Resetting of Timers
Many of the attributes defined herein represent the values 
of timers. They specify the interval between certain events 
in the operation of the routeing state machines. If the value 
of one of these characteristics is changed to a new value t 
while the routeing state machine is in operation the imple
mentation shall take the necessary actions to ensure that for 
any time interval which was in progress when the corre
sponding attribute was changed, the next expiration of that 
interval takes place t seconds from the original start of that 
interval, or immediately, whichever is the later.

Where this action is necessary it is indicated in the applica
ble behaviour clause of the GDMO.  See 11.2.16
11.1.3 Resource Limiting Characteristics
Certain attributes place limits on some resource, such as 
max

imum

SVC

Adjacencies. In general, implementa
tions may allocate memory resources up to this limit when 
the managed object is enabled and it may be impossible to 
change the allocation without first disabling and re-enabling 
the corresponding Network entity. Therefore this Interna
tional Standard only requires that system management shall 
be able to change these attributes when the managed object 
is disabled (i.e. in the state off).
However some implementations may be able to change the 
allocation of resources without first disabling the Network 
entity. In this case it is permitted to increase the value of 
the characteristic at any time, but it shall not be decreased 
below the currently used value of the resource. For exam
ple, maximumSVCAdjacencies shall not be decreased 
below the current number of SVCs which have been cre
ated.
Characteristics of this type are indicated in the behaviour 
clause of the GDMO.  See 11.2.16.

11.2 GDMO Definition
11.2.1 Name Bindings
iSO10589-NB NAME BINDING
SUBORDINATE OBJECT CLASS cLNS;
NAMED BY
SUPERIOR OBJECT CLASS 
"ISO/IEC xxxxx":networkEntity;
WITH ATTRIBUTE 
"ISO/IEC xxxxx":cLNS-MO-Name;
CREATE with-automatic-instance-naming 
iSO10589-NB-p1;
DELETE only-if-no-contained-objects;
REGISTERED AS {ISO10589-ISIS.nboi iSO10589-NB 
(1)};

level1ISO10589Circuit-NB NAME BINDING
SUBORDINATE OBJECT CLASS circuit;
NAMED BY
SUPERIOR OBJECT CLASS cLNS;
WITH ATTRIBUTE 
"ISO/IEC xxxxx":circuit-MO-Name;
CREATE with-reference-object  
iSO10589Circuit-MO-p1;
DELETE only-if-no-contained-objects;
REGISTERED AS {ISO10589-ISIS.nboi 
level1ISO10589Circuit-NB (2)};

destinationSystem-NB NAME BINDING
SUBORDINATE OBJECT CLASS destinationSystem;
NAMED BY
SUPERIOR OBJECT CLASS cLNS;
WITH ATTRIBUTE networkEntityTitle;
REGISTERED AS {ISO10589-ISIS.nboi 
destinationSystem-NB (3)};

destinationArea-NB NAME BINDING
SUBORDINATE OBJECT CLASS destinationArea;
NAMED BY
SUPERIOR OBJECT CLASS cLNS;
WITH ATTRIBUTE addressPrefix;
BEHAVIOUR destinationArea-NB-B BEHAVIOUR
DEFINED AS This name binding is only applicable 
where the superior object has an iSType of Level2;;
REGISTERED AS {ISO10589-ISIS.nboi 
destinationArea-NB (4)};

virtualAdjacency-NB NAME BINDING
SUBORDINATE OBJECT CLASS virtualAdjacency;
NAMED BY
SUPERIOR OBJECT CLASS cLNS;
WITH ATTRIBUTE networkEntityTitle;
BEHAVIOUR virtualAdjacency-NB-B BEHAVIOUR
DEFINED AS This name binding is only applicable 
where the superior  object has an iSType of Level2;;
REGISTERED AS {ISO10589-ISIS.nboi 
virtualAdjacency-NB (5)};

reachableAddress-NB NAME BINDING
SUBORDINATE OBJECT CLASS reachableAddress;
NAMED BY
SUPERIOR OBJECT CLASS circuit;
WITH ATTRIBUTE addressPrefix;
BEHAVIOUR reachableAddress-NB-B BEHAVIOUR
DEFINED AS This name binding is only applicable 
where the superior object of the Circuit instance is 
an object  with iSType level2IS;;
CREATE with-reference-object  reachableAddressP1 
reachableAddressP2;
DELETE only-if-no-contained-objects;
REGISTERED AS {ISO10589-ISIS.nboi 
reachableAddress-NB (6)};

adjacency-NB NAME BINDING
SUBORDINATE OBJECT CLASS adjacency;
NAMED BY
SUPERIOR OBJECT CLASS circuit;
WITH ATTRIBUTE adjacencyName;
REGISTERED AS {ISO10589-ISIS.nboi adjacency-NB 
(7)};

manualAdjacency-NB NAME BINDING
SUBORDINATE OBJECT CLASS manualAdjacency;
NAMED BY
SUPERIOR OBJECT CLASS circuit;
WITH ATTRIBUTE adjacencyName;
BEHAVIOUR manualAdjacency-NB-B BEHAVIOUR
DEFINED AS When an instance name is specified in 
the CREATE operation, that value shall be used for 
the adjacencyName, otherwise automatic instance 
naming shall be used;;
CREATE with-reference-object, 
with-automatic-instance-naming
manualAdjacencyP1  manualAdjacencyP2;
DELETE only-if-no-contained-objects;
REGISTERED AS {ISO10589-ISIS.nboi 
manualAdjacency-NB (8)};

11.2.2 The CLNS Managed Object for ISO 
10589
cLNS MANAGED OBJECT CLASS
DERIVED FROM "ISO/IEC xxxx":cLNS;
-- To be replaced by the number of the network layer 
MO definitions when assigned.
CONDITIONAL PACKAGES
level1ISO10589Package
PRESENT IF The Intermediate System is a Level 1 
Intermediate System,
level2ISO10589Package
PRESENT IF The Intermediate System is a Level 2 
Intermediate System (i.e. the value of iSType is 
Level2),
partitionRepairPackage
PRESENT IF The Intermediate System is a Level 2 
Intermediate System and the partition repair option 
is implemented,
level1AuthenticationPackage
PRESENT IF The authentication procedures are im
plemented,
level2AuthenticationPackage
PRESENT IF The Intermediate System is a Level 2 
Intermediate System and the authentication proce
dures are implemented;
REGISTERED AS {ISO10589-ISIS.moi cLNS (1)};

level1ISO10589Package PACKAGE
ATTRIBUTES
version GET,
iSType GET,
maximumPathSplits
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.maximumPathSplits-Default
PERMITTED VALUES 
ISO10589-ISIS.MaximumPathSplits-Permitted
GET-REPLACE,
maximumBuffers
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.maximumBuffers-Default
PERMITTED VALUES 
ISO10589-ISIS.MaximumBuffers-Permitted
GET-REPLACE,
minimumLSPTransmissionInterval
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.minimumLSPTransmissionInterval-
Default
PERMITTED VALUES 
ISO10589-ISIS.MinimumLSPTransmissionInterval-
Permitted
GET-REPLACE,
maximumLSPGenerationInterval
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.maximumLSPGenerationInterval-D
efault
PERMITTED VALUES 
ISO10589-ISIS.MaximumLSPGenerationInterval-Pe
rmitted
GET-REPLACE,
minimumBroadcastLSPTransmissionInterval
REPLACE-WITH-DEFAULT

DEFAULT VALUE 
ISO10589-ISIS.minimumBroadcastLSPTransmissio
nInterval-Default
PERMITTED VALUES 
ISO10589-ISIS.MinimumBroadcastLSPTransmissio
nInterval-Permitted
GET-REPLACE,
-- Note this is defined for all Circuits, but would only 
be required if one of them were a broadcast Circuit
completeSNPInterval
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.completeSNPInterval-Default
PERMITTED VALUES 
ISO10589-ISIS.CompleteSNPInterval-Permitted
GET-REPLACE,
-- Ditto
originatingL1LSPBufferSize
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.originatingL1LSPBufferSize-Defaul
t
PERMITTED VALUES 
ISO10589-ISIS.OriginatingL1LSPBufferSize-Permit
ted
GET-REPLACE,
-- Note: redirectHoldingTime moved to 
ISO9542ISPackage
manualAreaAddresses
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.manualAreaAddresses-Default
PERMITTED VALUES 
ISO10589-ISIS.ManualAreaAddresses-Permitted
GET ADD-REMOVE,
minimumLSPGenerationInterval
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.minimumLSPGenerationInterval-De
fault
PERMITTED VALUES 
ISO10589-ISIS.MinimumLSPGenerationInterval-Pe
rmitted
GET-REPLACE,
defaultESHelloTimer
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.defaultESHelloTime-Default
PERMITTED VALUES 
ISO10589-ISIS.DefaultESHelloTime-Permitted
GET-REPLACE,
pollESHelloRate
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.pollESHelloRate-Default
PERMITTED VALUES 
ISO10589-ISIS.PollESHelloRate-Permitted
GET-REPLACE,
partialSNPInterval
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.partialSNPInterval-Default
PERMITTED VALUES 
ISO10589-ISIS.PartialSNPInterval-Permitted
GET-REPLACE,
waitingTime
REPLACE-WITH-DEFAULT

DEFAULT VALUE 
ISO10589-ISIS.waitingTime-Default
PERMITTED VALUES 
ISO10589-ISIS.WaitingTime-Permitted
GET-REPLACE,
dRISISHelloTimer
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.dRISISHelloTimer-Default
PERMITTED VALUES 
ISO10589-ISIS.DRISISHelloTimer-Permitted
GET-REPLACE,
l1State GET,
areaAddresses GET,
-- PDUFormatErrors now in network layer MO
corruptedLSPsDetected GET,
lSPL1DatabaseOverloads GET,
manualAddressesDroppedFromArea GET,
attemptsToExceedMaximumSequenceNumber GET,
sequenceNumberSkips GET,
ownLSPPurges GET,
iDFieldLengthMismatches GET;
ATTRIBUTE GROUPS
counters
-- PDUFormatErrors now in Network Layer MO
corruptedLSPsDetected
lSPL1DatabaseOverloads
manualAddressesDroppedFromArea
attemptsToExceedMaximumSequenceNumber
sequenceNumberSkips
ownLSPPurges
iDFieldLengthMismatches;
-- activate and deactivate actions now in Network Layer 
MO
NOTIFICATIONS
"ISO/IEC xxxxx":pduFormatError 
notificationReceivingAdjacency,
-- extra parameter for ISO 10589
corruptedLSPDetected,
lSPL1DatabaseOverload,
manualAddressDroppedFromArea,
attemptToExceedMaximumSequenceNumber,
sequenceNumberSkip,
ownLSPPurge,
iDFieldLengthMismatch;
REGISTERED AS {ISO10589-ISIS.poi 
level1ISO10589Package (1)};

level2ISO10589Package PACKAGE
ATTRIBUTES
originatingL2LSPBufferSize
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.originatingL2LSPBufferSize-Defaul
t
PERMITTED VALUES 
ISO10589-ISIS.OriginatingL2LSPBufferSize-Permit
ted
GET-REPLACE,
l2State GET,
lSPL2DatabaseOverloads GET;
ATTRIBUTE GROUPS
counters
lSPL2DatabaseOverloads;
NOTIFICATIONS
lSPL2DatabaseOverload;

REGISTERED AS {ISO10589-ISIS.poi 
level2ISO10589Package (2)};

partitionRepairPackage PACKAGE
BEHAVIOUR DEFINITIONS partitionRepairPackage-B 
BEHAVIOUR
DEFINED AS Present when the partition repair option 
is implemented;;
ATTRIBUTES
maximumVirtualAdjacencies
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.maximumVirtualAdjacencies-Defau
lt
PERMITTED VALUES 
ISO10589-ISIS.MaximumVirtualAdjacencies-Permi
tted
GET-REPLACE,
partitionAreaAddresses GET,
partitionDesignatedL2IntermediateSystem GET,
partitionVirtualLinkChanges GET;
ATTRIBUTE GROUPS
counters
partitionVirtualLinkChanges;
NOTIFICATIONS
partitionVirtualLinkChange;
REGISTERED AS {ISO10589-ISIS.poi 
partitionRepairPackage (3)};

level1AuthenticationPackage PACKAGE
BEHAVIOUR DEFINITIONS 
level1AuthenticationPackage-B BEHAVIOUR
DEFINED AS Present when the authentication proce
dures option is implemented;;
ATTRIBUTES
areaTransmitPassword
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.password-Default
GET-REPLACE,
areaReceivePasswords
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.passwords-Default
GET-REPLACE
ADD-REMOVE,
authenticationFailures
GET;
ATTRIBUTE GROUPS
counters
authenticationFailures;
NOTIFICATIONS
authenticationFailure;
REGISTERED AS {ISO10589-ISIS.poi 
level1AuthenticationPackage (4)};

level2AuthenticationPackage PACKAGE
BEHAVIOUR DEFINITIONS 
level2AuthenticationPackage-B BEHAVIOUR
DEFINED AS Present when the authentication proce
dures option is implemented and the value of the 
iSType attribute is Level2;;
ATTRIBUTES
domainTransmitPassword
REPLACE-WITH-DEFAULT

DEFAULT VALUE 
ISO10589-ISIS.password-Default
GET-REPLACE,
domainReceivePasswords
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.passwords-Default
GET-REPLACE
ADD-REMOVE;
REGISTERED AS {ISO10589-ISIS.poi 
level2AuthenticationPackage (5)};

11.2.3 The Circuit Managed Object for ISO 
10589
circuit MANAGED OBJECT CLASS
DERIVED FROM "ISO/IEC xxxx":circuit;
-- xxxx to be replaced with the number of the network 
layer managed object definitions when one is 
assigned
CONDITIONAL PACKAGES
level1ISO10589CircuitPackage
PRESENT IF the Circuit is a level 1 ISO 10589 Cir
cuit,
level1ISO10589BroadcastCircuitPackage
PRESENT IF the Circuit is a level 1 ISO 10589 
broadcast Circuit,
level1ISO10589PtToPtCircuitPackage
PRESENT IF the Circuit is a level 1 ISO 10589 Point 
to Point Circuit,
level2ISO10589DACircuitPackage
PRESENT IF the Circuit is a level 2 ISO 10589 X.25 
DA Circuit,
level1ISO10589StaticCircuitPackage
PRESENT IF the Circuit is a level 1 ISO10589 X.25 
STATIC Circuit (IN or OUT),
level1ISO10589StaticOutCircuitPackage
PRESENT IF the Circuit is a level1 ISO 10589 X.25 
STATIC OUT SNAP,
level2ISO10589CircuitPackage
PRESENT IF the IS is a Level2 ISO 10589 IS,
level2ISO10589BroadcastCircuitPackage
PRESENT IF the Circuit is a level 1 ISO 10589 
broadcast Circuit and the IS is a L2 IS,
dACircuitCallEstablishmentMetricIncrementPackage
PRESENT IF the Circuit is an X.25 DA circuit and 
support is implemented for call establishement met
ric increment values greater than zero,
circuitAuthenticationPackage
PRESENT IF the authentication procedures are im
plemented on this IS;
REGISTERED AS {ISO10589-ISIS.moi circuit (2)};

level1ISO10589CircuitPackage PACKAGE
ATTRIBUTES
type GET,
helloTimer
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.helloTimer-Default
PERMITTED VALUES 
ISO10589-ISIS.HelloTimer-Permitted
GET-REPLACE,
l1DefaultMetric
REPLACE-WITH-DEFAULT

DEFAULT VALUE 
ISO10589-ISIS.defaultMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.DefaultMetric-Permitted
GET-REPLACE,
l1DelayMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.optionalMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.OptionalMetric-Permitted
GET-REPLACE,
l1ExpenseMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.optionalMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.OptionalMetric-Permitted
GET-REPLACE,
l1ErrorMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.optionalMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.OptionalMetric-Permitted
GET-REPLACE,
externalDomain
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.externalDomain-Default
GET-REPLACE,
circuitChanges GET,
changesInAdjacencyState GET,
initializationFailures GET,
rejectedAdjacencies GET,
controlPDUsSent GET,
controlPDUsReceived GET,
iDFieldLengthMismatches GET;
ATTRIBUTE GROUPS
counters
circuitChanges
changesInAdjacencyState
initializationFailures
rejectedAdjacencies
controlPDUsSent
controlPDUsReceived
iDFieldLengthMismatches;
-- Note: activate and deactivate are now imported from 
the network layer definition of circuit MO
NOTIFICATIONS
circuitChange,
adjacencyStateChange,
initializationFailure,
rejectedAdjacency,
iDFieldLengthMismatch;
REGISTERED AS {ISO10589-ISIS.poi 
level1ISO10589CircuitPackage (6)};

level1ISO10589BroadcastCircuitPackage PACKAGE
BEHAVIOUR DEFINITIONS 
level1BroadcastCircuitPackage-B BEHAVIOUR
DEFINED AS Present when the Circuit is of type 
Broadcast;;
ATTRIBUTES
iSISHelloTimer
REPLACE-WITH-DEFAULT

DEFAULT VALUE 
ISO10589-ISIS.iSISHelloTimer-Default
PERMITTED VALUES 
ISO10589-ISIS.ISISHelloTimer-Permitted
GET-REPLACE,
l1IntermediateSystemPriority
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.l1IntermediateSystemPriority-Defau
lt
PERMITTED VALUES 
ISO10589-ISIS.L1IntermediateSystemPriority-Perm
itted
GET-REPLACE,
l1CircuitID GET,
l1DesignatedIntermediateSystem GET,
lanL1DesignatedIntermediateSystemChanges GET;
ATTRIBUTE GROUPS
counters
lanL1DesignatedIntermediateSystemChanges;
NOTIFICATIONS
lanL1DesignatedIntermediateSystemChange;
REGISTERED AS {ISO10589-ISIS.poi 
level1ISO10589BroadcastCircuitPackage (7)};

level1ISO10589PtToPtCircuitPackage PACKAGE
BEHAVIOUR DEFINITIONS 
level1PtToPtCircuitPackage-B BEHAVIOUR
DEFINED AS Present when the Circuit is of type Pt
ToPt;;
ATTRIBUTES
ptPtCircuitID GET;
REGISTERED AS {ISO10589-ISIS.poi 
level1ISO10589PtToPtCircuitPackage (8)};

dACircuitCallEstablishmentMetricIncrementPackage 
PACKAGE
BEHAVIOUR DEFINITIONS 
dACircuitCallEstablishmentMetricIncrementPackag
e-B BEHAVIOUR
DEFINED AS Present when values of call establish
ment metric increment greater than zero are sup
ported and the parent iS MO has iSType Level2;;
ATTRIBUTES
callEstablishmentDefaultMetricIncrement
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.callEstablishmentMetricIncrement-
Default
PERMITTED VALUES 
ISO10589-ISIS.CallEstablishmentMetricIncrement-
Permitted
GET-REPLACE,
callEstablishmentDelayMetricIncrement
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.callEstablishmentMetricIncrement-
Default
PERMITTED VALUES 
ISO10589-ISIS.CallEstablishmentMetricIncrement-
Permitted
GET-REPLACE,
callEstablishmentExpenseMetricIncrement
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.callEstablishmentMetricIncrement-

Default
PERMITTED VALUES 
ISO10589-ISIS.CallEstablishmentMetricIncrement-
Permitted
GET-REPLACE,
callEstablishmentErrorMetricIncrement
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.callEstablishmentMetricIncrement-
Default
PERMITTED VALUES 
ISO10589-ISIS.CallEstablishmentMetricIncrement-
Permitted
GET-REPLACE;
REGISTERED AS {ISO10589-ISIS.poi 
dACircuitCallEstablishmentMetricIncrementPackag
e (9)};

level2ISO10589DACircuitPackage PACKAGE
BEHAVIOUR DEFINITIONS 
level2ISO10589DACircuitPackage-B 
BEHAVIOUR
DEFINED AS Present when the Circuit is of type DA, 
and the IS is operating as a L2 IS;;
-- Note: a DA Circuit is only permitted on an L2 IS
ATTRIBUTES
recallTimer
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.recallTimer-Default
PERMITTED VALUES 
ISO10589-ISIS.RecallTimer-Permitted
GET-REPLACE,
idleTimer
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.idleTimer-Default
PERMITTED VALUES 
ISO10589-ISIS.IdleTimer-Permitted
GET-REPLACE,
initialMinimumTimer
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.initialMinimumTimer-Default
PERMITTED VALUES 
ISO10589-ISIS.InitialMinimumTimer-Permitted
GET-REPLACE,
reserveTimer
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.reserveTimer-Default
PERMITTED VALUES 
ISO10589-ISIS.ReserveTimer-Permitted
GET-REPLACE,
maximumSVCAdjacencies
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.maximumSVCAdjacencies-Default
PERMITTED VALUES 
ISO10589-ISIS.MaximumSVCAdjacencies-Permitte
d
GET-REPLACE,
reservedAdjacency
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.reservedAdjacency-Default

GET-REPLACE,
-- Note: it is not clear that this attribute is required
callsPlaced GET,
callsFailed GET,
timesExceededMaximumSVCAdjacencies GET;
ATTRIBUTE GROUPS
counters
callsPlaced
callsFailed
timesExceededMaximumSVCAdjacencies;
NOTIFICATIONS
exceededMaximumSVCAdjacencies;
REGISTERED AS {ISO10589-ISIS.poi 
level2ISO10589DACircuitPackage (10)};

level1ISO10589StaticCircuitPackage PACKAGE
BEHAVIOUR DEFINITIONS 
level1StaticCircuitPackage-B BEHAVIOUR
DEFINED AS Present when the Circuit is of type 
Static;;
ATTRIBUTES
neighbourSNPAAddress
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.neighbourSNPAAddress-Default
GET-REPLACE,
-- Note: should this be handled by an X.25 IVMO?
ptPtCircuitID GET;
REGISTERED AS {ISO10589-ISIS.poi 
level1ISO10589StaticCircuitPackage (11)};

level1ISO10589StaticOutCircuitPackage PACKAGE
BEHAVIOUR DEFINITIONS 
level1StsticOutCircuitPackage-B BEHAVIOUR
DEFINED AS Present when the Circuit is of type Static 
Out;;
ATTRIBUTES
recallTimer
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.recallTimer-Default
PERMITTED VALUES 
ISO10589-ISIS.RecallTimer-Permitted
GET-REPLACE,
maximumCallAttempts
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.maximumCallAttempts-Default
PERMITTED VALUES 
ISO10589-ISIS.MaximumCallAttempts-Permitted
GET-REPLACE,
callsPlaced GET,
callsFailed GET,
timesExceededMaximumCallAttempts GET;
ATTRIBUTE GROUPS
counters
callsPlaced
callsFailed
timesExceededMaximumCallAttempts;
NOTIFICATIONS
exceededMaximumCallAttempts ;
REGISTERED AS {ISO10589-ISIS.poi 
level1ISO10589StaticOutCircuitPackage (12)};

level2ISO10589CircuitPackage PACKAGE
BEHAVIOUR DEFINITIONS level2CircuitPackage-B 
BEHAVIOUR
DEFINED AS Present when IS is an L2 IS;;
ATTRIBUTES
l2DefaultMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.defaultMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.DefaultMetric-Permitted
GET-REPLACE,
l2DelayMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.optionalMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.OptionalMetric-Permitted
GET-REPLACE,
l2ExpenseMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.optionalMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.OptionalMetric-Permitted
GET-REPLACE,
l2ErrorMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.optionalMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.OptionalMetric-Permitted
GET-REPLACE,
manualL2OnlyMode
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.manualL2OnlyMode-Default
GET-REPLACE;
REGISTERED AS {ISO10589-ISIS.poi 
level2ISO10589CircuitPackage (13)};

level2ISO10589BroadcastCircuitPackage PACKAGE
BEHAVIOUR DEFINITIONS 
level2BroadcastCircuitPackage-B BEHAVIOUR
DEFINED AS Present when the Circuit is of type 
Broadcast and the IS is an L2 IS;;
ATTRIBUTES
l2IntermediateSystemPriority
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.l2IntermediateSystemPriority-Defau
lt
PERMITTED VALUES 
ISO10589-ISIS.L2IntermediateSystemPriority-Perm
itted
GET-REPLACE,
l2CircuitID GET,
l2DesignatedIntermediateSystem GET,
lanL2DesignatedIntermediateSystemChanges GET;
ATTRIBUTE GROUPS
counters
lanL2DesignatedIntermediateSystemChanges;
NOTIFICATIONS
lanL2DesignatedIntermediateSystemChange;
REGISTERED AS {ISO10589-ISIS.poi 
level2ISO10589BroadcastCircuitPackage (14)};

circuitAuthenticationPackage PACKAGE
BEHAVIOUR DEFINITIONS 
circuitAuthenticationPackage-B BEHAVIOUR
DEFINED AS Present when the authentication proce
dures option is implemented;;
ATTRIBUTES
circuitTransmitPassword
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.password-Default
GET-REPLACE,
circuitReceivePasswords
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.passwords-Default
GET-REPLACE
ADD-REMOVE,
authenticationFailures GET;
ATTRIBUTE GROUPS
counters
authenticationFailures;
NOTIFICATIONS
authenticationFailure;
REGISTERED AS {ISO10589-ISIS.poi 
circuitAuthenticationPackage (15)};

11.2.4 The Adjacency managed Object
adjacency MANAGED OBJECT CLASS
DERIVED FROM "ISO/IEC 10165-2":top;
CHARACTERIZED BY adjacencyPackage PACKAGE
ATTRIBUTES
adjacencyName GET,
adjacencyState GET;
-- Note: this is NOT operational state
;;
CONDITIONAL PACKAGES
broadcastAdjacencyPackage
PRESENT IF the parent Circuit is of type broadcast,
dAAdjacencyPackage
PRESENT IF the parent Circuit is of type DA,
ptToPtAdjacencyPackage
PRESENT IF the parent Circuit is of type PtToPt or 
STATIC,
iSAdjacencyPackage
PRESENT IF the adjacency is to an IS (i.e the 
neighbourSystemType is Intermediate System L1 
Intermediate System or L2 Intermediate System),
broadcastISAdjacencyPackage
PRESENT IF the parent Circuit is of type broadcast 
and is to an IS as above,
eSAdjacencyPackage
PRESENT IF the adjacency is to an ES (i.e. the 
neighbourSystemType is EndSystem;
REGISTERED AS {ISO10589-ISIS.moi adjacency (3)};

broadcastAdjacencyPackage PACKAGE
BEHAVIOUR DEFINITIONS 
broadcastAdjacencyPackage-B BEHAVIOUR
DEFINED AS present if the parent Circuit is of type 
broadcast;;
ATTRIBUTES
neighbourLANAddress GET,
neighbourSystemType GET;
REGISTERED AS {ISO10589-ISIS.poi 
broadcastAdjacencyPackage (16)};

dAAdjacencyPackage PACKAGE
BEHAVIOUR DEFINITIONS dAAdjacencyPackage-B 
BEHAVIOUR
DEFINED AS present if the parent Circuit is of type 
DA;;
ATTRIBUTES
sNPAAddress GET;
REGISTERED AS {ISO10589-ISIS.poi 
dAAdjacencyPackage (17)};

ptToPtAdjacencyPackage PACKAGE
BEHAVIOUR DEFINITIONS 
ptToPtAdjacencyPackage-B BEHAVIOUR
DEFINED AS present if the parent Circuit is of type 
PtToPt;;
ATTRIBUTES
neighbourSystemType GET;
REGISTERED AS {ISO10589-ISIS.poi 
ptToPtAdjacencyPackage (18)};

iSAdjacencyPackage PACKAGE
BEHAVIOUR DEFINITIONS iSAdjacencyPackage-B 
BEHAVIOUR
DEFINED AS present if the adjacency is to an IS;;
ATTRIBUTES
adjacencyUsageType GET,
neighbourSystemID GET,
neighbourAreas GET,
holdingTimer GET;
REGISTERED AS {ISO10589-ISIS.poi 
iSAdjacencyPackage (19)};

broadcastISAdjacencyPackage PACKAGE
BEHAVIOUR DEFINITIONS 
broadcastISAdjacencyPackage-B BEHAVIOUR
DEFINED AS present if the parent Circuit is of type 
broadcast and the adjacency is to an IS;;
ATTRIBUTES
lANPriority GET;
REGISTERED AS {ISO10589-ISIS.poi 
broadcastISAdjacencyPackage (20)};

eSAdjacencyPackage PACKAGE
BEHAVIOUR DEFINITIONS eSAdjacencyPackage-B 
BEHAVIOUR
DEFINED AS present if the adjacency is to an ES;;
ATTRIBUTES
endSystemIDs GET;
REGISTERED AS {ISO10589-ISIS.poi 
eSAdjacencyPackage (21)};

11.2.5 The Manual Adjacency Managed 
Object
manualAdjacency MANAGED OBJECT CLASS
DERIVED FROM "ISO/IEC 10165-2":top;
CHARACTERIZED BY manualAdjacencyPackage 
PACKAGE
ATTRIBUTES
adjacencyName GET,
neighbourLANAddress GET,
endSystemIDs GET;
;;
REGISTERED AS {ISO10589-ISIS.moi 
manualAdjacency (4)};

11.2.6 The Virtual Adjacency managed Object
virtualAdjacency MANAGED OBJECT CLASS
DERIVED FROM "ISO/IEC 10165-2":top;
CHARACTERIZED BY virtualAdjacencyPackage 
PACKAGE
ATTRIBUTES
networkEntityTitle GET,
metric GET;
;;
REGISTERED AS {ISO10589-ISIS.moi virtualAdjacency 
(5)};

11.2.7 The Destination Managed Object
-- The destination MO class is never instantiated. It exists 
only to allow the destinationSystem and 
destinationArea MO classes to be derived from it.
destination MANAGED OBJECT CLASS
DERIVED FROM "ISO/IEC 10165-2":top;
CHARACTERIZED BY destinationPackage 
PACKAGE
ATTRIBUTES
defaultMetricPathCost GET,
defaultMetricOutputAdjacencies GET,
delayMetricPathCost GET,
delayMetricOutputAdjacencies GET,
expenseMetricPathCost GET,
expenseMetricOutputAdjacencies GET,
errorMetricPathCost GET,
errorMetricOutputAdjacencies GET;
;; -- no need for an object ID since it is never 
instantiated, but GDMO  needs one
REGISTERED AS {ISO10589-ISIS.moi destination (6)};

11.2.8 The Destination System Managed 
Object
destinationSystem MANAGED OBJECT CLASS
DERIVED FROM destination;
CHARACTERIZED BY destinationSystemPackage 
PACKAGE
ATTRIBUTES
networkEntityTitle GET;
;;
REGISTERED AS {ISO10589-ISIS.moi 
destinationSystem (7)};

11.2.9 The Destination Area Managed Object
destinationArea MANAGED OBJECT CLASS
DERIVED FROM destination;
CHARACTERIZED BY destinationAreaPackage 
PACKAGE
ATTRIBUTES
addressPrefix GET;
;;
REGISTERED AS {ISO10589-ISIS.moi destinationArea 
(8)};

11.2.10 The Reachable Address Managed 
Object
reachableAddress MANAGED OBJECT CLASS
DERIVED FROM "ISO/IEC 10165-2":top;
CHARACTERIZED BY reachableAddressPackage 
PACKAGE
ATTRIBUTES
addressPrefix GET,
defaultMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.defaultMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.DefaultMetric-Permitted
GET-REPLACE,
delayMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.optionalMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.OptionalMetric-Permitted
GET-REPLACE,
expenseMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.optionalMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.OptionalMetric-Permitted
GET-REPLACE,
errorMetric
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.optionalMetric-Default
PERMITTED VALUES 
ISO10589-ISIS.OptionalMetric-Permitted
GET-REPLACE,
defaultMetricType
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.metricType-Default
GET-REPLACE,
delayMetricType
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.metricType-Default
GET-REPLACE,
expenseMetricType
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.metricType-Default
GET-REPLACE,
errorMetricType

REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.metricType-Default
GET-REPLACE,
"ISO/IEC 10165-2":operationalState GET;
ACTIONS
activate,
deactivate;
;;
CONDITIONAL PACKAGES
mappingRAPackage
PRESENT IF the parent Circuit is of type broadcast 
or DA,
broadcastRAPackage
PRESENT IF the parent Circuit is of type broadcast 
and the value of mappingType is `manual',
dARAPackage
PRESENT IF the parent Circuit is of type DA and 
the value of mappingType is `manual';
REGISTERED AS {ISO10589-ISIS.moi 
reachableAddress (9)};

mappingRAPackage PACKAGE
BEHAVIOUR DEFINITIONS mappingRAPackage-B 
BEHAVIOUR
DEFINED AS When present, the NSAP to Circuit 
mapping is controlled by the value of the map
pingType attribute;;
ATTRIBUTES
mappingType GET;
REGISTERED AS {ISO10589-ISIS.poi 
mappingRAPackage (22)};

broadcastRAPackage PACKAGE
BEHAVIOUR DEFINITIONS broadcastRAPackage-B 
BEHAVIOUR
DEFINED AS When present, the remote SNPA address 
is determined by the value of the lANAddress attrib
ute;;
ATTRIBUTES
lANAddress
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.lANAddress-Default
GET-REPLACE;
REGISTERED AS {ISO10589-ISIS.poi 
broadcastRAPackage (23)};

dARAPackage PACKAGE
BEHAVIOUR DEFINITIONS dARAPackage-B 
BEHAVIOUR
DEFINED AS When present, the remote SNPA address 
is determined by the value of the sNPAAddresses at
tribute;;
ATTRIBUTES
sNPAAddresses
REPLACE-WITH-DEFAULT
DEFAULT VALUE 
ISO10589-ISIS.sNPAAddresses-Default
GET-REPLACE;
REGISTERED AS {ISO10589-ISIS.poi dARAPackage 
(24)};

11.2.11 Attribute Definitions
version ATTRIBUTE
WITH ATTRIBUTE SYNTAX ISO10589-ISIS.Version;
MATCHES FOR Equality, Ordering;
BEHAVIOUR version-B BEHAVIOUR
DEFINED AS The version number of this International 
Standard to which the implementation conforms;;
REGISTERED AS {ISO10589-ISIS.aoi version (1)};

iSType ATTRIBUTE
WITH ATTRIBUTE SYNTAX ISO10589-ISIS.ISType;
MATCHES FOR Equality;
BEHAVIOUR iSType-B BEHAVIOUR
DEFINED AS The type of this Intermediate System.  
The value of this attribute is only settable via the 
create parameter;;
REGISTERED AS {ISO10589-ISIS.aoi iSType (2)};

maximumPathSplits ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MaximumPathSplits;
MATCHES FOR Equality, Ordering;
BEHAVIOUR maximumPathSplits-B BEHAVIOUR
DEFINED AS Maximum number of paths with equal 
routeing metric value which it is permitted to split 
between;,
replaceOnlyWhileDisabled-B;
PARAMETERS constraintViolation;
REGISTERED AS {ISO10589-ISIS.aoi 
maximumPathSplits (3)};

maximumBuffers ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MaximumBuffers;
MATCHES FOR Equality, Ordering;
BEHAVIOUR maximumBuffers-B BEHAVIOUR
DEFINED AS Maximum guaranteed number of buffers 
for forwarding. This is the number of forwarding 
buffers that is to be reserved, more may be used if 
they are available. (See clause D.1.1);,
resourceLimiting-B;
PARAMETERS constraintViolation;
REGISTERED AS {ISO10589-ISIS.aoi maximumBuffers 
(4)};

minimumLSPTransmissionInterval ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MinimumLSPTransmissionInterval;
MATCHES FOR Equality, Ordering;
BEHAVIOUR minimumLSPTransmissionInterval-B 
BEHAVIOUR
DEFINED AS Minimum interval, in seconds, between 
re- transmissions of an LSP;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi 
minimumLSPTransmissionInterval (5)};

maximumLSPGenerationInterval ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MaximumLSPGenerationInterval;
MATCHES FOR Equality, Ordering;
BEHAVIOUR maximumLSPGenerationInterval-B 
BEHAVIOUR
DEFINED AS Maximum interval, in seconds, between 
generated LSPs by this system;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi 
maximumLSPGenerationInterval (6)};

minimumBroadcastLSPTransmissionInterval ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MinimumBroadcastLSPTransmissio
nInterval;
MATCHES FOR Equality, Ordering;
BEHAVIOUR 
minimumBroadcastLSPTransmissionInterval-B 
BEHAVIOUR
DEFINED AS Minimum interval, in milliseconds, be
tween transmission of LSPs on a broadcast circuit 
(See clause 7.3.15.6);,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi 
minimumBroadcastLSPTransmissionInterval (7)};

completeSNPInterval ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.CompleteSNPInterval;
MATCHES FOR Equality, Ordering;
BEHAVIOUR completeSNPInterval-B BEHAVIOUR
DEFINED AS Interval, in seconds, between generation 
of Complete Sequence Numbers PDUs by a Desig
nated Intermediate System on a broadcast circuit;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi 
completeSNPInterval (8)};

originatingL1LSPBufferSize ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.OriginatingLSPBufferSize;
MATCHES FOR Equality, Ordering;
BEHAVIOUR originatingL1LSPBufferSize-B 
BEHAVIOUR
DEFINED AS The maximum size of Level 1 LSPs and 
SNPs originated by this system;,
replaceOnlyWhileDisabled-B;
PARAMETERS constraintViolation;
REGISTERED AS {ISO10589-ISIS.aoi 
originatingL1LSPBufferSize (9)};

manualAreaAddresses ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.AreaAddresses;
MATCHES FOR Equality, Set Comparison, Set 
Intersection;
BEHAVIOUR manualAreaAddresses-B BEHAVIOUR
DEFINED AS Area Addresses to be used for this Inter
mediate System. At least one value must be sup
plied. The maximum number of Area Addresses 
which may exist in the set is MaximumAreaAd
dresses;;
REGISTERED AS {ISO10589-ISIS.aoi 
manualAreaAddresses (10)};

minimumLSPGenerationInterval ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MinimumLSPGenerationInterval;
MATCHES FOR Equality, Ordering;
BEHAVIOUR minimumLSPGenerationInterval-B 
BEHAVIOUR
DEFINED AS Maximum interval in seconds between 
successive generation of LSPs with the same LSPID 
by this IS;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi 
minimumLSPGenerationInterval (11)};

defaultESHelloTimer ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.DefaultESHelloTimer;
MATCHES FOR Equality, Ordering;
BEHAVIOUR defaultESHelloTimer-B BEHAVIOUR
DEFINED AS The value to be used for the suggested 
ES configuration timer in ISH PDUs when not solic
iting the ES configuration;;
REGISTERED AS {ISO10589-ISIS.aoi 
defaultESHelloTimer (12)};

pollESHelloRate ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.PollESHelloRate;
MATCHES FOR Equality, Ordering;
BEHAVIOUR pollESHelloRate-B BEHAVIOUR
DEFINED AS The value to be used for the suggested 
ES configuration timer in ISH PDUs when soliciting 
the ES configuration;;
REGISTERED AS {ISO10589-ISIS.aoi pollESHelloRate 
(13)};

partialSNPInterval ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.PartialSNPInterval;
MATCHES FOR Equality, Ordering;
BEHAVIOUR partialSNPInterval-B BEHAVIOUR
DEFINED AS Minimum interval between sending Par
tial Sequence Number PDUs;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi 
partialSNPInterval (14)};

waitingTime ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.WaitingTime;
MATCHES FOR Equality, Ordering;
BEHAVIOUR waitingTime-B BEHAVIOUR
DEFINED AS Number of seconds to delay in waiting 
state before entering On state;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi waitingTime 
(15)};

dRISISHelloTimer ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.DRISISHelloTimer;
MATCHES FOR Equality, Ordering;
BEHAVIOUR dRISISHelloTimer-B BEHAVIOUR
DEFINED AS The interval in seconds between the 
generation of IIH PDUs by the designated IS on a 
LAN;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi 
dRISISHelloTimer (16)};

l1State ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.DatabaseState;
MATCHES FOR Equality;
BEHAVIOUR l1State-B BEHAVIOUR
DEFINED AS The state of the Level 1 database;;
REGISTERED AS {ISO10589-ISIS.aoi l1State (17)};

areaAddresses ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.AreaAddresses;
MATCHES FOR Equality, Set Comparison, Set 
Intersection;
BEHAVIOUR areaAddresses-B BEHAVIOUR
DEFINED AS The union of the sets of manualAreaAd
dresses reported in all Level 1 Link State PDUs re
ceived by this Intermediate System;;
REGISTERED AS {ISO10589-ISIS.aoi areaAddresses 
(18)};

corruptedLSPsDetected ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR corruptedLSPsDetected-B BEHAVIOUR
DEFINED AS Number of Corrupted LSP Detected 
events generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
corruptedLSPsDetected (19)};

lSPL1DatabaseOverloads ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR lSPL1DatabaseOverloads-B 
BEHAVIOUR
DEFINED AS Number of times the LSP L1 Database 
Overload event has been generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
lSPL1DatabaseOverloads (20)};

manualAddressesDroppedFromArea ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR manualAddressesDroppedFromArea-B 
BEHAVIOUR
DEFINED AS Number of times the Manual Addresses 
Dropped From Area event has been generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
manualAddressesDroppedFromArea (21)};

attemptsToExceedMaximumSequenceNumber 
ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR 
attemptsToExceedMaximumSequenceNumber-B 
BEHAVIOUR
DEFINED AS Number of times the Attempt To Exceed 
Maximum Sequence Number event has been 
generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
attemptsToExceedMaximumSequenceNumber 
(22)};

sequenceNumberSkips ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR sequenceNumberSkips-B BEHAVIOUR
DEFINED AS Number of times the Sequence Number 
Skipped event has been generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
sequenceNumberSkips (23)};

ownLSPPurges ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR ownLSPPurges-B BEHAVIOUR
DEFINED AS Number of times the Own LSP Purged 
event has been generated;;
REGISTERED AS {ISO10589-ISIS.aoi ownLSPPurges 
(24)};

iDFieldLengthMismatches ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR iDFieldLengthMismatches-B 
BEHAVIOUR
DEFINED AS Number of times the iDFieldLengthMis
match event has been generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
iDFieldLengthMismatches (25)};

originatingL2LSPBufferSize ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.OriginatingLSPBufferSize;
MATCHES FOR Equality, Ordering;
BEHAVIOUR originatingL2LSPBufferSize-B 
BEHAVIOUR
DEFINED AS The maximum size of Level 2 LSPs and 
SNPs originated by this system;,
replaceOnlyWhileDisabled-B;
PARAMETERS constraintViolation;
REGISTERED AS {ISO10589-ISIS.aoi 
originatingL2LSPBufferSize (26)};

maximumVirtualAdjacencies ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MaximumVirtualAdjacencies;
MATCHES FOR Equality, Ordering;
BEHAVIOUR maximumVirtualAdjacencies-B 
BEHAVIOUR
DEFINED AS Maximum number of Virtual Adjacen
cies which may be created to repair partitioned 
Level 1 domains;;
REGISTERED AS {ISO10589-ISIS.aoi 
maximumVirtualAdjacencies (27)};

l2State ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.DatabaseState;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l2State-B BEHAVIOUR
DEFINED AS The state of the Level 2 database;;
REGISTERED AS {ISO10589-ISIS.aoi l2State (28)};

partitionAreaAddresses ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.AreaAddresses;
MATCHES FOR Equality, Set Comparison, Set 
Intersection;
BEHAVIOUR partitionAreaAddresses-B BEHAVIOUR
DEFINED AS The set union of all manualAreaAd
dresses of all Intermediate systems in the partition 
reachable by non-virtual links (calculated from their 
Level 1 LSPs);;
REGISTERED AS {ISO10589-ISIS.aoi 
partitionAreaAddresses (29)};

partitionDesignatedL2IntermediateSystem ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.SystemID;
MATCHES FOR Equality;
BEHAVIOUR 
partitionDesignatedL2IntermediateSystem-B 
BEHAVIOUR
DEFINED AS The ID of the Partition Designated 
Level 2 Intermediate System for this system;;
REGISTERED AS {ISO10589-ISIS.aoi 
partitionDesignatedL2IntermediateSystem (30)};

partitionVirtualLinkChanges ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR partitionVirtualLinkChanges-B 
BEHAVIOUR
DEFINED AS Number of times the Partition Virtual 
Link Change Notification has been generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
partitionVirtualLinkChanges (31)};

lSPL2DatabaseOverloads ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR lSPL2DatabaseOverloads-B 
BEHAVIOUR
DEFINED AS Number of times the LSP L2 Database 
Overload event has been generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
lSPL2DatabaseOverloads (32)};

type ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.CircuitType;
MATCHES FOR Equality;
BEHAVIOUR type-B BEHAVIOUR
DEFINED AS The type of the circuit. This attribute 
may only be set when the Circuit is created. Subse
quently it is read-only;;
REGISTERED AS {ISO10589-ISIS.aoi type (33)};

helloTimer ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HelloTimer;
MATCHES FOR Equality, Ordering;
BEHAVIOUR helloTimer-B BEHAVIOUR
DEFINED AS The period, in seconds, between ISH 
PDUs;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi helloTimer (34)};

l1DefaultMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l1defaultMetric-B BEHAVIOUR
DEFINED AS The default metric value of this circuit 
for Level 1 traffic. The value of zero is reserved to 
indicate that this metric is not supported;;
REGISTERED AS {ISO10589-ISIS.aoi l1DefaultMetric 
(35)};

l1DelayMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l1DelayMetric-B BEHAVIOUR
DEFINED AS The delay metric value of this circuit for 
Level 1 traffic. The value of zero is reserved to indi
cate that this metric is not supported;;
REGISTERED AS {ISO10589-ISIS.aoi l1DelayMetric 
(36)};

l1ExpenseMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l1ExpenseMetric-B BEHAVIOUR
DEFINED AS The expense metric value of this circuit 
for Level 1 traffic. The value of zero is reserved to 
indicate that this metric is not supported;;
REGISTERED AS {ISO10589-ISIS.aoi l1ExpenseMetric 
(37)};

l1ErrorMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l1ErrorMetric-B BEHAVIOUR
DEFINED AS The error metric value of this circuit for 
Level 1 traffic. The value of zero is reserved to indi
cate that this metric is not supported;;
REGISTERED AS {ISO10589-ISIS.aoi l1ErrorMetric 
(38)};

circuitChanges ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR circuitChanges-B BEHAVIOUR
DEFINED AS Number of times this Circuit state 
changed between On and Off and vice versa;;
REGISTERED AS {ISO10589-ISIS.aoi circuitChanges 
(39)};

changesInAdjacencyState ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR changesInAdjacencyState-B 
BEHAVIOUR
DEFINED AS Number of Adjacency State Change 
events generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
changesInAdjacencyState (40)};

initializationFailures ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR initializationFailures-B BEHAVIOUR
DEFINED AS Number of Initialization Failure events 
generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
initializationFailures (41)};

rejectedAdjacencies ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR rejectedAdjacencies-B BEHAVIOUR
DEFINED AS Number of Rejected Adjacency events 
generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
rejectedAdjacencies (42)};

controlPDUsSent ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR controlPDUsSent-B BEHAVIOUR
DEFINED AS Number of control PDUs sent on this 
circuit;;
REGISTERED AS {ISO10589-ISIS.aoi controlPDUsSent 
(43)};

controlPDUsReceived ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR controlPDUsReceived-B BEHAVIOUR
DEFINED AS Number of control PDUs received on 
this circuit;;
REGISTERED AS {ISO10589-ISIS.aoi 
controlPDUsReceived (44)};

iSISHelloTimer ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.ISISHelloTimer;
MATCHES FOR Equality, Ordering;
BEHAVIOUR iSISHelloTimer-B BEHAVIOUR
DEFINED AS The period, in seconds, between LAN 
Level 1 and Level 2 IIH PDUs.  It is also used as the 
period between ISH PDUs when polling the ES con
figuration;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi iSISHelloTimer 
(45)};

externalDomain ATTRIBUTE
WITH ATTRIBUTE SYNTAX ISO10589-ISIS.Boolean;
MATCHES FOR Equality;
BEHAVIOUR externalDomain-B BEHAVIOUR
DEFINED AS If TRUE, suppress notmal transmission 
of and interpretation of Intra-domain ISIS PDUs on 
this circuit.;;
REGISTERED AS {ISO10589-ISIS.aoi externalDomain 
(46)};

l1IntermediateSystemPriority ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.IntermediateSystemPriority;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l1IntermediateSystemPriority-B 
BEHAVIOUR
DEFINED AS Priority for becoming LAN Level 1 
Designated Intermediate System;;
REGISTERED AS {ISO10589-ISIS.aoi 
l1IntermediateSystemPriority (47)};

l1CircuitID ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.CircuitID;
MATCHES FOR Equality;
BEHAVIOUR l1CircuitID-B BEHAVIOUR
DEFINED AS The  LAN ID allocated by the LAN 
Level 1 Designated Intermediate System. Where this 
system is not aware of the value (because it is not 
participating in the Level 1 Designated Intermediate 
System election), this attribute has the value which 
would be proposed for this circuit. (i.e. the concate
nation of the local system ID and the one octet local 
Circuit ID for this circuit.;;
REGISTERED AS {ISO10589-ISIS.aoi l1CircuitID 
(48)};

l1DesignatedIntermediateSystem ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.SystemID;
MATCHES FOR Equality;
BEHAVIOUR l1DesignatedIntermediateSystem-B 
BEHAVIOUR
DEFINED AS The ID  of the LAN Level 1 Designated 
Intermediate System on this circuit. If, for any rea
son this system is not partaking in the relevant Des
ignated Intermediate System election process, then 
the value returned is zero;;
REGISTERED AS {ISO10589-ISIS.aoi 
l1DesignatedIntermediateSystem (49)};

lanL1DesignatedIntermediateSystemChanges 
ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR 
lanL1DesignatedIntermediateSystemChanges-B 
BEHAVIOUR
DEFINED AS Number of LAN L1 Designated Inter
mediate System Change events generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
lanL1DesignatedIntermediateSystemChanges (50)};

ptPtCircuitID ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.CircuitID;
MATCHES FOR Equality;
BEHAVIOUR ptPtCircuitID-B BEHAVIOUR
DEFINED AS The ID of the circuit allocated during 
initialization. If no value has been negotiated (either 
because the adjacency is to an End system, or 
because initialization has not yet successfully 
completed), this attribute has the value which would 
be proposed for this circuit. (i.e. the concatenation of 
the local system ID and the one octet local Circuit 
ID for this circuit.;;
REGISTERED AS {ISO10589-ISIS.aoi ptPtCircuitID 
(51)};

callEstablishmentDefaultMetricIncrement ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MetricIncrement;
MATCHES FOR Equality, Ordering;
BEHAVIOUR 
callEstablishmentDefaultMetricIncrement-B 
BEHAVIOUR
DEFINED AS Additional value to be reported for the 
default metric value of unestablished DA adjacen
cies;;
REGISTERED AS {ISO10589-ISIS.aoi 
callEstablishmentDefaultMetricIncrement (52)};

callEstablishmentDelayMetricIncrement ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MetricIncrement;
MATCHES FOR Equality, Ordering;
BEHAVIOUR 
callEstablishmentDelayMetricIncrement-B 
BEHAVIOUR
DEFINED AS Additional value to be reported for the 
delay metric value of unestablished DA adjacen
cies;;
REGISTERED AS {ISO10589-ISIS.aoi 
callEstablishmentDelayMetricIncrement (53)};

callEstablishmentExpenseMetricIncrement ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MetricIncrement;
MATCHES FOR Equality, Ordering;
BEHAVIOUR 
callEstablishmentExpenseMetricIncrement-B 
BEHAVIOUR
DEFINED AS Additional value to be reported for the 
Expense metric value of unestablished DA adjacen
cies;;
REGISTERED AS {ISO10589-ISIS.aoi 
callEstablishmentExpenseMetricIncrement (54)};

callEstablishmentErrorMetricIncrement ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MetricIncrement;
MATCHES FOR Equality, Ordering;
BEHAVIOUR callEstablishmentErrorMetricIncrement-B 
BEHAVIOUR
DEFINED AS Additional value to be reported for the 
Error metric value of unestablished DA adjacencies;;
REGISTERED AS {ISO10589-ISIS.aoi 
callEstablishmentErrorMetricIncrement (55)};

recallTimer ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.RecallTimer;
MATCHES FOR Equality, Ordering;
BEHAVIOUR recallTimer-B BEHAVIOUR
DEFINED AS Number of seconds that must elapse be
tween a call failure on a DED circuit and a recall;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi recallTimer 
(56)};

idleTimer ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.IdleTimer;
MATCHES FOR Equality, Ordering;
BEHAVIOUR idleTimer-B BEHAVIOUR
DEFINED AS Number of seconds of idle time before 
call is cleared;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi idleTimer (57)};

initialMinimumTimer ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.InitialMinimumTimer;
MATCHES FOR Equality, Ordering;
BEHAVIOUR initialMinimumTimer-B BEHAVIOUR
DEFINED AS Number of seconds that a call remains 
connected after being established, irrespective of 
traffic. (Note. This should be set small enough so 
that the call is cleared before the start of the next 
charging interval.);,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi 
initialMinimumTimer (58)};

reserveTimer ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.ReserveTimer;
MATCHES FOR Equality, Ordering;
BEHAVIOUR reserveTimer-B BEHAVIOUR
DEFINED AS Number of seconds, after call is cleared 
due to lack of traffic, during which the SVC remains 
reserved for the previous SNPA address;,
resettingTimer-B;
REGISTERED AS {ISO10589-ISIS.aoi reserveTimer 
(59)};

maximumSVCAdjacencies ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MaximumSVCAdjacencies;
MATCHES FOR Equality, Ordering;
BEHAVIOUR maximumSVCAdjacencies-B 
BEHAVIOUR
DEFINED AS Number of Adjacencies to reserve for 
SVCs for this circuit. This is the maximum number 
of simultaneous calls which are possible on this cir
cuit;,
resourceLimiting-B;
PARAMETERS constraintViolation;
REGISTERED AS {ISO10589-ISIS.aoi 
maximumSVCAdjacencies (60)};

reservedAdjacency ATTRIBUTE
WITH ATTRIBUTE SYNTAX ISO10589-ISIS.Boolean;
MATCHES FOR Equality;
BEHAVIOUR reservedAdjacency-B BEHAVIOUR
DEFINED AS When True, indicates that one SVC 
must be reserved for a connection to an Intermediate 
System;,
replaceOnlyWhileDisabled-B;
PARAMETERS constraintViolation;
REGISTERED AS {ISO10589-ISIS.aoi 
reservedAdjacency (61)};

callsPlaced ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR callsPlaced-B BEHAVIOUR
DEFINED AS Number of Call attempts (successful or 
unsuccessful);;
REGISTERED AS {ISO10589-ISIS.aoi callsPlaced (62)};

callsFailed ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR callsFailed-B BEHAVIOUR
DEFINED AS Number of Unsuccessful Call attempts;;
REGISTERED AS {ISO10589-ISIS.aoi callsFailed (63)};

timesExceededMaximumSVCAdjacencies ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR 
timesExceededMaximumSVCAdjacencies-B 
BEHAVIOUR
DEFINED AS Number of Exceeded Maximum SVC 
Adjacencies events generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
timesExceededMaximumSVCAdjacencies (64)};

neighbourSNPAAddress ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.SNPAAddress;
MATCHES FOR Equality;
BEHAVIOUR neighbourSNPAAddress-B 
BEHAVIOUR
DEFINED AS SNPA Address to call, or SNPA Ad
dress from which to accept call;;
REGISTERED AS {ISO10589-ISIS.aoi 
neighbourSNPAAddress (65)};

maximumCallAttempts ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MaximumCallAttempts;
MATCHES FOR Equality, Ordering;
BEHAVIOUR maximumCallAttempts-B BEHAVIOUR
DEFINED AS Maximum number of successive call 
failures before halting. (A value of zero means infi
nite retries.;;
REGISTERED AS {ISO10589-ISIS.aoi 
maximumCallAttempts (66)};

timesExceededMaximumCallAttempts ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR timesExceededMaximumCallAttempts-B 

BEHAVIOUR
DEFINED AS Number of Exceeded Maximum Call 
Attempts events generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
timesExceededMaximumCallAttempts (67)};

l2DefaultMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l2defaultMetric-B BEHAVIOUR
DEFINED AS The default metric value of this circuit 
for Level 2 traffic. The value of zero is reserved to 
indicate that this metric is not supported;;
REGISTERED AS {ISO10589-ISIS.aoi l2DefaultMetric 
(68)};

l2DelayMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l2DelayMetric-B BEHAVIOUR
DEFINED AS The delay metric value of this circuit for 
Level 2 traffic. The value of zero is reserved to indi
cate that this metric is not supported;;
REGISTERED AS {ISO10589-ISIS.aoi l2DelayMetric 
(69)};

l2ExpenseMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l2ExpenseMetric-B BEHAVIOUR
DEFINED AS The expense metric value of this circuit 
for Level 2 traffic. The value of zero is reserved to 
indicate that this metric is not supported;;
REGISTERED AS {ISO10589-ISIS.aoi l2ExpenseMetric 
(70)};

l2ErrorMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l2ErrorMetric-B BEHAVIOUR
DEFINED AS The error metric value of this circuit for 
Level 2 traffic. The value of zero is reserved to indi
cate that this metric is not supported;;
REGISTERED AS {ISO10589-ISIS.aoi l2ErrorMetric 
(71)};

manualL2OnlyMode ATTRIBUTE
WITH ATTRIBUTE SYNTAX ISO10589-ISIS.Boolean;
MATCHES FOR Equality;
BEHAVIOUR manualL2OnlyMode-B BEHAVIOUR
DEFINED AS When True, indicates that this Circuit is 
to be used only for Level 2;,
replaceOnlyWhileDisabled-B;
PARAMETERS constraintViolation;
REGISTERED AS {ISO10589-ISIS.aoi 
manualL2OnlyMode (72)};

l2IntermediateSystemPriority ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.IntermediateSystemPriority;
MATCHES FOR Equality, Ordering;
BEHAVIOUR l2IntermediateSystemPriority-B 
BEHAVIOUR
DEFINED AS Priority for becoming LAN Level 2 
Designated Intermediate System;;
REGISTERED AS {ISO10589-ISIS.aoi 
l2IntermediateSystemPriority (73)};

l2CircuitID ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.CircuitID;
MATCHES FOR Equality;
BEHAVIOUR l2CircuitID-B BEHAVIOUR
DEFINED AS The LAN ID allocated by the LAN 
Level 2 Designated Intermediate System. Where this 
system is not aware of the value (because it is not 
participating in the Level 2 Designated Intermediate 
System election), this attribute has the value which 
would be proposed for this circuit. (i.e. the concate
nation of the local system ID and the one octet local 
Circuit ID for this circuit.;;
REGISTERED AS {ISO10589-ISIS.aoi l2CircuitID 
(74)};

l2DesignatedIntermediateSystem ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.SystemID;
MATCHES FOR Equality;
BEHAVIOUR l2DesignatedIntermediateSystem-B 
BEHAVIOUR
DEFINED AS The ID  of the LAN Level 2 Designated 
Intermediate System on this circuit. If, for any rea
son this system is not partaking in the relevant Des
ignated Intermediate System election process, then 
the value returned is ;;
REGISTERED AS {ISO10589-ISIS.aoi 
l2DesignatedIntermediateSystem (75)};

lanL2DesignatedIntermediateSystemChanges 
ATTRIBUTE
DERIVED FROM nonWrappingCounter;
BEHAVIOUR 
lanL2DesignatedIntermediateSystemChanges-B 
BEHAVIOUR
DEFINED AS Number of LAN L2 Designated Inter
mediate System Change events generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
lanL2DesignatedIntermediateSystemChanges (76)};

adjacencyName ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.GraphicString;
MATCHES FOR Equality, Substrings;

BEHAVIOUR adjacencyName-B BEHAVIOUR
DEFINED AS A string which is the Identifier for the 
Adjacency and which is unique amongst the set of 
Adjacencies maintained for this Circuit. If this is a 
manually created adjacency (i.e. the type is Manual) 
it is set by the System Manager when the Adjacency 
is created, otherwise it is generated by the imple
mentation such that it is unique. The set of identifier 
containing the leading string "Auto" are reserved for 
Automatic Adjacencies. An attempt to create a Man
ual Adjacency with such an identifier will cause an 
exception to be raised;;
REGISTERED AS {ISO10589-ISIS.aoi adjacencyName 
(77)};

adjacencyState ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.AdjacencyState;
MATCHES FOR Equality;
BEHAVIOUR adjacencyState-B BEHAVIOUR
DEFINED AS The state of the adjacency;;
REGISTERED AS {ISO10589-ISIS.aoi adjacencyState 
(78)};

neighbourLANAddress ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.LANAddress;
MATCHES FOR Equality;
BEHAVIOUR neighbourLANAddress-B BEHAVIOUR
DEFINED AS The MAC address of the neighbour sys
tem on a broadcast circuit;,
replaceOnlyWhileDisabled-B;
PARAMETERS constraintViolation;
REGISTERED AS {ISO10589-ISIS.aoi 
neighbourLANAddress (79)};

neighbourSystemType ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.NeighbourSystemType;
MATCHES FOR Equality;
BEHAVIOUR neighbourSystemType-B BEHAVIOUR
DEFINED AS The type of the neighbour system  one 
of: Unknown End system Intermediate system L1 
Intermediate system L2 Intermediate system;;
REGISTERED AS {ISO10589-ISIS.aoi 
neighbourSystemType (80)};

sNPAAddress ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.SNPAAddress;
MATCHES FOR Equality;
BEHAVIOUR sNPAAddress-B BEHAVIOUR
DEFINED AS The SNPA Address of the neighbour 
system on an X.25 circuit;,
replaceOnlyWhileDisabled-B;
PARAMETERS constraintViolation;
REGISTERED AS {ISO10589-ISIS.aoi sNPAAddress 
(81)};

adjacencyUsageType ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.AdjacencyUsageType;
MATCHES FOR Equality;
BEHAVIOUR level-B BEHAVIOUR
DEFINED AS The usage of the Adjacency.  An 
Adjacency of type Level 1" will be used for Level 1 
traffic only.  An adjacency of type Level 2" will be 
used for Level 2 traffic only. An adjacency of type 
Level 1 and 2" will be used for both Level 1 and 
Level 2 traffic. There may be two adjacencies (of 
types Level 1" and Level 2" between the same pair 
of Intermediate Systems.;;
REGISTERED AS {ISO10589-ISIS.aoi 
adjacencyUsageType (82)};

neighbourSystemID ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.SystemID;
MATCHES FOR Equality;
BEHAVIOUR neighbourSystemID-B BEHAVIOUR
DEFINED AS The SystemID of the neighbouring In
termediate system from the Source ID field of the 
neighbour's IIH PDU. The Intermediate System ID 
for this neighbour is derived by appending zero to 
this value.;;
REGISTERED AS {ISO10589-ISIS.aoi 
neighbourSystemID (83)};

neighbourAreas ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.AreaAddresses;
MATCHES FOR Equality, Set Comparison, Set 
Intersection;
BEHAVIOUR neighbourAreas-B BEHAVIOUR
DEFINED AS This contains the Area Addresses of a 
neighbour Intermediate System from the IIH PDU.;;
REGISTERED AS {ISO10589-ISIS.aoi neighbourAreas 
(84)};

holdingTimer ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HoldingTimer;
MATCHES FOR Equality, Ordering;
BEHAVIOUR holdingTimer-B BEHAVIOUR
DEFINED AS Holding time for this adjacency updated 
from the IIH PDUs;;
REGISTERED AS {ISO10589-ISIS.aoi holdingTimer 
(85)};

lANPriority ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.IntermediateSystemPriority;
MATCHES FOR Equality, Ordering;
BEHAVIOUR lANPriority-B BEHAVIOUR
DEFINED AS Priority of neighbour on this adjacency 
for becoming LAN Level 1 Designated Intermediate 
System if adjacencyType is L1 Intermediate System 
or LAN Level 2 Designated Intermediate System if 
adjacencyType is L2 Intermediate System;;
REGISTERED AS {ISO10589-ISIS.aoi lANPriority 
(86)};

endSystemIDs ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.EndSystemIDs;
MATCHES FOR Equality, Set Comparison, Set 
Intersection;
BEHAVIOUR endSystemIDs-B BEHAVIOUR
DEFINED AS This contains the system ID(s) of a 
neighbour End system. Where (in a Intermediate 
System) an adjacency has been created manually, 
these will be the set of IDs given in the manualIDs 
parameter of the create directive.;;
REGISTERED AS {ISO10589-ISIS.aoi endSystemIDs 
(87)};

networkEntityTitle ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.NetworkEntityTitle;
MATCHES FOR Equality, Ordering;
BEHAVIOUR networkEntityTitle-B BEHAVIOUR
DEFINED AS The Network entity Title which is the 
destination of a Virtual link being used to repair a 
partitioned Level 1 area (see clause 7.2.10);;
REGISTERED AS {ISO10589-ISIS.aoi 
networkEntityTitle (88)};

metric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.PathMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR metric-B BEHAVIOUR
DEFINED AS Cost of least cost L2 path(s) to destina
tion area based on the default metric;;
REGISTERED AS {ISO10589-ISIS.aoi metric (89)};

defaultMetricPathCost ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.PathMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR defaultMetricPathCost-B BEHAVIOUR
DEFINED AS Cost of least cost path(s) using the de
fault metric to destination;;
REGISTERED AS {ISO10589-ISIS.aoi 
defaultMetricPathCost (90)};

defaultMetricOutputAdjacencies ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.OutputAdjacencies;
MATCHES FOR Equality, Set Comparison, Set 
Intersection;
BEHAVIOUR defaultMetricOutputAdjacencies-B 
BEHAVIOUR
DEFINED AS The set of Adjacency (or Reachable Ad
dress) managed object identifiers representing the 
forwarding decisions based upon the default metric 
for the destination;;
REGISTERED AS {ISO10589-ISIS.aoi 
defaultMetricOutputAdjacencies (91)};

delayMetricPathCost ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.PathMetric;
MATCHES FOR Equality, Ordering;

BEHAVIOUR delayMetricPathCost-B BEHAVIOUR
DEFINED AS Cost of least cost path(s) using the delay 
metric to destination;;
REGISTERED AS {ISO10589-ISIS.aoi 
delayMetricPathCost (92)};

delayMetricOutputAdjacencies ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.OutputAdjacencies;
MATCHES FOR Equality, Set Comparison, Set 
Intersection;
BEHAVIOUR delayMetricOutputAdjacencies-B 
BEHAVIOUR
DEFINED AS The set of Adjacency (or Reachable Ad
dress) managed object identifiers representing the 
forwarding decisions based upon the delay metric 
for the destination;;
REGISTERED AS {ISO10589-ISIS.aoi 
delayMetricOutputAdjacencies (93)};

expenseMetricPathCost ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.PathMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR expenseMetricPathCost-B BEHAVIOUR
DEFINED AS Cost of least cost path(s) using the ex
pense metric to destination;;
REGISTERED AS {ISO10589-ISIS.aoi 
expenseMetricPathCost (94)};

expenseMetricOutputAdjacencies ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.OutputAdjacencies;
MATCHES FOR Equality, Set Comparison, Set 
Intersection;
BEHAVIOUR expenseMetricOutputAdjacencies-B 
BEHAVIOUR
DEFINED AS The set of Adjacency (or Reachable Ad
dress) managed object identifiers representing the 
forwarding decisions based upon the expense metric 
for the destination;;
REGISTERED AS {ISO10589-ISIS.aoi 
expenseMetricOutputAdjacencies (95)};

errorMetricPathCost ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.PathMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR errorMetricPathCost-B BEHAVIOUR
DEFINED AS Cost of least cost path(s) using the error 
metric to destination;;
REGISTERED AS {ISO10589-ISIS.aoi 
errorMetricPathCost (96)};

errorMetricOutputAdjacencies ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.OutputAdjacencies;
MATCHES FOR Equality, Set Comparison, Set 
Intersection;
BEHAVIOUR errorMetricOutputAdjacencies-B 

BEHAVIOUR
DEFINED AS The set of Adjacency (or Reachable Ad
dress) managed object identifiers representing the 
forwarding decisions based upon the error metric for 
the destination;;
REGISTERED AS {ISO10589-ISIS.aoi 
errorMetricOutputAdjacencies (97)};

addressPrefix ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.AddressPrefix;
MATCHES FOR Equality, Substrings;
BEHAVIOUR addressPrefix-B BEHAVIOUR
DEFINED AS An Area Address (or prefix) of a desti
nation area;;
REGISTERED AS {ISO10589-ISIS.aoi addressPrefix 
(98)};

defaultMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR defaultMetric-B BEHAVIOUR
DEFINED AS The default metric value for reaching 
the specified prefix over this Circuit. If this attribute 
is changed while both the Reachable Address and 
the Circuit are Enabled (i.e. state On), the actions 
described in clause 8.3.5.4 must be taken. The value 
of zero is reserved to indicate that this metric is not 
supported;;
REGISTERED AS {ISO10589-ISIS.aoi defaultMetric 
(99)};

delayMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR delayMetric-B BEHAVIOUR
DEFINED AS The delay metric value for reaching the 
specified prefix over this Circuit.BEHAVIOURIf 
this attribute is changed while both the Reachable 
Address and the Circuit are Enabled (i.e. state On), 
the actions described in clause 8.3.5.4 must be taken. 
The value of zero is reserved to indicate that this 
metric is not supported;;
REGISTERED AS {ISO10589-ISIS.aoi delayMetric 
(100)};

expenseMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR expenseMetric-B BEHAVIOUR
DEFINED AS The expense metric value for reaching 
the specified prefix over this Circuit. If this attribute 
is changed while both the Reachable Address and 
the Circuit are Enabled (i.e. state On), the actions 
described in clause 8.3.5.4 must be taken. The value 
of zero is reserved to indicate that this metric is not 
supported;;
REGISTERED AS {ISO10589-ISIS.aoi expenseMetric 
(101)};

errorMetric ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.HopMetric;
MATCHES FOR Equality, Ordering;
BEHAVIOUR errorMetric-B BEHAVIOUR
DEFINED AS The error metric value for reaching the 
specified prefix over this Circuit. If this attribute is 
changed while both the Reachable Address and the 
Circuit are Enabled (i.e. state On), the actions de
scribed in clause 8.3.5.4 must be taken. The value of 
zero is reserved to indicate that this metric is not 
supported;;
REGISTERED AS {ISO10589-ISIS.aoi errorMetric 
(102)};

defaultMetricType ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MetricType;
MATCHES FOR Equality;
BEHAVIOUR defaultMetricType-B BEHAVIOUR
DEFINED AS Indicates whether the default metric is 
internal or external;;
REGISTERED AS {ISO10589-ISIS.aoi 
defaultMetricType (103)};

delayMetricType ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MetricType;
MATCHES FOR Equality;
BEHAVIOUR delayMetricType-B BEHAVIOUR
DEFINED AS Indicates whether the delay metric is in
ternal or external;;
REGISTERED AS {ISO10589-ISIS.aoi delayMetricType 
(104)};

expenseMetricType ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MetricType;
MATCHES FOR Equality;
BEHAVIOUR expenseMetricType-B BEHAVIOUR
DEFINED AS Indicates whether the expense metric is 
internal or external;;
REGISTERED AS {ISO10589-ISIS.aoi 
expenseMetricType (105)};

errorMetricType ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MetricType;
MATCHES FOR Equality;
BEHAVIOUR errorMetricType-B BEHAVIOUR
DEFINED AS Indicates whether the error metric is in
ternal or extternal;;
REGISTERED AS {ISO10589-ISIS.aoi errorMetricType 
(106)};

mappingType ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.MappingType;
MATCHES FOR Equality;

BEHAVIOUR mappingType-B BEHAVIOUR
DEFINED AS The type of mapping to be employed to 
ascertain the SNPA Address to which a call should 
be placed for this prefix. X.121 indicates that the 
X.121 address extraction algorithm is to be em
ployed. This will extract the SNPA address from the 
IDI of an X.121 format IDP of the NSAP address to 
which the NPDU is to be forwarded. Manual indi
cates that the set of addresses in the sNPAAddresses 
or LANAddresses characteristic are to be used. For 
Broadcast circuits, only the value Manual is permit
ted;;
REGISTERED AS {ISO10589-ISIS.aoi mappingType 
(107)};

lANAddress ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.LANAddress;
MATCHES FOR Equality;
BEHAVIOUR lANAddress-B BEHAVIOUR
DEFINED AS Asingle LAN addresses to which an 
NPDU may be directed in order to reach an address 
which matches the address prefix of the Reachable 
Address. An exception is raised if an attempt is 
made to enable the Reachable Address with the de
fault value;;
REGISTERED AS {ISO10589-ISIS.aoi lANAddress 
(108)};

sNPAAddresses ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.SNPAAddresses;
MATCHES FOR Equality;
BEHAVIOUR sNPAAddresses-B BEHAVIOUR
DEFINED AS A set of SNPA addresses to which a call 
may be directed in order to reach an address which 
matches the address prefix of the Reachable Ad
dress. Associated with each SNPA Address, but not 
visible to System Management, is a variable lastFail
ure of Type BinaryAbsoluteTime;;
REGISTERED AS {ISO10589-ISIS.aoi sNPAAddresses 
(109)};

nonWrappingCounter ATTRIBUTE
WITH ATTRIBUTE SYNTAX  
ISO10589-ISIS.NonWrappingCounter;
MATCHES FOR Equality, Ordering;
BEHAVIOUR nonWrappingCounter-B BEHAVIOUR
DEFINED AS Non-replaceable, non-wrapping 
counter;;
-- This attibute is only defined in order to allow other 
counter attributes to be derived from it.
REGISTERED AS {ISO10589-ISIS.aoi 
nonWrappingCounter (110)};

areaTransmitPassword ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.Password;
MATCHES FOR Equality;
BEHAVIOUR areaTransmitPassword-B BEHAVIOUR
DEFINED AS The value to be used as a transmit pass
word in Level 1 LSP, and SNP PDUs transmitted by 
this Intermediate System;;
REGISTERED AS {ISO10589-ISIS.aoi 
areaTransmitPassword (111)};

areaReceivePasswords ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.Passwords;
MATCHES FOR Equality;
BEHAVIOUR areaReceivePasswords-B BEHAVIOUR
DEFINED AS The values to be used as receive pass
words to check the receipt of Level 1 LSP, and SNP 
PDUs;;
REGISTERED AS {ISO10589-ISIS.aoi 
areaReceivePasswords (112)};

domainTransmitPassword ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.Password;
MATCHES FOR Equality;
BEHAVIOUR domainTransmitPassword-B 
BEHAVIOUR
DEFINED AS The value to be used as a transmit pass
word in Level 2 LSP, and SNP PDUs transmitted by 
this Intermediate System;;
REGISTERED AS {ISO10589-ISIS.aoi 
domainTransmitPassword (113)};

domainReceivePasswords ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.Passwords;
MATCHES FOR Equality;
BEHAVIOUR domainReceivePasswords-B 
BEHAVIOUR
DEFINED AS The values to be used as receive pass
words to check the receipt of Level 2 LSP, and SNP 
PDUs;;
REGISTERED AS {ISO10589-ISIS.aoi 
domainReceivePasswords (114)};

circuitTransmitPassword ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.Password;
MATCHES FOR Equality;
BEHAVIOUR circuitTransmitPassword-B 
BEHAVIOUR
DEFINED AS The value to be used as a transmit pass
word in IIH PDUs transmitted by this Intermediate 
System;;
REGISTERED AS {ISO10589-ISIS.aoi 
circuitTransmitPassword (115)};

circuitReceivePasswords ATTRIBUTE
WITH ATTRIBUTE SYNTAX 
ISO10589-ISIS.Passwords;
MATCHES FOR Equality;
BEHAVIOUR circuitReceivePasswords-B 
BEHAVIOUR
DEFINED AS The values to be used as receive pass
words to check the receipt of IIH PDUs;;
REGISTERED AS {ISO10589-ISIS.aoi 
circuitReceivePasswords (116)};

authenticationFailures ATTRIBUTE
DERIVED FROM nonWrappingCounter;

BEHAVIOUR authenticationFailures-B BEHAVIOUR
DEFINED AS Count of authentication Failure notifica
tions generated;;
REGISTERED AS {ISO10589-ISIS.aoi 
authenticationFailures (117)};

11.2.12 Notification Definitions
-- Note pduFormatError notification now included in 
Network layer definitions
corruptedLSPDetected NOTIFICATION
BEHAVIOUR corruptedLSPDetected-B BEHAVIOUR
DEFINED AS The Corrupted LSP Detected Notifica
tion is generated when a corrupted Link State PDU 
is detected in memory.  The occurance of this event 
is counted by the corruptedLSPsDetected counter.;;
MODE NON-CONFIRMED;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
corruptedLSPDetected (1)};

lSPL1DatabaseOverload NOTIFICATION
BEHAVIOUR lSPL1DatabaseOverload-B 
BEHAVIOUR
DEFINED AS The LSP L1 Database Overload Notifi
cation is generated when the l1State of the system 
changes between On and Waiting or Waiting and 
On. The stateChange argument is set to indicate the 
resulting state, and in the case of Waiting the sour
ceID is set to indicate the source of the LSP which 
precipitated the overload.  The occurance of this 
event is counted by the lSPL1DatabaseOverloads 
counter.;;
MODE NON-CONFIRMED;
PARAMETERS
notificationOverloadStateChange,
notificationSourceID;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
lSPL1DatabaseOverload (2)};

manualAddressDroppedFromArea NOTIFICATION
BEHAVIOUR manualAddressDroppedFromArea-B 
BEHAVIOUR
DEFINED AS The Manual Address Dropped From 
Area Notification is generated when one of the man
ualAreaAddresses (specified on this system) is ig
nored when computing partitionAreaAddresses or 
areaAddresses because there are more than Maximu
mAreaAddresses distinct Area Addresses. The 
areaAddress argument is set to the ignored Area Ad
dress. It is generated once for each Area Address in 
manualAreaAddresses which is dropped. It is not 
logged again for that Area Address until after it has 
been reinstated into areaAddresses (i.e. it is only the 
action of dropping the Area Address and not the 
state of being dropped, which causes the event to be 
generated). The occurance of this event is counted 
by the manualAddressDroppedFromAreas counter.;;
MODE NON-CONFIRMED;
PARAMETERS
notificationAreaAddress;
WITH INFORMATION SYNTAX 

ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
manualAddressDroppedFromArea (3)};

attemptToExceedMaximumSequenceNumber 
NOTIFICATION
BEHAVIOUR 
attemptToExceedMaximumSequenceNumber-B 
BEHAVIOUR
DEFINED AS The Attempt To Exceed Maximum Se
quence Number Notification is generated when an 
attempt is made to increment the sequence number 
of an LSP beyond the maximum sequence number. 
Following the generation of this event the operation 
of the Routeing state machine shall be disabled for at 
least (MaxAge + ZeroAgeLifetime) seconds.  The 
occurance of this event is counted by the 
attemptsToExceedMaximumSequenceNumber 
counter.;;
MODE NON-CONFIRMED;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
attemptToExceedMaximumSequenceNumber (4)};

sequenceNumberSkip NOTIFICATION
BEHAVIOUR sequenceNumberSkip-B BEHAVIOUR
DEFINED AS The Sequence Number Skipped Notifi
cation is generated when the sequence number of an 
LSP is incremented by more than one.  The occur
ance of this event is counted by the sequenceNum
berSkips counter.;;
MODE NON-CONFIRMED;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
sequenceNumberSkip (5)};

ownLSPPurge NOTIFICATION
BEHAVIOUR ownLSPPurge-B BEHAVIOUR
DEFINED AS The Own LSP Purged Notification is 
generated when a zero aged copy of a system's own 
LSP is received from some other system. This repre
sents an erroneous attempt to purge the local sys
tem's LSP.  The occurance of this event is counted 
by the ownLSPPurges counter.;;
MODE NON-CONFIRMED;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi ownLSPPurge 
(6)};

partitionVirtualLinkChange NOTIFICATION
BEHAVIOUR partitionVirtualLinkChange-B 
BEHAVIOUR
DEFINED AS The Partition Virtual Link Change Noti
fication is generated when a virtual link (for the pur
poses of Level 1 partition repair) is either created or 
deleted. The relative order of events relating to the 
same Virtual Link must be preserved.  The occur
ance of this event is counted by the partitionVirtual
LinkChanges counter.;;
MODE NON-CONFIRMED;
PARAMETERS
notificationVirtualLinkChange,

notificationVirtualLinkAddress;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
partitionVirtualLinkChange (7)};

lSPL2DatabaseOverload NOTIFICATION
BEHAVIOUR lSPL2DatabaseOverload-B 
BEHAVIOUR
DEFINED AS The LSP L2 Database Overload Notifi
cation is generated when the l2State of the system 
changes between On and Waiting or Waiting and 
On. The stateChange argument is set to indicate the 
resulting state, and in the case of Waiting the sour
ceID is set to indicate the source of the LSP which 
precipitated the overload.  The occurance of this 
event is counted by the lSPL2DatabaseOverloads 
counter.;;
MODE NON-CONFIRMED;
PARAMETERS
notificationOverloadStateChange,
notificationSourceID;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
lSPL2DatabaseOverload (8)};

iDFieldLengthMismatch NOTIFICATION
BEHAVIOUR iDFieldLengthMismatch-B 
BEHAVIOUR
DEFINED AS The iDFieldLengthMismatch Notifica
tion is generated when a PDU is received with a dif
ferent value for ID field length to that of the 
receiving Intermediate system. The occurance of this 
event is counted by the iDFieldLengthMismatches 
counter.;;
MODE NON-CONFIRMED;
PARAMETERS
notificationIDLength,
notificationSourceID;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
iDFieldLengthMismatch (9)};

circuitChange NOTIFICATION
BEHAVIOUR circuitChange-B BEHAVIOUR
DEFINED AS The Circuit Change Notification is gen
erated when the state of the Circuit changes from On 
to Off or from Off to On. The relative order of 
events relating to the same Circuit must be pre
served.  The occurance of this event is counted by 
the circuitChanges counter.;;
MODE NON-CONFIRMED;
PARAMETERS
notificationNewCircuitState;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi circuitChange 
(10)};

adjacencyStateChange NOTIFICATION
BEHAVIOUR adjacencyStateChange-B BEHAVIOUR
DEFINED AS The Adjacency State Change Notifica
tion is generated when the state of an Adjacency on 
the Circuit changes from Up to Down or Down to 
Up (in the latter case the Reason argument is omit
ted). For these purposes the states Up and 
Up/dormant are considered to be Up, and any other 
state is considered to be Down. The relative order of 
events relating to the same Adjacency must be pre
served.  The occurance of this event is counted by 
the adjacencyStateChanges counter.;;
MODE NON-CONFIRMED;
PARAMETERS
notificationAdjacentSystem,
notificationNewAdjacencyState,
notificationReason,
notificationPDUHeader,
notificationCalledAddress,
notificationVersion;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
adjacencyStateChange (11)};

initializationFailure NOTIFICATION
BEHAVIOUR initializationFailure-B BEHAVIOUR
DEFINED AS The Initialisation Failure Notification is 
generated when an attempt to initialise with an adja
cent system fails as a result of either Version Skew 
or Area Mismatch. In the case of Version Skew, the 
Adjacent system argument is not present.   The oc
curance of this event is counted by the initialization
Failures counter.;;
MODE NON-CONFIRMED;
PARAMETERS
notificationAdjacentSystem,
notificationReason,
notificationPDUHeader,
notificationCalledAddress,
notificationVersion;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
initializationFailure (12)};

rejectedAdjacency NOTIFICATION
BEHAVIOUR rejectedAdjacency-B BEHAVIOUR
DEFINED AS The Rejected Adjacency Notification is 
generated when an attempt to create a new adja
cency is rejected, because of a lack of resources.  
The occurance of this event is counted by the reject
edAdjacencies counter.;;
MODE NON-CONFIRMED;
PARAMETERS
notificationAdjacentSystem,
notificationReason,
notificationPDUHeader,
notificationCalledAddress,
notificationVersion;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
rejectedAdjacency (13)};

lanL1DesignatedIntermediateSystemChange 
NOTIFICATION
BEHAVIOUR 
lanL1DesignatedIntermediateSystemChange-B 
BEHAVIOUR
DEFINED AS The LAN L1 Designated Intermediate 
System Change Notification is generated when the 
local system either elects itself or resigns as being 
the LAN L1 Designated Intermediate System on this 
circuit. The relative order of these events must be 
preserved. The occurance of this event is counted by 
the lanL1DesignatedIntermediateSystemChanges 
counter.;;
MODE NON-CONFIRMED;
PARAMETERS
notificationDesignatedIntermediateSystemChange;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
lanL1DesignatedIntermediateSystemChange (14)};

exceededMaximumSVCAdjacencies NOTIFICATION
BEHAVIOUR exceededMaximumSVCAdjacencies-B 
BEHAVIOUR
DEFINED AS The Exceeded Maximum SVC Adjacen
cies Notification is generated when there is no free 
adjacency on which to establish an SVC for a new 
destination.(see clause 8.3.2.3)  The occurance of 
this event is counted by the 
timesExceededMaximumSVCAdjacencies counter.;;
MODE NON-CONFIRMED;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
exceededMaximumSVCAdjacencies (15)};

exceededMaximumCallAttempts NOTIFICATION
BEHAVIOUR exceededMaximumCallAttempts-B 
BEHAVIOUR
DEFINED AS The Exceeded Maximum Call Attempts 
Notification is generated when recallCount becomes 
equal to maximumCallAttempts.  The occurance of 
this event is counted by the timesExceededMaxi
mumCallAttempts counter.;;
MODE NON-CONFIRMED;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
exceededMaximumCallAttempts (16)};

lanL2DesignatedIntermediateSystemChange 
NOTIFICATION
BEHAVIOUR 
lanL2DesignatedIntermediateSystemChange-B 
BEHAVIOUR
DEFINED AS The LAN L2 Designated Intermediate 
System Change Notification is generated when the 
local system either elects itself or resigns as being 
the LAN L2 Designated Intermediate System on this 
circuit. The relative order of these events must be 
preserved. The occurance of this event is counted by 
the lanL2DesignatedIntermediateSystemChanges 
counter.;;
MODE NON-CONFIRMED;

PARAMETERS
notificationDesignatedIntermediateSystemChange;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
lanL2DesignatedIntermediateSystemChange (17)};

authenticationFailure NOTIFICATION
BEHAVIOUR authenticationFailure-B BEHAVIOUR
DEFINED AS Generated when a PDU is received with 
an incorrect Authentication information field;;
MODE NON-CONFIRMED;
PARAMETERS
notificationAdjacentSystem;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.NotificationInfo;
REGISTERED AS {ISO10589-ISIS.noi 
authenticationFailure (18)};

11.2.13 Action Definitions
-- Note:  The following actions have been proposed (in 
SC21 N4977) for inclusion in  DMI.  Until such time 
as this is completed, the definitions of these actions 
are given here.
--
activate ACTION
BEHAVIOUR activate-B BEHAVIOUR
DEFINED AS Sets OperationalState to `enabled' and 
commences operation;;
MODE CONFIRMED;
PARAMETERS successResponse, failureResponse, 
failureReason;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.ActionInfo;
WITH REPLY SYNTAX ISO10589-ISIS.ActionReply;
REGISTERED AS {ISO10589-ISIS.acoi activate (1)};

deactivate ACTION
BEHAVIOUR deactivate-B BEHAVIOUR
DEFINED AS Sets OperationalState to `disabled' and 
ceases operation;;
MODE CONFIRMED;
PARAMETERS successResponse, failureResponse, 
failureReason;
WITH INFORMATION SYNTAX 
ISO10589-ISIS.ActionInfo;
WITH REPLY SYNTAX ISO10589-ISIS.ActionReply;
REGISTERED AS {ISO10589-ISIS.acoi deactivate (2)};

11.2.14 Parameter Definitions
iSO10589-NB-p1 PARAMETER
CONTEXT CREATE-INFO;
WITH SYNTAX ISO10589-ISIS.ISType;
BEHAVIOUR iSO10589-NB-p1-B BEHAVIOUR
DEFINED AS The value to be given to the iStype at
tribute on MO creation. This parameter is manda
tory;;
REGISTERED AS {ISO10589-ISIS.proi 
iSO10589-NB-p1 (1)};

iSO10589Circuit-MO-p1 PARAMETER
CONTEXT CREATE-INFO;
WITH SYNTAX ISO10589-ISIS.CircuitType;
BEHAVIOUR iSO10589Circuit-MO-p1-B 
BEHAVIOUR
DEFINED AS The value to be given to the type attrib
ute on MO creation. This parameter is mandatory;;
REGISTERED AS {ISO10589-ISIS.proi 
iSO10589Circuit-MO-p1 (2)};

reachableAddressP1 PARAMETER
CONTEXT CREATE-INFO;
WITH SYNTAX ISO10589-ISIS.AddressPrefix;
BEHAVIOUR reachableAddressp1-B BEHAVIOUR
DEFINED AS The value to be given to the addressPre
fix attribute on MO creation. This parameter is man
datory;;
REGISTERED AS {ISO10589-ISIS.proi 
reachableAddressP1 (3)};

reachableAddressP2 PARAMETER
CONTEXT CREATE-INFO;
WITH SYNTAX ISO10589-ISIS.MappingType;
BEHAVIOUR reachableAddressp2-B BEHAVIOUR
DEFINED AS The value to be given to the map
pingType attribute on MO creation. This parameter 
is only permitted when the `type' of the parent cir
cuit is either `broadcast' or `DA'.  In those cases the 
default value is `manual';;
REGISTERED AS {ISO10589-ISIS.proi 
reachableAddressP2 (4)};

manualAdjacencyP1 PARAMETER
CONTEXT CREATE-INFO;
WITH SYNTAX ISO10589-ISIS.LANAddress;
BEHAVIOUR manualAdjacencyP1-B BEHAVIOUR
DEFINED AS The value to be given to the lANAd
dress attribute on MO creation;;
REGISTERED AS {ISO10589-ISIS.proi 
manualAdjacencyP1 (5)};

manualAdjacencyP2 PARAMETER
CONTEXT CREATE-INFO;
WITH SYNTAX ISO10589-ISIS.EndSystemIDs;
BEHAVIOUR manualAdjacencyP2-B BEHAVIOUR
DEFINED AS The value to be given to the endSys
temIDs attribute on MO creation;;
REGISTERED AS {ISO10589-ISIS.proi 
manualAdjacencyP2 (6)};

successResponse PARAMETER
CONTEXT ACTION-REPLY;
WITH SYNTAX ISO10589-ISIS.ResponseCode;
BEHAVIOUR successResponse-B BEHAVIOUR
DEFINED AS Returned in the responseCode field of 
an ActionReply when the action has completed suc
cessfully.;;
REGISTERED AS {ISO10589-ISIS.proi successResponse 
(7)};

failureResponse PARAMETER
CONTEXT ACTION-REPLY;
WITH SYNTAX ISO10589-ISIS.ResponseCode;

BEHAVIOUR failureResponse-B BEHAVIOUR
DEFINED AS Returned in the responseCode field of 
an ActionReply when the action failed to complete. 
The failureReason parameter is returned with this re
sponseCode, giving additional information;;
REGISTERED AS {ISO10589-ISIS.proi failureResponse 
(8)};

failureReason PARAMETER
CONTEXT ACTION-REPLY;
WITH SYNTAX ISO10589-ISIS.ActionFailureReason;
BEHAVIOUR failureReason-B BEHAVIOUR
DEFINED AS Gives the reason why an entity failed to 
activate or deactivate.;;
REGISTERED AS {ISO10589-ISIS.proi failureReason 
(9)};

constraintViolation PARAMETER
CONTEXT SPECIFIC-ERROR;
WITH SYNTAX 
ISO10589-ISIS.ConstraintViolationReason;
BEHAVIOUR constraintViolation-B BEHAVIOUR
DEFINED AS The specific error returned on failure of 
a REPLACE operation when the MO prohibits such 
operations under certain conditions, for example 
while the MO is in the disabled operational state.;;
REGISTERED AS {ISO10589-ISIS.proi 
constraintViolation (10)};

notificationReceivingAdjacency PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX  
ISO10589-ISIS.LocalDistinguishedName;
BEHAVIOUR notificationReceivingAdjacency-B 
BEHAVIOUR
DEFINED AS The local managed object name of the 
adjacency upon which the NPDU was received;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationReceivingAdjacency (11)};

notificationIDLength PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX  ISO10589-ISIS.IDLength;
BEHAVIOUR notificationIDLength-B BEHAVIOUR
DEFINED AS The IDLength specified in the ignored 
PDU;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationIDLength (12)};

notificationAreaAddress PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.AreaAddress;
BEHAVIOUR notificationAreaAddress-B BEHAVIOUR
DEFINED AS The Area Address which caused Maxi
mumAreaAddresses to be exceeded;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationAreaAddress (13)};

notificationSourceID PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.SourceID;

BEHAVIOUR notificationSourceID-B BEHAVIOUR
DEFINED AS The source ID of the LSP;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationSourceID (14)};

notificationVirtualLinkChange PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.VirtualLinkChange;
BEHAVIOUR notificationVirtualLinkChange-B 
BEHAVIOUR
DEFINED AS This indicates whether the event was 
genrated as a result of the creation or deletion of a 
Virtual Link between two Level 2 Intermediate Sys
tems.;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationVirtualLinkChange (15)};

notificationVirtualLinkAddress PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.NetworkEntityTitle;
BEHAVIOUR notificationVirtualLinkAddress-B 
BEHAVIOUR
DEFINED AS The Network Entity Title of the Level 2 
Intermediate System at the remote end of the virtual 
link;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationVirtualLinkAddress (16)};

notificationNewCircuitState PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.NewCircuitState;
BEHAVIOUR notificationNewCircuitState-B 
BEHAVIOUR
DEFINED AS The direction of the Circuit state change 
specified as the resulting state. i.e. a change from On 
to Off is specified as Off;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationNewCircuitState (17)};

notificationNewAdjacencyState PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.NewAdjacencyState;
BEHAVIOUR notificationNewAdjacencyState-B 
BEHAVIOUR
DEFINED AS The direction of the Adjacency state 
change specified as the resulting state.  i.e. a change 
from Up to Down is specified as Down.  Any state 
other than Up is considered to be Down.;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationNewAdjacencyState (18)};

notificationAdjacentSystem PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.SystemID;
BEHAVIOUR notificationAdjacentSystem-B 
BEHAVIOUR
DEFINED AS The system ID of the adjacent system;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationAdjacentSystem (19)};

notificationReason PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.Reason;

BEHAVIOUR notificationReason-B BEHAVIOUR
DEFINED AS The associated Reason;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationReason (20)};

notificationPDUHeader PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.PDUHeader;
BEHAVIOUR notificationPDUHeader-B BEHAVIOUR
DEFINED AS The header of the PDU which caused 
the notification;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationPDUHeader (21)};

notificationCalledAddress PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.SNPAAddress;
BEHAVIOUR notificationCalledAddres-B 
BEHAVIOUR
DEFINED AS The SNPA Address which was being 
called when the Adjacency was taken down as a re
sult of a call reject;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationCalledAddress (22)};

notificationVersion PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.Version;
BEHAVIOUR notificationVersion-B BEHAVIOUR
DEFINED AS The version number reported by the 
other system;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationVersion (23)};

notificationDesignatedIntermediateSystemChange 
PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.DesignatedISChange;
BEHAVIOUR 
notificationDesignatedIntermediateSystemChange-B 
BEHAVIOUR
DEFINED AS The direction of the change in Desig
nated Intermediate System status of this system;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationDesignatedIntermediateSystemChange 
(24)};

notificationOverloadStateChange PARAMETER
CONTEXT EVENT-INFO;
WITH SYNTAX ISO10589-ISIS.OverloadStateChange;
BEHAVIOUR notificationOverloadStateChange-B 
BEHAVIOUR
DEFINED AS The direction of the change in Overload 
status;;
REGISTERED AS {ISO10589-ISIS.proi 
notificationOverloadStateChange (25)};

11.2.15 Attribute Groups
counters ATTRIBUTE GROUP
DESCRIPTION  The group of all counters;
REGISTERED AS  {ISO10589-ISIS.agoi counters (1)};

11.2.16 Behaviour Definitions
resettingTimer-B BEHAVIOUR
DEFINED AS This attribute specifies the interval be
tween certain events in the operation of the protocol 
state machine.  If the value of this attribute is 
changed to a new value t  while the protocol state 
machine is in operation, the implementation shall 
take the necessary steps to ensure that for any time 
interval which was in progress when the correspond
ing attribute was changed, the next expiration of the 
that interval takes place t seconds from the original 
start of that interval, or immediately, whichever is 
later. The precision with which this time shall be im
plemented shall be  the same as that associated with 
the basic operation of the timer attribute;

replaceOnlyWhileDisabled-B BEHAVIOUR
DEFINED AS This attribute shall only permit the RE
PLACE operation to be performed on it while the 
MO is in the Disabled Operational State.  An at
tempt to perform a REPLACE operation while the 
MO is in the Enabled Operation State shall fail with 
the generation of the constraintViolation specific er
ror.;

resourceLimiting-B BEHAVIOUR
DEFINED AS This attribute places limits on some re
source".  In general implementations may allocate 
reources up to this limit when the managed object is 
enabled and it may be impossible to change the allo
cation without first disabling and re-enabling the 
managed object.  Therefore this International Stan
dard only requires that it shall be possible to perform 
a REPLACE operation on this attribute while the 
MO is disabled.  However some implementations 
may be able to to change the allocation of resources 
without first disabling the MO.  In this case it is per
mitted to increase the value of the atribute at any 
time, but it shall not be decreased below the cur
rently used" value of the resource. Where an at
tempt to perform a REPLACE operation fails either 
because the MO is enabled, or because an attempt 
has been made to decrease the value, the REPLACE 
operation shall fail with the generation of the con
straintViolation specific error.;
11.2.17 ASN1 Modules
ISO10589-ISIS{tbd1}
DEFINITIONS ::= BEGIN
-- object identifier definitions
sc6 OBJECT IDENTIFIER ::= {joint-iso-ccitt sc6(?)}
-- value to be assigned by SC21 secretariat
isisoi OBJECT IDENTIFIER ::= {sc6 iSO10589(?)}
-- value to be assigned by SC6 secretariat
moi OBJECT IDENTIFIER ::= {isisoi objectClass (3)}
poi OBJECT IDENTIFIER ::= {isisoi package (4)}
proi OBJECT IDENTIFIER ::= {isisoi parameter (5)}
nboi OBJECT IDENTIFIER ::= {isisoi nameBinding (6)}
aoi OBJECT IDENTIFIER ::= {isisoi attribute (7)}
agoi OBJECT IDENTIFIER ::= {isisoi attributeGroup 
(8)}
acoi OBJECT IDENTIFIER ::= {isisoi action (10)}
noi OBJECT IDENTIFIER ::= {isisoi notification (11)}

ActionFailureReason ::= ENUMERATED{
reason1(0),
reason2(1)}
-- Note: actual reasons TBS
ActionInfo ::= SET OF Parameter
ActionReply ::= SEQUENCE{
responseCode OBJECT IDENTIFIER,
responseArgs SET OF Parameter OPTIONAL}
AddressPrefix ::=  OCTETSTRING(SIZE(0..20))
AdjacencyState ::= ENUMERATED{
initializing(0),
up(1),
failed(2)}-- was 4 in N5821 , is it required at all?
AreaAddress ::=  OCTETSTRING(SIZE(1..20))
AreaAddresses ::= SET OF AreaAddress
Boolean ::= BOOLEAN
CircuitID ::=  OCTETSTRING(SIZE(1..10))
CompleteSNPInterval ::= INTEGER(1..600)
ConstraintViolationReason ::= OBJECT IDENTIFIER;
DRISISHelloTimer ::= INTEGER(1..65535)
DatabaseState ::= ENUMERATED{
off(0),
on(1),
waiting(2)}
DesignatedISChange ::= ENUMERATED{
resigned(0),
elected(1)}
DefaultESHelloTimer ::= INTEGER(1..65535)
EndSystemIDs ::= SET OF SystemID
GraphicString ::=  GRAPHICSTRING
HelloTimer ::= INTEGER(1..65535)
HoldingTimer ::= INTEGER(1..65535)
HopMetric ::= INTEGER(0..63)
ISISHelloTimer ::= INTEGER(1..65535)
IDLength ::= INTEGER(0..9)
IdleTimer ::= INTEGER(1..65535)
InitialMinimumTimer ::= INTEGER(1..65535)
IntermediateSystemPriority ::= INTEGER(1..127)
ISType ::= ENUMERATED{
level1IS(1),
level2IS(2)}
LANAddress ::=  OCTETSTRING(SIZE(6))
AdjacencyUsageType::= ENUMERATED{
undefined(0),
level1(1),
level2(2),
level1and2(3)}
LocalDistinguishedName ::= CMIP-1.ObjectInstance
-- A suitable free standing definition is requred
LSPID ::=  OCTETSTRING(SIZE(2..11))
MappingType ::= ENUMERATED{
manual(0),
x121(1)}
MaximumBuffers ::= INTEGER(1..65535)
MaximumCallAttempts ::= INTEGER(1..65535)
MaximumLSPGenerationInterval ::= INTEGER(1..65535)
MaximumPathSplits ::= INTEGER(1..32)
MaximumSVCAdjacencies ::= INTEGER(1..65535)
MaximumVirtualAdjacencies ::= INTEGER(0..32)
MetricIncrement ::= INTEGER(0..63)
MetricType ::= ENUMERATED{
internal(0),
external(1)}
MinimumBroadcastLSPTransmissionInterval ::= 
INTEGER(1..65535)
MinimumLSPGenerationInterval ::= INTEGER(1..65535)
MinimumLSPTransmissionInterval ::= 

INTEGER(1..65535)
NeighbourSystemType ::= ENUMERATED{
unknown(0),
endSystem(1),
intermediateSystem(2),
l1IntermediateSystem(3),
l2IntermediateSystem(4)}
NetworkEntityTitle ::=  OCTETSTRING(SIZE(1..19))
NewAdjacencyState ::= ENUMERATED{
down(0),
up(1)}
NewCircuitState ::= ENUMERATED{
off(0),
on(1)}
NonWrappingCounter ::= INTEGER(0..264-1)
NotificationInfo ::= SET OF Parameter
NSAPAddress ::=  OCTETSTRING(SIZE(1..20))
OctetString ::=  OCTETSTRING
OriginatingLSPBufferSize ::= INTEGER(512..1492)
OutputAdjacencies ::= SET OF LocalDistinguishedName
OverloadStateChange ::= ENUMERATED{
on(0),
waiting(1)}
Parameter ::= SEQUENCE{
paramIdOBJECT IDENTIFIER,
paramInfoANY DEFINED BY paramID}
PartialSNPInterval ::= INTEGER(1..65535)
Password ::=  OCTETSTRING(SIZE(0..254)
Passwords ::= SET OF Password
PathMetric ::= INTEGER(0..1023)
PDUHeader ::=  OCTETSTRING(SIZE(0..255))
PollESHelloRate ::= INTEGER(1..65535)
Reason ::= ENUMERATED{
holdingTimerExpired(0),
checksumError(1),
oneWayConnectivity(2),
callRejected(3),
reserveTimerExpired(4),
circuitDisabled(5),
versionSkew(6),
areaMismatch(7),
maximumBroadcastIntermediateSystemsExceeded(8),
maximumBroadcastEndSystemsExceeded(9),
wrongSystemType(10)}
ResponseCode ::= OBJECT IDENTIFIER
RecallTimer ::= INTEGER(1..65535)
ReserveTimer ::= INTEGER(1..65535)
SNPAAddress ::=  
NUMERICSTRING(FROM("0"|"1"|"2"|"3"|"4"|"5"|
"6"|"7"|"8"|"9"))(SIZE(0..15))
-- Up to 15 Digits 0..9
SNPAAddresses ::= SET OF SNPAAddress
CircuitType ::= ENUMERATED{
broadcast(0),
ptToPt(1),
staticIN(2),
staticOut(3),
dA(4)}
SourceID ::=  OCTETSTRING(SIZE(1..10))
SystemID ::=  OCTETSTRING(SIZE(0..9))
VirtualLinkChange ::= ENUMERATED{
deleted(0),
created(1)}
Version ::=  GRAPHICSTRING
WaitingTime ::= INTEGER(1..65535)
maximumPathSplits-Default INTEGER ::= 2
MaximumPathSplits-Permitted ::= INTEGER(1..32)

maximumBuffers-Default INTEGER ::= ImpSpecific
MaximumBuffers-Permitted ::= INTEGER(1..ImpSpecific)
minimumLSPTransmissionInterval-Default INTEGER ::= 
5
MinimumLSPTransmissionInterval-Permitted ::= 
INTEGER(5..30)
maximumLSPGenerationInterval-Default INTEGER ::= 
900
MaximumLSPGenerationInterval-Permitted ::= 
INTEGER(60..900)
minimumBroadcastLSPTransmissionInterval-Default 
INTEGER ::=33
MinimumBroadcastLSPTransmissionInterval-Permitted ::= 
INTEGER(1..65535)
completeSNPInterval-Default INTEGER ::= 10
CompleteSNPInterval-Permitted ::= INTEGER(1..600)
originatingL1LSPBufferSize-Default INTEGER ::= 
receiveLSPBufferSize
OriginatingL1LSPBufferSize-Permitted ::= 
INTEGER(512..receiveLSPBufferSize)
manualAreaAddresses-Default AreaAddresses ::= {}
ManualAreaAddresses-Permitted ::= AreaAddresses 
(SIZE(0..MaximumAreaAddresses))
minimumLSPGenerationInterval-Default INTEGER ::= 30
MinimumLSPGenerationInterval-Permitted ::= 
INTEGER(5..300)
defaultESHelloTime-Default INTEGER ::= 600
DefaultESHelloTime-Permitted ::= INTEGER(1..65535)
pollESHelloRate-Default INTEGER ::= 50
PollESHelloRate-Permitted ::= INTEGER(1..65535)
partialSNPInterval-Default INTEGER ::= 2
PartialSNPInterval-Permitted ::= INTEGER(1..65535)
waitingTime-Default INTEGER ::= 60
WaitingTime-Permitted ::= INTEGER(1..65535)
dRISISHelloTimer-Default INTEGER ::= 1
DRISISHelloTimer-Permitted ::=  INTEGER(1..65535)
originatingL2LSPBufferSize-Default INTEGER ::= 
receiveLSPBufferSize
OriginatingL2LSPBufferSize-Permitted ::= 
INTEGER(512..receiveLSPBufferSize)
maximumVirtualAdjacencies-Default INTEGER ::= 2
MaximumVirtualAdjacencies-Permitted ::= 
INTEGER(0..32)
helloTimer-Default INTEGER ::= 10
HelloTimer-Permitted ::= INTEGER(1..21845)
defaultMetric-Default INTEGER ::= 20
DefaultMetric-Permitted ::= INTEGER(1..MaxLinkMetric)
optionalMetric-Default INTEGER ::= 0
OptionalMetric-Permitted ::= 
INTEGER(0..MaxLinkMetric)
metricType-Default MetricType ::= Internal
iSISHelloTimer-Default INTEGER ::= 3
ISISHelloTimer-Permitted ::= INTEGER(1..21845)
externalDomain-Default BOOLEAN ::= TRUE
l1IntermediateSystemPriority-Default INTEGER ::= 64
L1IntermediateSystemPriority-Permitted ::= 
INTEGER(1..127)
callEstablishmentMetricIncrement-Default INTEGER ::= 0
CallEstablishmentMetricIncrement-Permitted ::= 
INTEGER(0..MaxLinkMetric)
idleTimer-Default INTEGER ::= 30
IdleTimer-Permitted ::= INTEGER(0..65535)
initialMinimumTimer-Default INTEGER ::= 55
InitialMinimumTimer-Permitted ::= INTEGER(1..65535)
reserveTimer-Default INTEGER ::= 600
ReserveTimer-Permitted ::= INTEGER(1..65535)
maximumSVCAdjacencies-Default INTEGER ::= 1

MaximumSVCAdjacencies-Permitted ::= 
INTEGER(1..65535)
reservedAdjacency-Default BOOLEAN ::= FALSE
neighbourSNPAAddress-Default INTEGER ::= 0
recallTimer-Default INTEGER ::= 60
RecallTimer-Permitted ::= INTEGER(0..65535)
maximumCallAttempts-Default INTEGER ::= 10
MaximumCallAttempts-Permitted ::= INTEGER(0..255)
manualL2OnlyMode-Default BOOLEAN ::= FALSE
l2IntermediateSystemPriority-Default INTEGER ::= 64
L2IntermediateSystemPriority-Permitted ::= 
INTEGER(1..127)
lANAddress-Default LANAddress ::= 000000000000
sNPAAddresses-Default SNPAAddresses::= {}
password-Default Password ::= {}
passwords-Default Passwords ::= {} -- The empty set
END

12 Conformance
12.1 Static Conformance Requirements
12.1.1 Protocol Implementation Conformance 
Statement
A Protocol Implementation Conformance Statement (PICS) 
shall be completed in respect of any claim for conformance 
of an implementation to this International Standard: the 
PICS shall be produced in accordance with the relevant 
PICS pro-forma in Annex A. 
12.1.2 Static Conformance for all ISs
A system claiming conformance to this International Stan
dard shall be capable of:
a)calculating a single minimum cost route to each desti
nation according to 7.2.6 for the default metric speci
fied in 7.2.2;
b)utilising Link State information from a system only 
when an LSP with LSP number 0 and remaining life
time>0 is present according to 7.2.5;
c)removing excess paths according to 7.2.7
d)performing the robustness checks according to 7.2.8;
e)constructing a forwarding database according to 7.2.9;
f)if (and only if) Area Partition Repair is supported, 
1)performing the operations according to 7.2.10;
2)performing the encapsulation operations in the for
warding process according to 7.4.3.2; and
3)performing the decapsulation operations in the re
ceive process according to 7.4.4;
TEMPORARY NOTE  may need to reor
ganise clause 7.4.4 in order to make it crystal 
clear what is required in the receive process in 
the presence/absence of partition repair
g)computing area addresses according to 7.2.11;
h)generating local Link State information as required by 
7.3.2;
i)including information from Manual Adjacencies ac
cording to 7.3.3.1;
j)if (and only if) Reachable Addresses are supported, in
cluding information from Reachable Addresses ac
cording to 7.3.3.2;
k)generating multiple LSPs according to 7.3.4;
l)generating LSPs periodically according to 7.3.5;
m)generating LSPs on the occurrence of events accord
ing to 7.3.6;

n)generating an LSP checksum according to 7.3.11;
o)operating the Update Process according to 7.3.12
7.3.17 including controlling the rate of LSP transmis
sion only for each broadcast circuit (if any) according 
to 7.3.15.6;
p)operating the LSP database overload procedures ac
cording to 7.3.19.1;
q)selecting the appropriate forwarding database accord
ing to 7.4.2;
r)forwarding ISO 8473 PDUs according to 7.4.3.1 and 
7.4.3.3;
s)operating the receive process according to 7.4.4;
TEMPORARY NOTE  item 1 of the second bulleted 
list is only required if you implement partition repair. 
We need to reorganise the structure so we can pull 
this out.
t)performing on each supported Point-to-Point circuit (if 
any):
1)forming and maintaining adjacencies according to 
8.2;
u)performing on each supported ISO 8208 circuit (if 
any)
1)SVC establishment according to 8.3.2.1 using the 
network layer protocols according to 8.3.1;
2)If  Reachable Addresses are supported, the opera
tions specified in 8.3.2.2  8.3.5.6.
3)If call

Estab

lish

ment

Met

ricIncrement greater 
than zero are supported, the operations specified in 
8.3.5.3.
4)If the Reverse Path Cache is supported, the opera
tions specified in 8.3.3
v)performing on each supported broadcast circuit (if 
any)
1)the pseudonode operations according to 7.2.3;
2)controlling the rate of LSP transmission according 
to 7.3.15.6;
3)the operations specified in  8.4.18.4.4 and 8.4.6;
4)the operations specified in 8.4.5.
w)constructing and correctly parsing all PDUs according 
to clause 9;
x)providing a system environment in accordance with 
clause 10;
y)being managed via the system management attributes 
defined in clause 11. For all attributes referenced inthe 
normative text, the default value (if any) shall be sup
ported. Other values shall be supported if referenced 
in a  REQUIRED VALUES clause of the GDMO 
definition;

z)If authentication procedures are implemented:
1)the authentication field processing functions of 
clauses 7.3.77.3.10, 7.3.15.17.3.15.4, 8.2.3
8.2.4, and 8.4.1.1;
2)the Authentication Information field of the 
PDU in clauses 9.59.13.
12.1.3  Static Conformance Requirements for 
level 1 ISs
A system claiming conformance to this International Stan
dard as a level 1 IS shall conform to the requirements of 
12.1.2 and in addition shall be capable of
a)identifying the nearest Level 2 IS according to 7.2.9.1;
b)generating Level 1 LSPs according to 7.3.7;
c)generating Level 1 pseudonode LSPs for each sup
ported broadcast circuit (if any) according to 7.3.8;
d)performing the actions in Level 1 Waiting State ac
cording to 7.3.19.2
12.1.4 Static Conformance Requirements for 
level 2 ISs
A system claiming conformance to this International Stan
dard as a level 2 IS shall conform to the requirements of 
12.1.2 and in addition shall be capable of
a)setting the attached flag according to 7.2.9.2;
b)generating Level 2 LSPs according to 7.3.9;
c)generating Level 2 pseudonode LSPs for each sup
ported broadcast circuit (if any) according to 7.3.10;
d)performing the actions in Level 2 Waiting State ac
cording to 7.3.19.3.
12.2 Dynamic Conformance
12.2.1 Receive Process Conformance 
Requirements
Any protocol function supported shall be implemented in 
accordance with 7.4.4.
12.2.2 Update Process Conformance 
Requirements
Any protocol function supported shall be implemented in 
accordance with 7.3 and its subclauses.
Any PDU transmitted shall be constructed in accordance 
with the appropriate subclauses of 9.

12.2.3 Decision Process Conformance 
Requirements
Any protocol function supported shall be implemented in 
accordance with 7.2 and its subclauses.
12.2.4 Forwarding Process Conformance 
Requirements
Any protocol function supported shall be implemented in 
accordance with 7.4  and its subclauses.
12.2.5 Performance Requirements
This International Standard requires that the following per
formance criteria be met. These requirements apply regard
less of other demands on the system; if an Intermediate sys
tem has other tasks as well, those will only get resources 
not required to meet these criteria. 
Each Intermediate system implementation shall specify (in 
its PICS):
a)the maximum number of other Intermediate systems it 
can handle. (For L1 Intermediate systems that means 
Intermediate systems in the area; for L2 Intermediate 
systems that is the sum of Intermediate systems in the 
area and Intermediate systems in the L2 subdomain.) 
Call this limit N.
b)the maximum supported forwarding rate in ISO 8473 
PDUs per second.
12.2.5.1 Performance requirements on the Update 
process
The implementation shall guarantee the update process 
enough resources to process N LSPs per 30 seconds. (Re
sources = CPU, memory, buffers, etc.)
In a stable topology the arrival of a single new LSP on a 
circuit shall result in the propagation of that new LSP over 
the other circuits of the IS within one second, irrespective 
of the forwarding load for ISO 8473 data PDUs.    
12.2.5.2 Performance requirement on the Decision 
process   
The implementation shall guarantee the decision process 
enough resources to complete (i.e. start to finish) within 5 
seconds, in a stable topology while forwarding at the maxi
mum rate. (For L2 Intermediate Systems, this applies to the 
two levels together, not each level separately.)    
12.2.5.3 Reception and Processing of PDUs
An ideal Intermediate system would be able to correctly 
process all PDUs, both control and data, with which it was 
presented, while simultaneously running the decision proc
ess and responding to management requests. However, in 
the implementations of real Intermediate systems some 
compromises must be made. The way in which these com
promises are made can dramatically affect the correctness 

of operation of the Intermediate system. The following gen
eral principles apply.
a)A stable topology should result in stable routes when 
forwarding at the maximum rated forwarding rate. 
b)Some forwarding progress should always be made (al
beit over incorrect routes) even in the presence of a 
maximally unstable topology.
In order to further characterise the required behaviour, it is 
necessary to identify the following types of traffic. 
a)IIH traffic. This traffic is important for maintaining In
termediate system adjacencies and hence the Interme
diate system topology. In order to prevent gratuitous 
topology changes it is essential that Intermediate sys
tem adjacencies are not caused to go down errone
ously. In order to achieve this no more than 
ISISHoldingMultiplier - 1 IIH PDUs may be 
dropped between any pair of Intermediate systems. A 
safer requirement is that no IIH PDUs are dropped. 
The rate of arrival of IIH PDUs is approximately con
stant and is limited on Pointto-Point links to 1/iSIS

Hello

Timer and on LANs to a value of approxi
mately 2(n/iSIS

Hello

Timer) + 2, where n is the 
number of Intermediate systems on the LAN (assum
ing the worst case that they are all Level 2 Intermedi
ate systems). 
b)ESH PDU traffic. This traffic is important for main
taining End system adjacencies, and has relatively low 
processing latency. As with IIH PDUs, loss of End 
system adjacencies will cause gratuitous topology 
changes which will result in extra control traffic.
The rate of arrival of ESH PDUs on Pointto-Point 
links is limited to approximately 1/Default

ES

Hello

Timer under all conditions. On LANs the background 
rate is approximately n/DefaultESHelloTimer 
where n is the number of End systems on the LAN. 
The maximum rate during polling is limited to ap
proximately n/pollESHelloRate averaged over a pe
riod of about 2 minutes. (Note that the actual peak ar
rival rate over a small interval may be much higher 
than this.) 
c)LSP (and SNP) traffic. This traffic will be 
retransmitted indefinitely by the update process if it is 
dropped, so there is no requirement to be able to proc
ess every received PDU. However, if a substantial 
proportion are lost, the rate of convergence to correct 
routes will be affected, and bandwidth and processing 
power will be wasted.
On Point-to-Point links the peak rate of arrival is lim
ited only by the speed of the data link and the other 
traffic flowing on that link. The maximum average 
rate is determined by the topology.
On LANs the rate is limited at a first approximation to 
a maximum rate of  1000/min

i

mum

Broad

cast

LSP

Trans

mis

sion

Int

er

val, however it is possible that 
this may be multiplied by a factor of up to n, where n 
is the number of Intermediate systems on the LAN, for 

short periods. A Intermediate system shall be able to 
receive and process at least the former rate without 
loss, even if presented with LSPs at the higher rate. 
(i.e. it is permitted to drop LSPs, but must process at 
least 1000/min

i

mum

Broad

cast

LSP

Trans

mis

sion

Int

er

val per second of those presented.)
The maximum background rate of LSP traffic (for a 
stable topology) is dependent on the maximum sup
ported configuration size and the settings of 
maximumLSPGenerationInterval. For these pur
poses the default value of 900 seconds can be as
sumed. The number of LSPs per second is then very 
approximately (n1 + n2 +ne/x)/900 where n1 is the 
number of level 1 Intermediate systems, n2 the num
ber of level 2 Intermediate systems, ne the number of 
End system IDs and x the number of ID which can be 
fitted into a single LSP.
NOTE  This gives a value around 1 per second for 
typical maximum configurations of:
4000 IDs
100 L1 Intermediate systems per area
400 L2 Intermediate systems.
d)Data Traffic. This is theoretically unlimited and can 
arrive at the maximum data rate of the Pointto-Point 
link or LAN (for ISO 8802.3 this is 14,000 PDUs per 
second). In practice it will be limited by the operation 
of the congestion avoidance and control algorithms, 
but owing to the relatively slow response time of these 
algorithms, substantial peaks are likely to occur.
An Intermediate system shall state in its PICS its 
maximum forwarding rate. This shall be quoted under 
at least the following conditions.
1)A stable topology of maximum size. 
2)A maximally unstable topology. This figure shall 
be non-zero, but may reasonably be as low as 1 
PDU per second.
The following constraints must be met.
a)The implementation shall be capable of receiving the 
maximum rate of ISH PDUs without loss whenever 
the following conditions hold
1)The data forwarding traffic rate averaged over any 
period of one second does not exceed the rate 
which the implementation claims to support
2)The ESH and LSP rates do not exceed the back
ground (stable topology) rate.
b)If it is unavoidable that PDUs are dropped, it is a goal 
that the order of retaining PDUs shall be as follows 
(i.e. It is least desirable for IIH PDUs to be dropped).
1)IIH PDUs 
2)ESH PDUs 
3)LSPs and SNPs 
4)data PDUs.

However, no class of traffic shall be completely 
starved. One way to achieve this is to allocate a queue 
of suitable length to each class of traffic and place the 
PDUs onto the appropriate queue as they arrive. If the 
queue is full the PDUs are discarded. Processor re
sources shall be allocated to the queues to ensure that 
they all make progress with the same priorities as 
above. This model assumes that an implementation is 
capable of receiving PDUs and selecting their correct 
queue at the maximum possible data rate (14,000 
PDUs per second for a LAN). If this is not the case, 
reception of data traffic at a rate greater than some 
limit (which must be greater than the maximum rated 
limit) will cause loss of some IIH PDUs even in a sta
ble topology. This limit shall be quoted in the PICS if 
it exists. 
NOTE - Starting from the stable topology condition at maxi
mum data forwarding rate, an increase in the arrival rate of 
data PDUs will initially only cause some data NPDUs to be 
lost. As the rate of arrival of data NPDUs is further in
creased a point may be reached at which random PDUs are 
dropped. This is the rate which must be quoted in the PICS
12.2.5.4 Transmission
Sufficient processor resources shall be allocated to the 
transmission process to enable it to keep pace with recep
tion for each PDU type. Where prioritisation is required, the 
same order as for reception of PDU types applies.

Annex A 
PICS Proforma
(This annex is normative) 

A.1 Introduction
The supplier of a protocol implementation which is claimed 
to conform to International Standard ISO 10589, whether as 
a level 1 or level 2 Intermediate system implementation, 
shall complete the applicable Protocol Implementation 
Conformance Statement (PICS) proforma.
A completed PICS proforma is the PICS for the implemen
tation in question. The PICS is a statement of which capa
bilities and options of the protocol have been implemented. 
The PICS can have a number of uses, including use:
-by the protocol implementor, as a check-list to reduce 
the risk of failure to conform to the standard through 
oversight;
-by the supplier and acquirer  or potential acquirer 
 of the implementation, as a detailed indication of 
the capabilities of the implementation, stated relative 
to the common basis for understanding provided by 
the standard PICS proforma;
-by the user  or potential user  of the implementa
tion, as a basis for initially checking the possibility of 
interworking with another implementation (note that, 
while interworking can never be guaranteed, failure to 
interwork can often be predicted from incompatible 
PICS's);
-by a protocol tester, as the basis for selecting appropri
ate tests against which to assess the claim for 
conformance of the implementation.
A.2 Abbreviations and Special Symbols
A.2.1 Status-related symbols
M	mandatory
O	optional
O.<n>	optional, but support of at least one of the 
group of options labelled by the same numeral 
<n> is required.
X	prohibited
	not applicable
c.<p>	conditional requirement, according to condi
tion <p>

A.3 Instructions for Completing the 
PICS Proformas
A.3.1 General structure of the PICS proforma
The first part of the PICS proforma  Implementation 
Identification and Protocol Summary  is to be completed 
as indicated with the information necessary to identify fully 
both the supplier and the implementation.
The main part of the PICS proforma is a fixed-format ques
tionnaire divided into subclauses each containing a group of 
individual items. Answers to the questionnaire items are to 
be provided in the rightmost column, either by simply 
marking an answer to indicate a restricted choice (usually 
Yes or No), or by entering a value or a set or range of val
ues. (Note that there are some items where two or more 
choices from a set of possible answers can apply: all rele
vant choices are to be marked.)
Each item is identified by an item reference in the first col
umn; the second column contains the question to be an
swered; the third column contains the reference or refer
ences to the material that specifies the item in the main 
body of the standard. the remaining columns record the 
status of the item  whether support is mandatory, optional 
or conditional  and provide the space for the answers: see 
A.3.4  below.
A supplier may also provide  or be required to provide  
further information, categorised as either Additional Infor
mation or Exception Information. When present, each kind 
of further information is to be provided in a further sub
clause of items labelled A<i> or X<i> respectively for 
cross-referencing purposes, where <i> is any unambiguous 
identification for the item (e.g. simply a number): there are 
no other restrictions on its format and presentation.
A completed PICS proforma, including any Additional In
formation and Exception Information, is the Protocol Im
plementation Conformance Statement for the implementa
tion in question.
NOTE - Where an implementation is capable of being con
figured in more than one way, a single PICS may be able to 
describe all such configurations. However, the supplier has 
the choice of providing more than one PICS, each covering 
some subset of the implementation's configuration capabili
ties, in case this makes for easier and clearer presentation of 
the information.
A.3.2 Additional Information
Items of Additional Information allow a supplier to provide 
further information intended to assist the interpretation of 
the PICS. It is not intended or expected that a large quantity 
will be supplied, and a PICS can be considered complete 

without any such information. Examples might be an out
line of the ways in which a (single) implementation can be 
set up to operate in a variety of environments and configu
rations.
References to items of Additional information may be en
tered next to any answer in the questionnaire, and may be 
included in items of Exception Information.
A.3.3 Exception Information
It may occasionally happen that a supplier will wish to an
swer an item with mandatory or prohibited status (after any 
conditions have been applied) in a way that conflicts with 
the indicated requirement. No pre-printed answer will be 
found in the Support column for this, but the Supplier may 
write the desired answer into the Support column. If this is 
done, the supplier is required to provide an item of Excep
tion Information containing the appropriate rationale, and a 
cross-reference from the inserted answer to the Exception 
item.
An implementation for which an Exception item is required 
in this way does not conform to ISO 10589.
NOTE - A possible reason for the situation described above 
is that a defect report is being progressed, which is expected 
to change the requirement that is not met by the implemen
tation.
A.3.4 Conditional Status
A.3.4.1 Conditional items
The PICS proforma contains a number of conditional items. 
These are items for which the status  mandatory, optional 
or prohibited  that applies is dependent upon whether or 
not certain other items are supported, or upon the values 
supported for other items. In many cases, whether or not the 
item applies at all is conditional in this way, as well as the 
status when the item does apply.
Individual conditional items are indicated by a conditional 
symbol in the Status column as described in A.3.4.2 below. 
Where a group of items are subject to the same condition 
for applicability, a separate preliminary question about the 
condition appears at the head of the group, with an instruc
tion to skip to a later point in the questionnaire if the Not 
Applicable answer is selected.
A.3.4.2 Conditional symbols and conditions
A conditional symbol is of the form c.<n> or c.G<n> where 
<n> is a numeral. For the first form, the numeral identifies 
a condition appearing in a list at the end of the subclause 
containing the item. For the second form, c.G<n>, the nu
meral identifies a condition appearing in the list of global 
conditions at the end of the PICS.
A simple condition is of the form:if <p> then <s1> else <s2>

where <p> is a predicate (see A.3.4.3 below), and <s1> and 
<s2> are either basic status symbols (M,O,O.<n>, or X) or 

the symbol . An extended condition is of the formif <p1> then <s1> else <s2>
else if <p2> then <s2>
[else if <p3> ...]
else <sn>

where <p1> etc. are predicates and <s1> etc. are basic 
status symbols or .
The status symbol applicable to an item governed by a sim
ple condition is <s1> if the predicate of the condition is 
true, and <s2> otherwise; the status symbol applicable to an 
item governed by an extended condition is <si> where <pi> 
is the first true predicate, if any, in the sequence <p1>, 
<p2>..., and <sn> if no predicate is true.
A.3.4.3 Predicates
A simple predicate in a condition is either
a)a single item reference; or
b)a relation containing a comparison operator (=, <, etc.) 
with one (or both) of its operands being an item refer
ence for an item taking numerical values as its answer.
In case (a) the predicate is true if the item referred to is 
marked as supported, and false otherwise. In case (b), the 
predicate is true if the relation holds when each item refer
ence is replaced by the value entered in the Support column 
as answer to the item referred to.
Compound predicates are boolean expressions constructed 
by combining simple predicates using the boolean operators 
AND, OR and NOT, and parentheses, in the usual way. A 
compound predicate is true if and only if the boolean ex
pression evaluates to true when the simple predicates are in
terpreted as described above.
Items whose references are used in predicates are indicated 
by an asterisk in the Item column.
A.3.4.4 Answering conditional items
To answer a conditional item, the predicate(s) of the condi
tion is (are) evaluated as described in A.3.4.3 above, and 
the applicable status symbol is determined as described in 
A.3.4.2. If the status symbol is  this indicates that the 
item is to be marked in this case; otherwise, the Support 
column is to be completed in the usual way.
When two or more basic status symbols appear in a condi
tion for an item, the Support column for the item contains 
one line for each such symbol, labelled by the relevant sym
bol. the answer for the item is to be marked in the line la
belled by the symbol selected according to the value of the 
condition (unselected lines may be crossed out for added 
clarity).
For example, in the item illustrated below, the N/A column 
would be marked if neither predicate were true; the answer 

line labelled M: would be marked if item A4 was marked as supported,
and the answer line labelled O: would be marked if 
the condition including items D1 and B52 applied.Item

References
Status
N/A
Support
H3
Is ... supported?
42.3(d)
C.1

M: Yes  
O:  Yes    No 
C.1if A4 then M
else if D1 AND (B52 < 3) then O else 

A.4 Identification
A.4.1 Implementation IdentificationSupplierContact point for
queriesabout this PICSImplementation Name(s)and Version(s)Operating
systemName(s and Version(s)Other Hardware and Operating
SystemsClaimedSystem Name(s)(if different)Notes:
a)Only the first three items are required for all implementations; others may be 
completed as appropriate in meeting the requirements for full identification.
b)The terms Name and Version should be interpreted appropriately to correspond 
with a supplier's terminology (using, e.g., Type, Series, Model)

A.4.2 Protocol Summary: ISO 10589:19xxProtocol VersionAddenda
Implemented(if applicable)AmmendmentsImplementedDate of StatementHave
any Exception items been required (see A.3.3)?	No 	Yes 
(The answer Yes means that the implementation does not conform to ISO 10589)

PICS Proforma: Item

References
Status
N/A
Support
AllIS
Are all basic ISIS routeing functions 
implemented?
12.1.2
M

M: Yes  

C.1if L2IS then O else 
C.2if 8208 then O else 
PartitionRe
pair
Is Level 1 Partition Repair imple
mented?
12.1.2.f
C.1

O:  Yes    No 
L1IS
Are Level 1 ISIS routeing functions 
implemented?
12.1.3
M

M: Yes  
L2IS
Are Level 2 ISIS routeing functions 
implemented?
12.1.4
O

O:  Yes    No 
PtPt
Are point-to-point circuits imple
mented?
12.1.2.t
O.1

O:  Yes    No 
8208
Are ISO 8208  circuits implemented?
12.1.2.u
O.1

O:  Yes    No 
LAN
Are broadcast  circuits implemented?
12.1.2.v
O.1

O:  Yes    No 
EqualCost
Paths
Is computation of equal minimum cost 
paths implemented?
7.2.6
O

O:  Yes    No 
Downstream
Is computation of downstream routes 
implemented?
7.2.6
O

O:  Yes    No 
DelayMetric
Is path computation based on the delay 
metric implemented?
7.2.2
O

O:  Yes    No 
ExpenseMet
ric
Is path computation based on the Ex
pense metric implemented?
7.2.2
O

O:  Yes    No 
Prefixes
Are Reachable Address Prefixes imple
mented?
12.1.2.j
C.1

O:  Yes    No 
Forward

ingRate
How many ISO 8473 PDUs can the im
plementation forward per second?
12.2.5.1.b
M

	           PDUs/sec            
L2 ISCount
How many Level 2 ISs does the imple
mentation support?
12.2.5.1.
C.1

N =            
call

Estab

lish

ment

Met

ricIncrement
Are non-zero values of the call

Estab

lish

ment

Met

ricIncrement supported?
12.1.2.u.3
C.2

O:  Yes    No 
L1 ISCount
How many Level 1 ISs does the imple
mentation support?
12.2.5.1.
M

N =            
ReversePath
Cache
Is the 8208 Reverse Path Cache sup
ported?
12.1.2.u.4
C.2

O:  Yes    No 
ErrorMetric
Is path computation based on the Error 
metric implemented?
7.2.2
O

O:  Yes    No 
ISO 10589:19xx

PICS Proforma: Item

References
Status
N/A
Support
C.1if L2IS then O else 
C.2if 8208 then O else 
ID field 
Length
What values of the routeingDomain

ID

Length are supported by this imple
mentation?
7.1.1
M

Values =            
Is the value Se
table by System 
Man

agement?
Yes    No 
PDU Authen
tication
Is PDU Authentication based on Pass
words implemented?
12.1.2.z
O

O:  Yes    No 
ISO 10589:19xx (continued)

Annex B 
Supporting Technical Material
(This annex is informative) 

B.1 Matching of Address Prefixes
The following example shows how address prefixes may be 
matched according to the rules defined in 7.1.4.
The prefix 
	37-123 
matches both the full NSAP addresses 
	37-1234::AF< and 
	37-123::AF< 
which are encoded as
	3700000000001234AF< and
	3700000000000123AF< 
respectively.
This can be achieved by first converting the address to be 
compared to an internal decoded form (i.e. any padding, as 
indicated by the particular AFI, is removed), which corre
sponds to the external representation of the address. The 
position of the end of the IDP must be marked, since it can 
no longer be deduced. This is done by inserting the semi-
octet F after the last semi-octet of the IDP. (There can be 
no confusion, since the abstract syntax of the IDP is deci
mal digits).
Thus the examples above become in decoded form
	371234FAF< and 
	37123FAF<
 and the prefix 37-123 matches as a leading sub-string of 
both of them.
For comparison purposes the prefix is converted to the in
ternal decoded form as above.
B.2 Addressing and Routeing
In order to ensure the unambiguous identification of Net
work and Transport entities across the entire OSIE, some 
form of address administration is mandatory. ISO 
8348/Add.2 specifies a hierarchical structure for network 
addresses, with a number of top-level domains responsible 
for administering addresses on a world-wide basis. These 
address registration authorities in turn delegate to sub-
authorities the task of administering portions of the address 
space. There is a natural tendency to repeat this sub-
division to a relatively fine level of granularity in order to 
ease the task of each sub-authority, and to assign responsi
bility for addresses to the most localised administrative 

body feasible. This results in (at least in theory) reduced 
costs of address administration and reduced danger of mas
sive address duplication through administrative error. Fur
thermore, political factors come into play which require the 
creation of sub-authorities in order to give competing inter
ests the impression of hierarchical parity. For example at 
the top level of the ISO geographic address space, every 
country is assigned an equally-sized portion of the address 
space even though some countries are small and might in 
practice never want to undertake administration of their 
own addresses. Other examples abound at lower levels of 
the hierarchy, where divisions of a corporation each wish to 
operate as an independent address assignment authority 
even though this is inefficient operationally and may waste 
monumental amounts of potential address space.
If network topologies and traffic matrices aligned naturally 
with the hierarchical organisation of address administration 
authorities, this profligate use of hierarchy would pose little 
problem, given the large size (20 octets) of the N-address 
space. Unfortunately, this is not usually the case, especially 
at higher levels of the hierarchy. Network topologies may 
cross address administration boundaries in many cases, for 
example:
-Multi-national Corporations with a backbone network 
that spans several countries
-Community-of-interest networks, such as academic or 
research networks, which span organisations and ge
ographies
-Military networks, which follow treaty alignments 
rather than geographic or national administrations
-Corporate networks where divisions at times operate 
as part of a contractor's network, such as with trade 
consortia or government procurements.
These kinds of networks also exhibit rich internal topolo
gies and large scale (105 systems), which require sophisti
cated routeing technology such as that provided by this In
ternational Standard. In order to deploy such networks ef
fectively, a considerable amount of address space must be 
left over for assignment in a way which produces efficient 
routes without undue consumption of memory and 
bandwidth for routeing overhead11This is just a fancy way of saying
that hierarchical routing, with its natural effect on address
assignment, is a mandatory requirement for such net
works.
.
Similarly important is the inter-connection of these net
works via Inter-domain routeing technology. If all of the as
signment flexibility of the addressing scheme is exhausted 
in purely administrative hierarchy (at the high-order end of 
the address) and in Intra-Domain routeing assignment (at 
the low end of the address) there may be little or no address 

space left to customise to the needs of inter-domain routing. 
The considerations for how addresses may be structured for 
the Intra- and Inter-domain cases are discussed in more de
tail in the following two clauses.
B.2.1 Address Structure for Intra-domain 
Routeing
The IS-IS Intra-domain routeing protocol uses a preferred 
addressing scheme. There are a number of reasons the de
signers of this protocol chose to specify a single address 
structure, rather than leaving the matter entirely open to the 
address assignment authorities and the routeing domain ad
ministrators:
a)If one address structure is very common and known a 
priori, the forwarding functions can be made much 
faster;
b)If part of the address is known to be assigned locally 
to an end system, then the routeing can be simpler, use 
less memory, and be potentially faster, by not having 
to discriminate based on that portion of the address.
c)If part of the address can be designated as globally 
unique by itself (as opposed to only the entire address 
having this property) a number of benefits accrue:
1)Errors in address administration causing duplicate 
addresses become much less likely
2)Automatic and dynamic NSAP address assignment 
becomes feasible without global knowledge or 
synchronisation
3)Routeing on this part of the address can be made 
simple and fast, since no address collisions will oc
cur in the forwarding database.
d)If a part of the address can be reserved for assignment 
purely on the basis of topological efficiency (as op
posed to political or address administration ease), hier
archical routeing becomes much more memory and 
bandwidth efficient, since the addresses and the topol
ogy are in close correspondence.
e)If an upper bound can be placed on the amount of ad
dress space consumed by the Intra-domain routeing 

scheme, then the use of address space by Inter-domain 
routeing can be made correspondingly more flexible.
The preferred address format of the Intra-domain ISIS 
protocol achieves these goals by being structured into two 
fixed-sized fields as follows shown in figure 91#ID#81Used by level 1
routeingKey:Used by level 2 routeingID
SEL
HO-DSP
IDP
IDP	Initial Domain Part
HO-DSP	High Order Domain Specific Part
ID	System Identifier
SEL	NSAP Selector
Figure 9 - Preferred Address Format

 below:
The field marked IDP in the figure is precisely the IDP 
specified in  ISO 8348/Add.2. The field marked HO-DSP 
is that portion of the DSP from ISO 8348/Add.2 whose 
structure, assignment, and meaning are not specified or 
constrained by the Intra-domain ISIS routeing protocol. 
However, the design presumes that the routeing domain ad
ministrator has at least some flexibility in assigning a por
tion of the HO-DSP field. The purpose and usage of the 
fields specified by the Intra-domain ISIS routeing protocol 
is explained in the following paragraphs.
B.2.1.1 The IDP + HO-DSP
Since the Intra-domain ISIS protocol is customised for op
eration with ISO 8473, all addresses are specified to use the 
preferred binary encoding of ISO 8348/Add.2.
B.2.1.2 The Selector (SEL) Field
The SEL field is intended for two purposes. Its main use is 
to allow for multiple higher-layer entities in End systems 
(such as multiple transport entities) for those systems which 
need this capability. This allows up to 256 NSAPs in a sin
gle End system. The advantage of reserving this field exclu
sively for local system administration the Intra-domain 
routing functions need not store routeing information about, 
nor even look at this field. If each individual NSAP were 
represented explicitly in routing tables, the size of these ta
bles would grow with the number of NSAPs, rather than 
with the number of End systems. Since Intra-domain rout
ing routes to systems, explicit recording of each NSAP 
brings no efficiency benefit and potentially consumes large 
amounts of memory in the Intermediate systems.
A second use for the SEL field is in Intermediate systems. 
Certain ISIS functions require that PDUs be encapsulated 
and sent to the Network Entity in an Intermediate system 
rather than to an NSAP and upward to a Transport entity. 
An example of this is the Partition Repair function of this 
International Standard. In order to use a level 2 path as if it 
were a single subnetwork in a level 1 area, PDUs are encap

sulated and addressed to an IS on the other side of the parti
tion11This is a gross oversimplification for the purpose of
illustrating the need for the SEL field. See 7.2.10.
.  By reserving certain values of the SEL field in Inter
mediate systems for direct addressing of Intermediate sys
tem Network entities, the normal addressing and relaying 
functions of other Intermediate systems can be transpar
ently used for such purposes.
B.2.1.3 The Identifier (ID) Field
The ID field is a flat, large identifier space for identifying 
OSI systems. The purpose of this field is to allow very fast, 
simple routeing to a large (but not unconstrained) number 
of End systems in a routeing domain. The Intra-Domain IS
IS protocol uses this field for routeing within a area. While 
this field is only required to be unambiguous within a single 
area, if the values are chosen to be globally unambiguous 
the Intra-domain ISIS design can exploit this fact in the 
following ways.
First, a certain amount of parallelism can be obtained dur
ing relaying. An IS can be simultaneously processing the ID 
field along with other fields (i.e. IDP, HO-DSP). If the ID 
is found in the forwarding table, the IS can initiate forward
ing while checking to make sure that the other fields have 
the expected value. Conversely, if the ID is not found the 
IS can assume that either the addressed NSAP is unreach
able or exists only in some other area or routeing domain. 
In the case where the ID is not globally unique, the for
warding table can indicate this fact and relaying delayed 
until the entire address is analysed and the route looked up.
Second, a considerable savings can be obtained in manual 
address administration for all systems in the routeing do
main. If the ID is chosen from the ISO 8802 48-bit address 
space, the ID is known to be globally unique. Furthermore, 
since LAN systems conforming to ISO 8802 often have 
their 48-bit MAC address stored in ROM locally, each sys
tem can be guaranteed to have a globally unambiguous 
NET and NSAP(s) without centralised address administra
tion at the area level.22Note, however, that the use of the ISO 8802
addresses does not avoid the necessity to run ISO 9542 or to maintain
tables mapping NSAP addresses to 
MAC (i.e. SNPA) addresses on the ISO 8802 subnetwork. This is because
there is no guarantee that a particular MAC address is always enabled (the LAN 
controller may be turned off) or that a system has only a single MAC address.
  This not only eliminates administra
tive overhead, but also drastically reduces the possibility of 
duplicate NSAP addresses, which are illegal, difficult to di
agnose, and often extremely difficult to isolate.
An alternative to a large, flat space for the lowest level of 
routeing would be to hierarchically subdivide this field to 
allow more levels of routeing within a single routeing do
main. The designers of the Intra-domain ISIS protocol 
considered that this would lead to an inferior routeing archi
tecture, since:
a)The cost of memory in the ISs was sufficiently reason
able that large (e.g. 104 system) areas were quite fea
sible, thus requiring at least 2 octets per level to ad
dress
b)Two levels of routeing within a routeing domain were 
sufficient (allowing domains of 106107 systems) be
cause it was unlikely that a single organisation would 
wish to operate and manage a routeing domain much 
larger than that.

c)Administrative boundaries often become the dominant 
concern once routeing domains reach a certain size.
d)The additional burdens and potential for error in man
ual address assignment were deemed serious enough 
to permit the use of a large, flat space. 
B.3 Use of the HO-DSP field in 
Intra-domain routeing
Use of a portion of the HO-DSP field provides for hierar
chical routeing within a routeing domain. A value is as
signed to a set of ISs in order to group the ISs into a single 
area for the usual benefits of hierarchical routeing:
a)Limiting the size of routeing tables in the ISs;
b)conserving bandwidth by hierarchical summarisation 
of routeing information;
c)designating portions of the network which are to have 
optimal routeing within themselves; and
d)moderate firewalling of portions of the routeing do
main from failures in other portions.
It is important to note that the assignment of HO-DSP val
ues is intended to provide the routeing domain administra
tor with a mechanism to optimise the routeing within a 
large routeing domain. The Intra-domain ISIS designers 
did not intend the HO-DSP to be entirely consumed by 
many levels of address registration authority. Reserving the 
assignment of a portion of the HO-DSP field to the route
ing domain administrator also allows the administrator to 
start with a single assigned IDP+HO-DSP and run the 
routing domain as a single area. As the routeing domain 
grows, the routeing domain administrator can then add ar
eas without the need to go back to the address administra
tion authority for further assignments. Areas can be added 
and re-assigned within the routeing domain without involv
ing the external address administration authority.
A useful field to reserve as part of the HO-DSP would be 2 
octets,permitting up to 65,536 areas in a routeing domain. 
This is viewed as a reasonable compromise between route
ing domain size and address space consumption. The field 
may be specified as flat for the same reasons that the ID 
field may be flat.
B.3.1 Addressing considerations for 
Inter-domain Routeing
It is in the Inter-domain arena where the goals of routeing 
efficiency and administrative independence collide most 
strongly. Although the OSI Routeing Framework explicitly 
gives priority in Inter-domain routeing to considerations of 
autonomy and firewalls over efficiency, it must be feasible 
to construct an Inter-Domain topology that both produces 
isolable domains and relays data at acceptable cost. Since 

no routeing information is exchanged across domain 
boundaries with static routeing, the practicality of a given 
Inter-domain topology is essentially determined by the size 
of the routeing tables that are present at the boundary ISs. If 
these tables become too large, the memory needed to store 
them, the processing needed to search them, and the 
bandwidth needed to transmit them  within the routeing do
main all combine to disallow certain forms of 
interconnection.
Inter-domain routeing primarily computes routes to other 
routeing domains33This International Standard also uses static
Inter-domain tables for routeing to individual End systems across
dynamically assigned circuits, and also to 
End systems whose addresses do not conform to the address construction rules. 
. If there is no correspondence between 
the address registration hierarchy and the organisation of 
routeing domains (and their interconnection) then the task 
of static table maintenance quickly becomes a nightmare, 
since each and every routeing domain in the OSIE would 
need a table entry potentially at every boundary IS of every 
other routeing domain. Luckily, there is some reason to be
lieve that a natural correspondence exists, since at least at 
the global level the address registration authorities fall 
within certain topological regions. For example, most of the 
routeing domains which obtained their IDP+HO-DSP 
from a hierarchy of French authorities are likely to reside in 
France and be more strongly connected with other routeing 
domains in France that with routeing domains in other 
countries.
There are enough exceptions to this rule, however, to be a 
cause for concern. The scenarios cited in B.2 all exist today 
and may be expected to remain common for the foreseeable 
future. Consider as a practical case the High Energy Phys
ics Network (HEPnet), which contains some 17000 End 
systems, and an unknown number of intermediate systems44The number of
ISs is hard to estimate since some ISs and links are in fact shared
with other networks, such as the similarly organised NASA Space 
Physics network, or SPAN.
. 
This network operates as a single routeing domain in order 
to provide a known set of services to a known community 
of users, and is funded and cost-justified on this basis. This 
network is international in scope (at least 10 countries in 
North America, Europe, and the far east) and yet its topol
ogy does not map well onto existing national boundaries. 
Connectivity is richer between CERN and FERMIlab, for 
example than between many points within the U.S.
More importantly, this network has rich connectivity with a 
number of other networks, including the PDNs of the vari
ous countries, the NSFnet in the U.S., the international 
ESnet (Energy Sciences Network), the general research 
Internet, and military networks in the U.S. and elsewhere. 
None of these other networks shares a logical part of the 
NSAP address hierarchy with HEPnet55It is conceivable that ISO would
sanction such networks by assigning a top-level IDI from the ISO
non-geographic AFI, but this is unlikely and would 
only exacerbate the problem if many such networks were assigned
top-level registrations.
 .  If the only method 
of routing from the HEPnet to these other networks was to 
place each within one and only one of the existing registra
tion authorities, and to build static tables showing these re
lationships, the tables would clearly grow as O(n2). 
It seems therefore, that some means must be available to as
sign addresses in a way that captures the Inter-Domain to
pology, and which co-exists cleanly with both the adminis
trative needs of the registration authorities, and the algo
rithms employed  by both the Intra- and Inter-domain 

routeing protocols. As alluded to in an earlier clause, it 
seems prudent to leave some portion of the address space 
(most likely from the HO-DSP part) sufficiently undefined 
and flexible that various Inter-domain topologies may be 
efficiently constructed.

Annex C 
Implementation Guidelines and Examples
(This annex is informative) 

C.1  Routeing Databases
Each database contains records as defined in the following 
sub-clauses. The following datatypes are defined.
FROM CommonMgmt IMPORT NSAPAddress, 
AddressPrefix, BinaryAbsoluteTime; 
PDU Type

lspID = ARRAY [0..7] OF Octet; 
systemID = ARRAY [0..5] OF Octet;
octetTimeStamp = BinaryAbsoluteTime;

C.1.1 Level 1 Link State Database
This database is kept by Level 1 and Level 2 Intermediate 
Systems, and consists of the latest Level 1 Link State PDUs 
from each Intermediate System (or pseudonode) in the area. 
The Level 1 Link State PDU lists Level 1 links to the Inter
mediate System that originally generated the Link State 
PDU. 
RECORD 
adr: lspID; 	(* 8 octet ID of LSP originator 
*)  
type: (Level1IntermediateSystem,
AttachedLevel2IntermediateSystem,
UnattachedLevel2IntermediateSystem);  
seqnum: [0..SequenceModulus  1];  
LSPage: [0..MaxAge]; 	(*Remaining Lifetime *)  

expirationTime: TimeStamp; 	
(*Time at which LSP age 
became zero (see 7.3.16.4). *)
SRMflags: ARRAY[1..(maximumCircuits + 
maximumVirtualAdjacencies)]
OF BOOLEAN;
(*Indicates this LSP to be sent on this circuit. Note 
that level 2 Intermediate systems may send level 1 
LSPs to other partitions (if any exist). Only one level 
2 Intermediate system per partition does this. For 
level 1 Intermediate Systems the array is just 
maximumCircuits long. *)
SSNflags: ARRAY[1..maximumCircuits + 
maximumVirtualAdjacencies] 
OF BOOLEAN; 
(*Indicates that information about this LSP shall be 
included in the next partial sequence number PDU 
transmitted on this circuit. *) 
POINTER TO LSP;	(*The received LSP *)
END;

C.1.2 Level 2 Link State Database
This database is kept by Level 2 Intermediate Systems, and 
consists of the latest Level 2 Link State PDUs from each 
Level 2 Intermediate System (or pseudonode) in the do
main.  The Level 2 Link State PDU lists Level 2 links to the 
Intermediate System that originally generated the Link 
State PDU. 
RECORD  
adr: lspID;  (* 8 octet ID of LSP originator *)  
type: (AttachedLevel2IntermediateSystem,
UnattachedLevel2IntermediateSystem);  
seqnum: [0..SequenceModulus  1];  
LSPage: [0..MaxAge];  (*Remaining Lifetime *)  
expirationTime: TimeStamp;	
(*Time at which LSP age 
became zero (see 7.3.16.4). *)
SRMflags: ARRAY[1..(maximumCircuits)] OF 
BOOLEAN;
(*Indicates this LSP to be sent on this circuit. *)
SSNflags: ARRAY[1..maximumCircuits] OF 
BOOLEAN; 
(*Indicates that information about this LSP must be 
included in the next partial sequence number PDU 
transmitted on this circuit. *)
POINTER TO LSP; (*The received LSP *)
END;
C.1.3 Adjacency Database
 This database is kept by all systems. Its purpose is to keep 
track of neighbours.
For Intermediate systems, the adjacency database comprises 
a database with an entry for each:
-Adjacency on a Point to Point circuit. 
-Broadcast Intermediate System Adjacency. (Note that 
both a Level 1 and a Level 2 adjacency can exist be
tween the same pair of systems.)
-Broadcast End system Adjacency. 
-potential SVC on a DED circuit (max

i

mum

SVC

Adja

cencies for a DA circuit, or 1 for a Static cir
cuit). 
-Virtual Link Adjacency.
Each entry contains the parameters in Clause 11 for the Ad
jacency managed object. It also contains the variable used 
to store the remaining holding time for each Adjacency 
IDEntry and NETEntry entry, as defined below. 

IDEntry =  RECORD
ID: systemID;
(* The 6 octet System ID of a neighbour End system 
extracted from the SOURCE ADDRESS field of its 
ESH PDUs. *)
entryRemainingTime: Unsigned [1..65535]
(* The remaining holding time in seconds for this 
entry.  This value is not accessible to system 
management. An implementation may choose to 
implement the timer rules without an explicit 
remainingTime being maintained. For example by 
the use of asynchronous timers. It is present here in 
order to permit a consistent description of the timer 
rules. *)
END

NETEntry =  RECORD
NET: NetworkEntityTitle;
(* The NET of a neighbour Intermediate system 
as reported in its IIH PDUs. *)
entryRemainingTime: Unsigned [1..65535]
 (* The remaining holding time in seconds for this 
entry.  This value is not accessible to system 
management.  An implementation may choose to 
implement the timer rules without an explicit 
remainingTime being maintained.  For example by 
the use of asynchronous timers. It is present here in 
order to permit a consistent description of the timer 
rules. *)
END;
C.1.4 Circuit Database
This database is kept by all systems. Its purpose is to keep 
information about a circuit. It comprises an AR
RAY[1..maximumCircuits].
Each entry contains the parameters in Clause 11 for a Cir
cuit managed object (see 11.3). It also contains the remain
ingHelloTime (WordUnsigned [1..65535] seconds) vari
able for the Circuit. This variable not accessible to system 
management. An implementation may choose to implement 
the timer rules without an explicit remainingHelloTime 
being maintained. For example by the use of asynchronous 
timers. It is present here in order to permit a consistent de
scription of the timer rules. Additionally, for Circuits of 
type  X.25 Static Outgoing or X.25 DA, it contains the 
recallCount (Unsigned[0..255]) variable for the Circuit. 
This variable is not accessible to system management. It 
used to keep track of recall attempts. 
C.1.5 Level 1 Shortest Paths Database 
This database is kept by Level 1 and Level 2 Intermediate 
Systems (unless each circuit is Level 2 Only).  It is com
puted by the Level 1 Decision Process, using the Level 1 
Link State Database. The Level 1 Forwarding Database is a 
subset of this database. 
RECORD
adr: systemId; (*6 octet ID of destination system *)
cost: [1..MaxPathMetric];
(*Cost of best path to destination system *)
adjacencies: ARRAY[1..max

i

mum

Path

Splits] 
OF POINTER TO Adjacency;

(*Pointer to adjacency for forwarding to system adr 
*)
END;
C.1.6 Level 2 Shortest Paths Database 
This database is kept by Level 2 Intermediate Systems. It is 
computed by the Level 2 Decision Process, using the 
Level 2 Link State Database. The Level 2 Forwarding Data
base is a subset of this database. 
RECORD 
adr: AddressPrefix;	(*destination prefix *)
cost: [1..MaxPathMetric];	
(*Cost of best path to destination prefix *)
adjacencies: ARRAY[1..max

i

mum

Path

Splits] 
OF  POINTER TO Adjacency;
(*Pointer to adjacency for forwarding to prefix adr 
*)
END;

C.1.7 Level 1 Forwarding Database 
This database is kept by Level 1 and Level 2 Intermediate 
Systems (unless each circuit is Level 2 Only).  It is used 
to determine where to forward a data NPDU with destina
tion within this system's area. It is also used to determine 
how to reach a Level 2 Intermediate System within the area, 
for data PDUs with destinations outside this system's area.
RECORD  
adr:systemId;
(*6 octet ID of destination system. Destination 
0 is special, meaningnearest level 2 
Intermediate system *)
splits: [0..max

i

mum

Path

Splits];
(* Number of valid output adj's for reachingadr 
(0 indicates it is unreachable) *)
 nextHop: ARRAY[1..max

i

mum

Path

Splits] OF 
POINTER TO adjacency; 
(*Pointer to adjacency for forwarding to destination 
system *)
END;

C.1.8 Level 2 Forwarding Database
This database is kept by Level 2 Intermediate systems. It is 
used to determine where to forward a data NPDU with des
tination outside this system's area.
RECORD  
adr: AddressPrefix; 	(*address of destination area. 
*)
splits: [0..max

i

mum

Path

Splits];
(*Number of valid output adj's for reaching adr 
(0 indicates it is unreachable) *)
nextHop: ARRAY[1..max

i

mum

Path

Splits] OF 
POINTER TO adjacency;  
(*Pointer to adjacency for forwarding to destination 
area. *)
END;

C.2 SPF Algorithm for Computing 
Equal Cost Paths
An algorithm invented by Dijkstra (see references) known 
as shortest path first (SPF), is used as the basis for the 
route calculation. It has a computational complexity of the 
square of the number of nodes, which can be decreased to 
the number of links in the domain times the log of the num
ber of nodes for sparse networks (networks which are not 
highly connected). 
A number of additional optimisations are possible: 
a)If the routeing metric is defined over a small finite 
field (as in this International Standard), the factor of 
log n may be removed by using data structures which 
maintain a separate list of systems for each value of 
the metric rather than sorting the systems by logical 
distance. 
b)Updates can be performed incrementally without re
quiring a complete recalculation. However, a full up
date must be done periodically to recover from data 
corruption, and studies suggest that with a very small 
number of link changes (perhaps 2) the expected com
putation complexity of the incremental update exceeds 
the complete recalculation. Thus, this International 
Standard specifies the algorithm only for the full up
date. 
c)If only End system LSP information has changed, it is 
not necessary to re-compute the entire Dijkstra tree for 
the IS. If the proper data structures exist, End Systems 
may be attached and detached as leaves of the tree and 
their forwarding information base entries altered as 
appropriate
The original SPF algorithm does not support load splitting 
over multiple paths. The algorithm in this International 
Standard does permit load splitting by identifying a set of 
equal cost paths to each destination rather than a single 
least cost path.
C.2.1 Databases
PATHS  This represents an a

cyclic directed graph of 
shortest paths from the system S performing the cal
culation. It is stored as a set of triples of the form 
aN,d(N),{Adj(N)}q, where:
	N is a system Identifier. In the level 1 algorithm, N is 
a 7 octet ID. For a non-pseudonode it is the 6 octet 
system ID, with a 0 appended octet.  For a 
pseudonode it is a true 7 octet quantity, comprised of 
the 6 octet Designated Intermediate System ID and 
the extra octet assigned by the Designated Interme
diate System.  In the level 2 algorithm it is either a 
7 octet Intermediate System or pseudonode ID (as in 
the level 1 algorithm), or it is a variable length ad
dress prefix (which will always be a leaf, i.e. End 
system, in PATHS).
	d(N) is N's distance from S (i.e. the total metric 
value from N to S). 

{Adj(N)} is a set of valid adjacencies that S may use 
for forwarding to N.
	When a system is placed on PATHS, the path(s) 
designated by its position in the graph is guaranteed 
to be a shortest path.
TENT  This is a list of triples of the form 
aN,d(N),{Adj(N)}q, where N, d(N) and {Adj(N)} are 
as defined above for PATHS.
	TENT can intuitively be thought of as a tentative 
placement of a system in PATHS. In other words, 
the triple aN,x,{A}q in TENT means that if N were 
placed in PATHS, d(N) would be x, but N cannot be 
placed on PATHS until it is guaranteed that no path 
shorter than x exists.
	The triple aN,x,{A,B}q in TENT means that if N 
were placed in PATHS, d(N) would be x via either 
adjacency A or B
NOTE - As described above, (see 7.2.6), it is suggested that 
the implementation keep the database TENT as a set of lists 
of triples of the form a*,Dist,*q, for each possible distance 
Dist.  In addition it is necessary to be able to process those 
systems which are pseudonodes before any non-
pseudonodes at the same distance Dist.
C.2.2 Use of Metrics in the SPF Calculation
Internal metrics are not comparable to external metrics. 
Therefore, the cost of the path from N to S for external 
routes (routes to destinations outside of the routing domain) 
may include both internal and external metrics. The cost of 
the path from N to S (called d(N) below in database 
PATHS) may therefore be maintained as a two-
dimensioned vector quantity (specifying internal and exter
nal metric values). In incrementing d(N) by 1, if the internal 
metric value is less than the maximum value 
MaxPathMetric, then the internal metric value is incre
mented by one and the external metric value left un
changed; if the internal metric value is equal to the maxi
mum value MaxPathMetric, then the internal metric value 
is set to 0 and the external metric value is incremented by 1. 
Note that this can be implemented in a straightforward 
manner by maintaining the external metric as the high order 
bits of the distance.
NOTE - In the code of the algorithm below, the current path 
length is held in a variable tentlength. This variable is a 
two-dimensional quantity tentlength=(internal,external) 
and is used for comparing the current path length with d(N) 
as described above.
C.2.3 Overview of the Algorithm
The basic algorithm, which builds PATHS from scratch, 
starts out by putting the system doing the computation on 
PATHS (no shorter path to SELF can possibly exist).  
TENT is then pre-loaded from the local adjacency data
base.
Note that a system is not placed in PATHS unless no 
shorter path to that system exists. When a system N is 
placed in PATHS, the path to each neighbour M of N, 

through N, is examined, as the path to N plus the link from 
N to M. If aM,*,*q is in PATHS, this new path will be 
longer, and thus ignored.
If aM,*,*q is in TENT, and the new path is shorter, the old 
entry is removed from TENT and the new path is placed in 
TENT. If the new path is the same length as the one in 
TENT, then the set of potential adjacencies {adj(M)}  is set 
to the union of the old set (in TENT) and the new set 
{adj(N)}. If M is not in TENT, then the path is added to 
TENT.
Next the algorithm finds the triple aN,x,{Adj(N)}q in 
TENT, with minimal x. 
NOTE - This is done efficiently because of the optimisation 
described above. When the list of triples for distance Dist is 
exhausted, the algorithm then increments Dist until it finds a 
list with a triple of the form a*,Dist,*q. 
N is placed in PATHS. We know that no path to N can be 
shorter than x at this point because all paths through sys
tems already in PATHS have already been considered, and 
paths through systems in TENT will have to be greater than 
x because x is minimal in TENT.
When TENT is empty, PATHS is complete.
C.2.4 The Algorithm
The Decison Process Algorithm must be run once for each 
supported routeing metric.  A Level 1 Intermediate System 
runs the algorithm using the Level 1 LSP database to com
pute Level 1 paths.  In addition a Level 2 Intermediate Sys
tem runs the algorithm using the Level 2 LSP database to 
compute Level 2 paths.
If this system is a Level 2 Intermediate System which sup
ports the partition repair optional function the Decision 
Process algorithm for computing Level 1 paths must be run 
twice for the default  metric.  The first execution is done to 
determine which of the area's manual

Area

Addresses 
are reachable in this partition, and elect a Partition Desig
nated Level 2 Intermediate System for the partition.  The 
Partition Designated Level 2 Intermediate System will de
termine if the area is partitioned and will create virtual 
Level 1 links to the other Partition Designated Level 2 In
termediate Systems in the area in order to repair the Level 1 
partition.  This is further described in 7.2.10. 
Step 0:  Initialise TENT and PATHS to empty.  Initialise 
tentlength to (0,0).
(tentlength is the pathlength of elements in TENT 
we are examining.)
a)Add aSELF, 0, Wq to PATHS, where W is a special 
value indicating traffic to SELF is passed up to Trans
port (rather than forwarded).
b)Now pre-load TENT with the local adjacency data
base. (Each entry made to TENT must be marked as 
being either an End system or an Intermediate System 
to enable the check at the end of Step 2 to be made 
correctly.) For each adjacency Adj(N), (including 
Manual Adjacencies, or for Level 2 enabled Reach

able Addresses) on enabled circuits, to system N of 
SELF in state Up, compute 
d(N) = cost of the parent circuit of the adjacency 
(N), obtained from metrick, where k = one of de
fault metric, delay metric, monetary metric, er
ror metric.
Adj(N) =  the adjacency number of the adjacency 
to N
c)If a triple aN,x,{Adj(M)}q is in TENT,  then:
If x = d(N), then Adj(M) , {Adj(M)} H Adj(N).
d)If there are now more adjacencies in {Adj(M)} than 
max

i

mum

Path

Splits, then remove excess adjacen
cies as described in 7.2.7.
e)If x < d(N), do nothing.
f)If x > d(N),  remove aN,x,{Adj(M)}q from TENT and 
add  the triple aN,d(N),Adj(N)q.
g)If no triple aN, x,{Adj(M)}q is in TENT, then add aN, 
d(N),Adj(N)q to TENT. 
h)Now add any systems to which the local Intermediate 
system does not have adjacencies, but which are men
tioned in neighbouring pseudonode LSPs. The adja
cency for such systems is set to that of the Designated 
Intermediate System.
i)For all broadcast circuits in state On, find the LSP 
with LSP number zero and with the first  7 octets of 
LSPID equal to the LnCircuitID for that circuit (i.e. 
pseudonode LSP for that circuit). If it is present, for 
all the neighbours N reported in all the LSPs of this 
pseudonode which do not exist in TENT add an entry 
aN,d(N),Adj(N)q to TENT, where
d(N) = metrick of the circuit.
Adj(N) = the adjacency number of the adjacency to the 
DR.
j)Go to Step 2.
Step 1: Examine the zeroth Link State PDU of P, the sys
tem just placed on PATHS (i.e. the Link State PDU with 
the same first 7 octets of LSPID as P, and LSP number 
zero). 
a)If this LSP is present, and the LSP Database Over
load bit is clear, then for each LSP of P (i.e. all the 
Link State PDUs with the same first 7 octets of LSPID 
as P, irrespective of the value of LSP number) com
pute 
dist(P,N) = d(P) + metrick(P,N).

for each neighbour N (both Intermediate System and 
End system) of the system P. If the LSP Database 
Overload bit is set, only consider the End system 
neighbours of the system P. d(P) is the second ele
ment of the triple 

aP,d(P),{Adj(P)q

and metrick(P,N) is the cost of the link from P to N as 
reported in P's Link State PDU
b)If dist(P,N) > MaxPathMetric, then do nothing.
c)If aN,d(N),{Adj(N)}q is in  PATHS, then do nothing.
NOTE  d(N) must be less than dist(P,N), or else N 
would not have been put into PATHS. An additional san
ity check may be done here to ensure d(N) is in fact less 
than dist(P,N).
d)If a triple aN,x,{Adj(N)}q is in TENT,  then:
1)If x = dist(P,N), then Adj(N) , {Adj(N)} H 
Adj(P).
2)If there are now more adjacencies in {Adj(N)} than 
max

i

mum

Path

Splits, then remove excess adja
cencies, as described in 7.2.7.
3)If x < dist(P,N), do nothing.
4)If x > dist(P,N),  remove aN,x,{Adj(N)}q from 
TENT and add  aN,dist(P,N),{Adj(P)}q.
e)If no triple aN, x,{Adj(N)}q is in TENT, then add aN, 
dist(P,N),{P}q to TENT.
Step 2: If TENT is empty, stop, else:
a)Find the element aP,x,{Adj(P)}q,  with minimal x as 
follows:
1)If an element a*,tentlength,*q remains in TENT  
in the list for tentlength, choose that element. If 
there are  more than one elements in the list for 
tentlength, choose one of  the elements (if any) 
for a system which is a pseudonode in preference  
to one for a non-pseudonode. If there are no more 
elements in the list  for tentlength increment ten
tlength and repeat  Step 2. 
2)Remove aP,tentlength,{Adj(P)}q from TENT.
3)Add aP,d(P),{Adj(P)}q  to PATHS.
4)If this is the Level 2 Decision Process running, and 
the system just added to PATHS listed itself as 
Partition Designated Level 2 Intermediate system, 
then additionally add aAREA.P, d(P), {adj(P)}q to 
PATHS, where AREA.P is the Network Entity 
Title of the other end of the Virtual Link, obtained 
by taking the first AREA listed in P's Level 2 LSP 
and appending P's ID.
5)If the system just added to PATHS was an End 
system, go to Step 2, Else go to Step 1.
NOTE - In the Level 2 context, the End systems are the 
set of Reachable Address Prefixes and the set of area ad
dresses with zero cost. 

C.3 Forwarding Process
C.3.1 Example pseudo-code for the forwarding 
procedure described in 7.4.3
This procedure chooses, from the Level 1 forwarding data
base  if level is level1, or from the Level 2 forwarding 
database  if level is level2, an adjacency on which to for
ward PDUs for destination dest. A pointer to the adjacency 
is returned in adj, and the procedure returns the value 
True. If no suitable adjacency exists the procedure returns 
the value False, in which case a call should be made to 
Drop(Destination Address Unreachable, octetNumber).
If queue length values are available to the forwarding proc
ess, the minimal queue length of all candidate circuits is 
chosen, otherwise, they are used in round robin fashion.
PROCEDURE Forward(
level: (level1, level2),  
dest: NetworkLayerAddress, 
VAR adj: POINTER TO adjacency) : 
BOOLEAN

VAR  
adjArray: ARRAY OF 
ForwardingDatabaseRecords; 
temp, index, minQueue: CARDINAL; 

BEGIN 
(*Set adjArray to appropriate database} *) 
IF level = level1 THEN 
adjArray := level1ForwardingDatabase 
ELSE 
adjArray := level2ForwardingDatabase 
END;
 (*Perform appropriate hashing function to obtain an 
index into the database *)
 IF Hash(level, dest, index) THEN 
IF adjArray[index].splits > 0 THEN
(*Find minimum queue size for all equal cost 
paths *)
minQueue := MaxUnsigned;
temp := adjArray[index].lastChosen + 1; 	
(*start off after last time *)
FOR i := 1 TO adjArray[index].splits DO
(*for all equal cost paths to dest *)
IF temp > adjArray[index].splits THEN
(*after end of valid entries, wrap to first 
*)
temp := 1
ELSE
temp := temp + 1
END;
IF 
QueueSize(adjArray[index].nextHop[temp]) 
< minQueue THEN
minQueue := 
QueueSize(adjArray[index].nextHop[tem
p]);
adj := adjArray[index].nextHop[temp];
adjArray[index].lastChosen := temp;
END;
Forward := true
END;

ELSE
Forward := false (*There must be at least one 
valid output adjacency *)
END
ELSE 
Forward := false (*Hash returned destination 
unknown *)
END 
END forward; 

Annex D 
Congestion Control and Avoidance
(This annex is informative) 

D.1 Congestion Control
The transmit management subroutine handles congestion 
control. Transmit management consists of the following 
components:
Square root limiter. Reduces buffer occupancy 
time per PDU by using a square root limiter algo
rithm. The square root limiter also queues PDUs for 
an output circuit, and prevents buffer deadlock by 
discarding PDUs when the buffer pool is exhausted. 
Clause D.1.1 specifies the Square Root Limiter 
Process. 
Originating PDU limiter. Limits originating NPDU 
traffic when necessary to ensure that transit NPDUs 
are not rejected. An originating NPDU is an NPDU 
resulting from an NSDU from the Transport at this 
ES. A transit NPDU is an NPDU from another sys
tem to be relayed to another destination ES.
Flusher. Flushes PDUs queued for an adjacency that 
has gone down.
Information for higher layer (Transport) congestion control 
procedures is provided by the setting of the congestion ex
perienced bit in the forwarded data NPDUs.
D.1.1 Square Root Limiter
The square root limiter discards a data NPDU by calling the 
ISO 8473 discard PDU function with the reason  PDU 
Discarded due to Congestion when the number of data 
NPDUs on the circuit output queue exceeds the discard 
threshold, Ud.  Ud is given as follows:=
where:
Nb = Number of Routeing Layer buffers 
(maximumBuffers) for all output circuits.
Nc = Number of active output circuits (i.e. Circuits in state 
On).
The output queue is a queue of buffers containing data 
NPDUs which have been output to that circuit by the for
warding process, and which have not yet been transmitted 
by the circuit. It does not include NPDUs which are held 
by the data link layer for the purpose of retransmission.
Where a data NPDU is to be fragmented by this Intermedi
ate system over this circuit, each fragment shall occupy a 

separate buffer and shall be counted as such in the queue 
length. If the addition of all the buffers required for the 
fragmentation of a single input data NPDU would cause the 
discard threshold for that queue to be exceeded, it is recom
mended that all those fragments (including those which 
could be added without causing the threshold to be ex
ceeded) be discarded. 
D.1.2 Originating PDU Limiter
TEMPORARY NOTE - Strictly this function is an End Sys
tem function. However it is closely coupled to the routeing 
function, particularly in the case of real systems which are 
performing the functions of both an Intermediate System 
and an End System (i.e. systems which can both initiate and 
terminate data NPDUs and perform relaying functions). 
Therefore, until a more appropriate location for this infor
mation can be determined, this function is described here. 
The originating PDU limiter first distinguishes between 
originating NPDUs and transit NPDUs. It then imposes a 
limit on the number of buffers that originating NPDUs can 
occupy on a per circuit basis. In times of heavy load, origi
nating NPDUs may be rejected while transit NPDUs con
tinue to be routed. This is done because originating NPDUs 
have a relatively short wait, whereas transit NPDUs, if re
jected, have a long wait  a transport retransmission period. 
The originating PDU limiter accepts as input:
-An NSDU received from Transport Layer 
-A transmit complete signal from the circuit for an ISO 
8473 Data PDU.
The originating PDU limiter produces the following as out
put:
-PDU accepted 
-PDU rejected 
-Modifications to originating PDU counter
There is a counter, N, and an originating PDU limit, 
originatingQueueLimit, for each active output circuit. 
Each N is initialised to 0. The originatingQueueLimit is 
set by management to the number of buffers necessary to 
prevent the circuit from idling. 
D.1.3 Flusher
The flusher ensures that no NPDU is queued on a circuit 
whose state is not ON, or on a non-existent adjacency, or 
one whose state is not  Up. 

D.2 Congestion Avoidance
D.2.1 Buffer Management
The Forwarding Process supplies and manages the buffers 
necessary for relaying. PDUs shall be discarded if buffer 
thresholds are exceeded. If the average queue length on the 
input circuit or the forwarding processor or the output cir
cuit exceeds QueueThreshold, the congestion experi
enced bit shall be set in the QoS maintenance option of the 
forwarded data PDU (provided the QoS maintenance option 
is present). 

Security Considerations

  Security issues are not discussed in this memo.

Author's Address

   David R. Oran
   Digital Equipment Corporation
   LKG 1-2/a 19
   550 King Street
   Littleton, MA 01460

   Email: Oran@Oran.enet.dec.com

   Phone:  (508) 4866-7377

 

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