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RFC 3786 - Extending the Number of Intermediate System to Interm

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Network Working Group                                        A. Hermelin
Request for Comments: 3786                                 Montilio Inc.
Category: Informational                                       S. Previdi
                                                                M. Shand
                                                           Cisco Systems
                                                                May 2004

                        Extending the Number of
          Intermediate System to Intermediate System (IS-IS)
          Link State PDU (LSP) Fragments Beyond the 256 Limit

Status of this Memo

   This memo provides information for the Internet community.  It does
   not specify an Internet standard of any kind.  Distribution of this
   memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2004).  All Rights Reserved.


   This document describes a mechanism that allows a system to originate
   more than 256 Link State PDU (LSP) fragments, a limit set by the
   original Intermediate System to Intermediate System (IS-IS) Routing
   protocol, as described in ISO/IEC 10589.  This mechanism can be used
   in IP-only, OSI-only, and dual routers.

Table of Contents

   1.  Introduction .................................................  2
       1.1.  Keywords ...............................................  2
       1.2.  Definitions of Commonly Used Terms .....................  2
       1.3.  Operation Modes ........................................  3
       1.4.  Overview ...............................................  4
   2.  IS Alias ID TLV (IS-A) .......................................  5
   3.  Generating LSPs ..............................................  6
       3.1.  Both Operation Modes ...................................  6
       3.2.  Operation Mode 1 Additives .............................  8
   4.  Purging Extended LSP Fragments ............................... 10
   5.  Modifications to LSP handling in SPF ......................... 10
   6.  Forming Adjacencies .......................................... 11
   7.  Interoperating between extension-capable and non-capable ISs . 11
   8.  Security Considerations ...................................... 12
   9.  Acknowledgements ............................................. 12

   10. References ................................................... 12
   11. Authors' Addresses ........................................... 13
   12. Full Copyright Statement ..................................... 14

1.  Introduction

   In the Intermediate System to Intermediate System (IS-IS) protocol, a
   system floods its link-state information in Link State PDU (LSP) Data
   Units, or LSPs for short.  These logical LSPs can become quite large,
   therefore the protocol specifies a means of fragmenting this
   information into multiple LSP fragments.  The number of fragments a
   system can generate is limited by ISO/IEC 10589 [ISIS-ISO] to 256
   fragments, where each fragment's size is also limited.  Hence, there
   is a limit on the amount of link-state information a system can

   A number of factors can contribute to exceeding this limit:

   -  Introduction of new TLVs and sub-TLVs to be included in LSPs.
   -  The use of LSPs to propagate various types of information (such as
      traffic-engineering information).
   -  The increasing number of destinations and AS topologies.
   -  Finer granularity routing, and the ability to inject external
      routes into areas [DOMAIN-WIDE].
   -  Other emerging technologies, such as optical, IPv6, etc.

   This document describes mechanisms to relax the limit on the number
   of LSP fragments.

1.1.  Keywords

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in BCP 14, RFC 2119

1.2.  Definitions of Commonly Used Terms

   This section provides definitions for terms that are used throughout
   the text.

      Originating System
         A router physically running the IS-IS protocol.  As this
         document describes methods allowing a single IS-IS process to
         advertise its LSPs as multiple "virtual" routers, the
         Originating System represents the single "physical" IS-IS

      Normal system-id
         The system-id of an Originating System.

      Additional system-id
         An Additional system-id that is assigned by the network
         administrator.  Each Additional system-id allows generation of
         256 additional, or extended, LSP fragments.  The Additional
         system-id, like the Normal system-id, must be unique throughout
         the routing domain.

      Virtual System
         The system, identified by an Additional system-id, advertised
         as originating the extended LSP fragments.  These fragments
         specify the Additional system-id in their LSP IDs.

      Original LSP
         An LSP using the Normal system-id in its LSP ID.

      Extended LSP
         An LSP using an Additional system-id in its LSP ID.

      LSP set
         Logical LSP.  This term is used only to resolve the ambiguity
         between a logical LSP and an LSP fragment, both of which are
         sometimes termed "LSP".

      Extended LSP set
         A group of LSP fragments using an Additional system-id, and
         originated by the Originating System.

      Extension-capable IS
         An IS implementing the mechanisms described in this document.

1.3.  Operation Modes

   Two administrative operation modes are provided:

   -  Operation Mode 1 provides behavior that allows implementations
      that don't support this extension, to correctly process the
      extended fragment information, without any modifications.  This
      mode has some restrictions on what may be advertised in the
      extended LSP fragments.  Namely, only leaf information may be
      advertised in the extended LSPs.

   -  Operation Mode 2 extends the previous mode and relaxes its
      advertisement restrictions.  Any link-state information may be
      advertised in the extended LSPs.  However, it mandates a change to
      the way LSPs are considered during the SPF algorithm, in a way
      that is not compatible with previous implementations.

   These modes are configured on a per-level and area basis.  That is,
   all LSPs considered in the same SPF instance MUST use the same Mode.
   There is no restriction that an L1/L2 IS operates in the same mode,
   for both its L1 and L2 instances.  It can use Mode 1 for its L1 LSPs,
   and Mode 2 for its L2 LSPs, or vice versa.

   Mode 1 has the only advantage of being backwards compatible with
   older implementations.  It does have restrictions which are
   considered drawbacks.  Therefore, routers should operate in Mode 1
   only if backwards compatibility is desired.  Otherwise, it is
   recommended to run in Mode 2.

   Routers MAY implement Operational Mode 2 without supporting running
   in Operational Mode 1.  They will still interoperate correctly with
   routers that support both modes.

1.4.  Overview

   Using Additional system-ids assigned by the administrator, the
   Originating System can advertise the excess link-state information in
   extended LSPs under these Additional system-ids.  It would do so as
   if other routers, or "Virtual Systems", were advertising this
   information.  These extended LSPs will also have the specified limit
   on their LSP fragments; however, the Originating System may generate
   extended LSPs under numerous Virtual Systems.

   For Operation Mode 1, 0-cost adjacencies are advertised from the
   Originating System to its Virtual System(s).  No adjacencies (other
   than back to the Originating System) are advertised in the extended
   LSPs.  As a consequence, the Virtual Systems are 'stub', meaning they
   can only be reached through their Originating System.  Therefore,
   older implementations do not need modifications in order to correctly
   process these extended LSPs.

   For both modes, each LSP (set) created by a node will contain in its
   fragment-0 a new TLV (IS Alias ID TLV) that contains the Normal
   system-id and PN Number of the Original LSP created by the router.
   Extension-capable ISs can then use this information and store the
   original and extended LSPs as one logical LSP.

   The only sections that deal only with Mode 1 additions are 3.2,
   3.2.1, and 3.2.2.  All other sections relate to both modes.

2.  IS Alias ID TLV (IS-A)

   The proposed IS-A TLV allows extension-capable ISs to recognize all
   LSPs of an Originating System, and combine the original and extended
   LSPs for the purpose of SPF computation.  It identifies the Normal
   system-id of the Originating System.

   The proposed IS Alias ID TLV is type 24, and its format is as

   x CODE   - 24.

   x LENGTH - total length of the value field.

   x VALUE  -

                            No. of Octets
      | Normal system-id  |      6
      | Pseudonode number |      1
      | Sub-TLVs length   |      1
      |                   |     0-247
      : Sub-TLVs          :
      :                   :
      |                   |

   Normal system-id
      The Normal system-id of the LSP set, as described in section 1.2
      of this document.

   Pseudonode number
      The Pseudonode number of the LSP set.  LSPs with the same Normal
      system-id and Pseudonode number are considered in SPF as one
      logical LSP, as described in section 5 of this document.

   Sub-TLVs length
      Total length of all sub-TLVs.

   A series of tuples with the following format:

                         No. of Octets
   | Sub-type          |      1
   | Length            |      1
   |                   |     0-245
   : Value             :
   :                   :
   |                   |

      Type of the sub-TLV

      Total length of the value field

      Type-specific TLV payload.

   For an explanation on sub-TLV handling, see [ISIS-TE].

   Without sub-TLVs, this structure consumes 8 octets per LSP set.  This
   TLV MUST be included in fragment 0 of every LSP set belonging to an
   Originating System running in either Mode 1 or Mode 2.  Currently,
   there are no sub-TLVs defined.

   For a complete list of used IS-IS TLV numbers, see [ISIS-CODES].

3.  Generating LSPs

3.1.  Both Operation Modes

   Under both modes, the Originating System MUST include information
   binding the Original LSP and the Extended ones.  It can do this since
   it is trivially an extension-capable IS.  This is to ensure other
   extension-capable routers correctly process the extra information in
   their SPF calculation.  This binding is advertised via a new IS Alias
   ID TLV, which is advertised in all fragment 0 of Original and
   Extended LSPs.

   |  Originating System                         |
   |  system-id   = S                            |
   |  is-alias-id = S                            |

   +-------------------+     +-------------------+
   |  Virtual System   |     |  Virtual System   |
   |  system-id   = S' |     |  system-id   = S''|
   |  is-alias-id = S  |     |  is-alias-id = S  |
   +-------------------+     +-------------------+

   Figure 1. Advertising binding between all of a system's LSPs
             (both modes).  S' and S'' are configured as Additional

   When new extended LSP fragments are generated, these fragments should
   be generated as specified in ISO/IEC 10589 [ISIS-ISO].  Furthermore,
   a system SHOULD treat its extended LSPs the same as it treats its
   original LSPs, with the exceptions noted in the following sections.
   Specifically, creating, flooding, renewing, purging and all other
   operations are similar for both Original and Extended LSPs, unless
   stated otherwise.  The Extended LSPs will use one of the Additional
   system-ids configured for the router, in their LSP ID.

   Extended LSPs fragment zero should be regarded in the same special
   manner as specified in ISO/IEC 10589 for LSPs with number zero, and
   should include the same type of extra information as specified in
   ISO/IEC 10589 and RFC 1195 [ISIS-IP].  So, for example, when a system
   reissues its LSP fragment zero due to an area address change, it
   should reissue all extended LSPs fragment zero as well.

   An extended LSP fragment zero MUST be generated for every extended
   LSP set, to allow a router's SPF calculation to consider those
   fragments in that set.  See section 5 for details.

3.1.1.  The Attached Bits

   The Attached (ATT) bits SHOULD be set to zero for all four metric
   types, on all Extended LSPs.  This is due to the following: if a
   Virtual System is reachable, so is its Originating System.  It is
   preferable, then, that an L1 IS chooses the Originating System and
   not the Virtual System as its nearest L2 exit point, as connectivity
   to the Virtual System has a higher probability of being lost (as a
   result of the extended LSP no longer being advertised).  This could
   cause unnecessary computations on some implementations.

3.1.2.  The Partition Repair Bit

   The Partition Repair (P) bit SHOULD be set to zero on all extended
   LSPs.  This is for the same reasons as for the Attached bits.

3.1.3.  ES Neighbors TLV

   ISO/IEC 10589 [ISIS-ISO] section 7.3.7 specifies inserting an ES
   Neighbor TLV in L1 LSPs, with the system ID of the router.  RFC 1195
   [ISIS-IP] relieves IP-only routers of this requirement.  However, for
   routers that do insert this ESN TLV in L1 LSPs (whether IP-only or
   OSI-capable), then in an extended LSP, the ESN TLV should include the
   relevant Additional system-id.  Furthermore, OSI-capable routers
   should accept packets destined for this Additional system-id.

3.1.4.  Overload Bit

   The overload bit should be set consistently across all LSPs, original
   and extended, belonging to an Originating System, and should reflect
   the Originating System's overload state.

3.1.5.  Other Fields and TLVs

   Other fields and TLVs not mentioned above remain the same, both for
   original and extended LSPs.

3.2.  Operation Mode 1 Additions

   The following additions apply only to routers generating LSPs in Mode
   1.  Routers, which are configured to operate in Operation Mode 2,
   SHOULD NOT apply these additions to their advertisements.

   Under Operation Mode 1, adjacencies from the Originating System to
   its Virtual Systems are advertised using the standard neighbor TLVs.
   The metric for these connections MUST be zero, since the cost of
   reaching a Virtual System is the same as the cost of reaching its
   Originating System.

   To older implementations, Virtual Systems would appear reachable only
   through their Originating System, hence loss of connectivity to the
   Originating System means loss of connectivity to all of its
   information, including that advertised in its extended LSPs.
   Furthermore, the cost of reaching information advertised in non-
   extended LSPs is the same as the cost of reaching information
   advertised in the new extended LSPs, with an additional hop.

   |         Originating System                  |
   |         system-id = S                       |
   |         is-alias-id = S                     |
          |    /\                    |    /\
   cost=0 |    |cost=max-1    cost=0 |    |cost=max-1
          |    |                     |    |
          \/   |                     \/   |
   +-------------------+     +-------------------+
   |  Virtual System   |     |  Virtual System   |
   |  system-id   = S' |     |  system-id   = S''|
   |  is-alias-id = S  |     |  is-alias-id = S  |
   +-------------------+     +-------------------+

   Figure 2. Advertising connections to Virtual Systems under
             Operation Mode 1.  S' and S'' are configured as
             Additional system-ids.

   Under Operation Mode 1, only "leaf" information, i.e., information
   that serves only as leaves in a shortest path tree, can be advertised
   in extended LSPs.

   When an Extended LSP belonging to Additional system-id S' is first
   created, the Original LSP MUST specify S' as a neighbor, with metric
   set to zero.  This is in order to consider the cost of reaching the
   Virtual System S' the same as the cost of reaching its Originating
   System.  Furthermore, the Extended LSP MUST specify the Normal
   system-id as a neighbor.  The metric SHOULD be set to MaxLinkMetric -
   1 (this is only for uniformity purpose, any metric greater than zero
   is acceptable).  This in order to satisfy the two-way connectivity
   check on other routers.  Where relevant, this adjacency SHOULD be
   considered as point-to-point.

   Note, that the restriction specified in ISO/IEC 10589 section 7.2.5
   holds:  if an LSP Number zero of the Originating System is not
   present, none of that system's neighbor entries would be processed
   during SPF, hence none of its extended LSPs would be processed as

3.2.1.  IS Neighbors TLV (Mode 1 Only)

   An Extended LSP must specify only the Originating System as a
   neighbor, with the metric set to (MaxLinkMetric - 1).  Where
   relevant, this adjacency should be considered as point-to-point.
   Other neighbors MUST NOT be specified in an Extended LSP, because

   those other neighbors would only specify the Originating System and
   not the Virtual System, and hence would not satisfy the bi-
   directionality check in the SPF computation.

3.2.2.  Originating System in the Overload State in (Mode 1 Only)

   If the Originating System is in the overload state, information in
   the extended LSPs will not be processed by other routers in their SPF
   computation.  This is because in Mode 1, extended LSPs are reachable
   only through adjacencies from the Original LSP.  If this LSP has set
   its OL bit, adjacencies will not be processed in the SPF computation.

   This side effect should be taken into consideration when operating in
   Mode 1.

4.  Purging Extended LSP Fragments

   ISO/IEC 10589 [ISIS-ISO] section note 25 suggests that an
   implementation keeps the number of LSP fragments within a certain
   limit based on the optimal (minimal) number of fragments needed.
   Section also recommends that an IS purge its empty LSPs to
   conserve resources.  These recommendations hold for the extended LSP
   fragments as well.  However, an extended LSP fragment zero should not
   be purged until all of the fragments in its set (i.e., belonging to a
   particular Additional system-id), are empty as well.  This is to
   ensure implementations consider the fragments in their SPF
   computations, as specified in section 7.2.5.

   In Operational Mode 1, when all the extended LSP fragments of a
   particular Additional system-id S' have been purged, the Originating
   System SHOULD remove the neighbor information to S' from its original

5.  Modifications to LSP handling in SPF

   This section describes modifications to the way extension-capable ISs
   handle LSPs for the SPF computation.

   When considering LSPs of an extension-capable IS (identified by the
   inclusion of the IS Alias ID TLV), the original and extended LSPs are
   combined to form one large logical LSP.  If the LSP belongs to an IS
   running Operational Mode 1, there might be adjacencies between the
   original and extended LSPs.  These are trivially ignored (since when
   processing them the large logical LSP is already on PATHS), and does
   not complicate the SPF.  Furthermore, this check should already be
   implemented (this scenario could occur on error, without this

   If LSP fragment 0 of the Original LSP set is missing or its
   RemainingLifetime is zero, all of the LSPs generated by that
   Originating System (Extended as well) MUST NOT be considered in the
   SPF.  That is, the large logical LSP is not considered in the SPF.
   The original LSP fragments are identified when the is-alias-id value
   is the same as the system-id of those LSPs.  If an LSP fragment 0 of
   an extended LSP set is missing or its RemainingLifetime is zero, only
   that LSP set MUST NOT be considered in the SPF.  These rules are
   present to ensure consistent SPF results on Mode 1 and Mode 2 LSPs.

   Note, that the above behavior is consistent with how previous
   implementations will interpret Mode 1 LSPs.

6.  Forming Adjacencies

   It should be noted, that an IS MUST use the system-id of the LSP that
   will include a neighbor, when forming an adjacency with that
   neighbor.  That is, if a neighbor is to be included in extended LSP
   S', then S' should be used as the system-id in IS Hellos [3] and IS-
   IS Hellos when forming an adjacency with that neighbor.  This is
   regardless of the Operational Mode.  Of course, in Mode 1 this means
   that only the Normal system-id will be used when sending hellos.

7.  Interoperating between extension-capable and non-extension-capable

   In order to correctly advertise link-state information under
   Operation Mode 2, all ISs in an area must be extension-capable.
   However, it is possible to not upgrade every router in the network,
   if the extended information is not routing information, but rather
   data that is of use to only a subset of routers (e.g., optical
   switches using IS-IS could carry optical-specific information in
   extended LSPs)

   If a live network contains routers exceeding the 256 fragment limit,
   and for some reason the upgrade has to be done incrementally, it is
   possible to transition the network, using the following steps:

   -  Upgrade the routers, one-by-one, to run this extension in
      Operation Mode 1.  The other non-extension-capable routers will
      interoperate correctly.

   -  When all routers are extension-capable, configure them one-by-one
      to run in Operation Mode 2.  All extension-capable routers
      interoperate correctly, regardless of what mode they are run in.

   Implementations SHOULD support a configuration parameter controlling
   the LSP origination behavior.  The default value of this parameter
   SHOULD correspond to the behavior described in [ISIS-ISO], i.e.,
   neither of the two modes described in this document should be enabled
   without explicit configuration when the router software is upgraded
   with this extension.

8.  Security Considerations

   This document raises no new security issues for IS-IS.

9.  Acknowledgments

   The authors would like to thank Tony Li and Radia Perlman for helpful
   comments and suggestions on the subject.

10.  References

10.1.  Normative References

   [ISIS-ISO]    "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)",
                 ISO/IEC 10589:2002, Second Edition.

   [ISIS-IP]     Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
                 dual environments", RFC 1195, December 1990.

   [ISIS-TE]     Smit, H. and T. Li, "Intermediate System to
                 Intermediate System (IS-IS) Extensions for Traffic
                 Engineering (TE)", RFC 3784, May 2004.

   [BCP14]       Bradner, S., "Key words for use in RFCs to Indicate
                 Requirement Levels", BCP 14, RFC 2119, March 1997.

10.2.  Informative References

   [DOMAIN-WIDE] Li, T., Przygienda, T. and H. Smit, "Domain-wide Prefix
                 Distribution with Two-Level IS-IS", RFC 2966, October

   [ISIS-CODES]  Przygienda, T., "Reserved Type, Length and Value (TLV)
                 Codepoints in Intermediate System to Intermediate
                 System", RFC 3359, August 2002.

11.  Authors' Addresses

   Amir Hermelin
   Montilio Inc.
   1 Maskit St.
   POB 12253
   Herzelia, 46733

   Phone: +972 9 9511944
   Fax: +972 9 9542430
   EMail: amir@montilio.com

   Stefano Previdi
   Cisco Systems, Inc.
   Via Del Serafico 200
   00142 Roma

   Phone: +39 06 5164 4491
   EMail: sprevidi@cisco.com

   Mike Shand
   Cisco Systems
   250, Longwater Avenue,
   Green Park,
   RG2 6GB,

   Phone: +44 20 8824 8690
   EMail: mshand@cisco.com

12.  Full Copyright Statement

   Copyright (C) The Internet Society (2004).  This document is subject
   to the rights, licenses and restrictions contained in BCP 78, and
   except as set forth therein, the authors retain all their rights.

   This document and the information contained herein are provided on an

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