faqs.org - Internet FAQ Archives

RFC 2740 - OSPF for IPv6


Or Display the document by number




Network Working Group                                          R. Coltun
Requests for Comments: 2740                                Siara Systems
Category: Standards Track                                    D. Ferguson
                                                        Juniper Networks
                                                                  J. Moy
                                                       Sycamore Networks
                                                           December 1999

                             OSPF for IPv6

Status of this Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

   This document describes the modifications to OSPF to support version
   6 of the Internet Protocol (IPv6).  The fundamental mechanisms of
   OSPF (flooding, DR election, area support, SPF calculations, etc.)
   remain unchanged. However, some changes have been necessary, either
   due to changes in protocol semantics between IPv4 and IPv6, or simply
   to handle the increased address size of IPv6.

   Changes between OSPF for IPv4 and this document include the
   following. Addressing semantics have been removed from OSPF packets
   and the basic LSAs. New LSAs have been created to carry IPv6
   addresses and prefixes. OSPF now runs on a per-link basis, instead of
   on a per-IP-subnet basis. Flooding scope for LSAs has been
   generalized. Authentication has been removed from the OSPF protocol
   itself, instead relying on IPv6's Authentication Header and
   Encapsulating Security Payload.

   Most packets in OSPF for IPv6 are almost as compact as those in OSPF
   for IPv4, even with the larger IPv6 addresses. Most field-XSand
   packet-size limitations present in OSPF for IPv4 have been relaxed.
   In addition, option handling has been made more flexible.

   All of OSPF for IPv4's optional capabilities, including on-demand
   circuit support, NSSA areas, and the multicast extensions to OSPF
   (MOSPF) are also supported in OSPF for IPv6.

Table of Contents

   1        Introduction ........................................... 4
   1.1      Terminology ............................................ 4
   2        Differences from OSPF for IPv4 ......................... 4
   2.1      Protocol processing per-link, not per-subnet ........... 5
   2.2      Removal of addressing semantics ........................ 5
   2.3      Addition of Flooding scope ............................. 5
   2.4      Explicit support for multiple instances per link ....... 6
   2.5      Use of link-local addresses ............................ 6
   2.6      Authentication changes ................................. 7
   2.7      Packet format changes .................................. 7
   2.8      LSA format changes ..................................... 8
   2.9      Handling unknown LSA types ............................ 10
   2.10     Stub area support ..................................... 10
   2.11     Identifying neighbors by Router ID .................... 11
   3        Implementation details ................................ 11
   3.1      Protocol data structures .............................. 12
   3.1.1    The Area Data structure ............................... 13
   3.1.2    The Interface Data structure .......................... 13
   3.1.3    The Neighbor Data Structure ........................... 14
   3.2      Protocol Packet Processing ............................ 15
   3.2.1    Sending protocol packets .............................. 15
   3.2.1.1  Sending Hello packets ................................. 16
   3.2.1.2  Sending Database Description Packets .................. 17
   3.2.2    Receiving protocol packets ............................ 17
   3.2.2.1  Receiving Hello Packets ............................... 19
   3.3      The Routing table Structure ........................... 19
   3.3.1    Routing table lookup .................................. 20
   3.4      Link State Advertisements ............................. 20
   3.4.1    The LSA Header ........................................ 21
   3.4.2    The link-state database ............................... 22
   3.4.3    Originating LSAs ...................................... 22
   3.4.3.1  Router-LSAs ........................................... 25
   3.4.3.2  Network-LSAs .......................................... 27
   3.4.3.3  Inter-Area-Prefix-LSAs ................................ 28
   3.4.3.4  Inter-Area-Router-LSAs ................................ 29
   3.4.3.5  AS-external-LSAs ...................................... 29
   3.4.3.6  Link-LSAs ............................................. 31
   3.4.3.7  Intra-Area-Prefix-LSAs ................................ 32
   3.5      Flooding .............................................. 35
   3.5.1    Receiving Link State Update packets ................... 36
   3.5.2    Sending Link State Update packets ..................... 36
   3.5.3    Installing LSAs in the database ....................... 38

   3.6      Definition of self-originated LSAs .................... 39
   3.7      Virtual links ......................................... 39
   3.8      Routing table calculation ............................. 39
   3.8.1    Calculating the shortest path tree for an area ........ 40
   3.8.1.1  The next hop calculation .............................. 41
   3.8.2    Calculating the inter-area routes ..................... 42
   3.8.3    Examining transit areas' summary-LSAs ................. 42
   3.8.4    Calculating AS external routes ........................ 42
   3.9      Multiple interfaces to a single link .................. 43
            References ............................................ 44
   A        OSPF data formats ..................................... 46
   A.1      Encapsulation of OSPF packets ......................... 46
   A.2      The Options field ..................................... 47
   A.3      OSPF Packet Formats ................................... 48
   A.3.1    The OSPF packet header ................................ 49
   A.3.2    The Hello packet ...................................... 50
   A.3.3    The Database Description packet ....................... 52
   A.3.4    The Link State Request packet ......................... 54
   A.3.5    The Link State Update packet .......................... 55
   A.3.6    The Link State Acknowledgment packet .................. 56
   A.4      LSA formats ........................................... 57
   A.4.1    IPv6 Prefix Representation ............................ 58
   A.4.1.1  Prefix Options ........................................ 58
   A.4.2    The LSA header ........................................ 59
   A.4.2.1  LS type ............................................... 60
   A.4.3    Router-LSAs ........................................... 61
   A.4.4    Network-LSAs .......................................... 64
   A.4.5    Inter-Area-Prefix-LSAs ................................ 65
   A.4.6    Inter-Area-Router-LSAs ................................ 66
   A.4.7    AS-external-LSAs ...................................... 67
   A.4.8    Link-LSAs ............................................. 69
   A.4.9    Intra-Area-Prefix-LSAs ................................ 71
   B        Architectural Constants ............................... 73
   C        Configurable Constants ................................ 73
   C.1      Global parameters ..................................... 73
   C.2      Area parameters ....................................... 74
   C.3      Router interface parameters ........................... 75
   C.4      Virtual link parameters ............................... 77
   C.5      NBMA network parameters ............................... 77
   C.6      Point-to-MultiPoint network parameters ................ 78
   C.7      Host route parameters ................................. 78
            Security Considerations ............................... 79
            Authors' Addresses .................................... 79
            Full Copyright Statement .............................. 80

1.  Introduction

   This document describes the modifications to OSPF to support version
   6 of the Internet Protocol (IPv6).  The fundamental mechanisms of
   OSPF (flooding, DR election, area support, SPF calculations, etc.)
   remain unchanged. However, some changes have been necessary, either
   due to changes in protocol semantics between IPv4 and IPv6, or simply
   to handle the increased address size of IPv6.

   This document is organized as follows. Section 2 describes the
   differences between OSPF for IPv4 and OSPF for IPv6 in detail.
   Section 3 provides implementation details for the changes. Appendix A
   gives the OSPF for IPv6 packet and LSA formats. Appendix B lists the
   OSPF architectural constants. Appendix C describes configuration
   parameters.

1.1.  Terminology

   This document attempts to use terms from both the OSPF for IPv4
   specification ([Ref1]) and the IPv6 protocol specifications
   ([Ref14]). This has produced a mixed result. Most of the terms used
   both by OSPF and IPv6 have roughly the same meaning (e.g.,
   interfaces). However, there are a few conflicts. IPv6 uses "link"
   similarly to IPv4 OSPF's "subnet" or "network". In this case, we have
   chosen to use IPv6's "link" terminology. "Link" replaces OSPF's
   "subnet" and "network" in most places in this document, although
   OSPF's Network-LSA remains unchanged (and possibly unfortunately, a
   new Link-LSA has also been created).

   The names of some of the OSPF LSAs have also changed. See Section 2.8
   for details.

2.  Differences from OSPF for IPv4

   Most of the algorithms from OSPF for IPv4 [Ref1] have preserved in
   OSPF for IPv6. However, some changes have been necessary, either due
   to changes in protocol semantics between IPv4 and IPv6, or simply to
   handle the increased address size of IPv6.

   The following subsections describe the differences between this
   document and [Ref1].

2.1.  Protocol processing per-link, not per-subnet

   IPv6 uses the term "link" to indicate "a communication facility or
   medium over which nodes can communicate at the link layer" ([Ref14]).
   "Interfaces" connect to links. Multiple IP subnets can be assigned to
   a single link, and two nodes can talk directly over a single link,
   even if they do not share a common IP subnet (IPv6 prefix).

   For this reason, OSPF for IPv6 runs per-link instead of the IPv4
   behavior of per-IP-subnet. The terms "network" and "subnet" used in
   the IPv4 OSPF specification ([Ref1]) should generally be relaced by
   link. Likewise, an OSPF interface now connects to a link instead of
   an IP subnet, etc.

   This change affects the receiving of OSPF protocol packets, and the
   contents of Hello Packets and Network-LSAs.

2.2.  Removal of addressing semantics

   In OSPF for IPv6, addressing semantics have been removed from the
   OSPF protocol packets and the main LSA types, leaving a network-
   protocol-independent core. In particular:

   o   IPv6 Addresses are not present in OSPF packets, except in
       LSA payloads carried by the Link State Update Packets. See
       Section 2.7 for details.

   o   Router-LSAs and Network-LSAs no longer contain network
       addresses, but simply express topology information. See
       Section 2.8 for details.

   o   OSPF Router IDs, Area IDs and LSA Link State IDs remain at
       the IPv4 size of 32-bits. They can no longer be assigned as
       (IPv6) addresses.

   o   Neighboring routers are now always identified by Router ID,
       where previously they had been identified by IP address on
       broadcast and NBMA "networks".

2.3.  Addition of Flooding scope

   Flooding scope for LSAs has been generalized and is now explicitly
   coded in the LSA's LS type field. There are now three separate
   flooding scopes for LSAs:

   o   Link-local scope. LSA is flooded only on the local link, and
       no further. Used for the new Link-LSA (see Section A.4.8).

   o   Area scope. LSA is flooded throughout a single OSPF area
       only. Used for Router-LSAs, Network-LSAs, Inter-Area-Prefix-
       LSAs, Inter-Area-Router-LSAs and Intra-Area-Prefix-LSAs.

   o   AS scope. LSA is flooded throughout the routing domain. Used
       for AS-external-LSAs.

2.4.  Explicit support for multiple instances per link

   OSPF now supports the ability to run multiple OSPF protocol instances
   on a single link. For example, this may be required on a NAP segment
   shared between several providers -- providers may be running separate
   OSPF routing domains that want to remain separate even though they
   have one or more physical network segments (i.e., links) in common.
   In OSPF for IPv4 this was supported in a haphazard fashion using the
   authentication fields in the OSPF for IPv4 header.

   Another use for running multiple OSPF instances is if you want, for
   one reason or another, to have a single link belong to two or more
   OSPF areas.

   Support for multiple protocol instances on a link is accomplished via
   an "Instance ID" contained in the OSPF packet header and OSPF
   interface structures. Instance ID solely affects the reception of
   OSPF packets.

2.5.  Use of link-local addresses

   IPv6 link-local addresses are for use on a single link, for purposes
   of neighbor discovery, auto-configuration, etc. IPv6 routers do not
   forward IPv6 datagrams having link-local source addresses [Ref15].
   Link-local unicast addresses are assigned from the IPv6 address range
   FF80/10.

   OSPF for IPv6 assumes that each router has been assigned link-local
   unicast addresses on each of the router's attached physical segments.
   On all OSPF interfaces except virtual links, OSPF packets are sent
   using the interface's associated link-local unicast address as
   source.  A router learns the link-local addresses of all other
   routers attached to its links, and uses these addresses as next hop
   information during packet forwarding.

   On virtual links, global scope or site-local IP addresses must be
   used as the source for OSPF protocol packets.

   Link-local addresses appear in OSPF Link-LSAs (see Section 3.4.3.6).
   However, link-local addresses are not allowed in other OSPF LSA
   types.  In particular, link-local addresses must not be advertised in
   inter-area-prefix-LSAs (Section 3.4.3.3), AS-external-LSAs (Section
   3.4.3.5) or intra-area-prefix-LSAs (Section 3.4.3.7).

2.6.  Authentication changes

   In OSPF for IPv6, authentication has been removed from OSPF itself.
   The "AuType" and "Authentication" fields have been removed from the
   OSPF packet header, and all authentication related fields have been
   removed from the OSPF area and interface structures.

   When running over IPv6, OSPF relies on the IP Authentication Header
   (see [Ref19]) and the IP Encapsulating Security Payload (see [Ref20])
   to ensure integrity and authentication/confidentiality of routing
   exchanges.

   Protection of OSPF packet exchanges against accidental data
   corruption is provided by the standard IPv6 16-bit one's complement
   checksum, covering the entire OSPF packet and prepended IPv6 pseudo-
   header (see Section A.3.1).

2.7.  Packet format changes

   OSPF for IPv6 runs directly over IPv6. Aside from this, all
   addressing semantics have been removed from the OSPF packet headers,
   making it essentially "network-protocol-independent".  All addressing
   information is now contained in the various LSA types only.

   In detail, changes in OSPF packet format consist of the following:

   o  The OSPF version number has been increased from 2 to 3.

   o  The Options field in Hello Packets and Database description Packet
      has been expanded to 24-bits.

   o  The Authentication and AuType fields have been removed from the
      OSPF packet header (see Section 2.6).

   o  The Hello packet now contains no address information at all, and
      includes an Interface ID which the originating router has assigned
      to uniquely identify (among its own interfaces) its interface to
      the link.  This Interface ID becomes the Netowrk-LSA's Link State
      ID, should the router become Designated-Router on the link.

   o  Two option bits, the "R-bit" and the "V6-bit", have been added to
      the  Options field for processing Router-LSAs during the SPF
      calculation (see Section A.2).  If the "R-bit" is clear an OSPF
      speaker can participate in OSPF topology distribution without
      being used to forward transit traffic; this can be used in multi-
      homed hosts that want to participate in the routing protocol. The
      V6-bit specializes the R-bit; if the V6-bit is clear an OSPF
      speaker can participate in OSPF topology distribution without
      being used to forward IPv6 datagrams. If the R-bit is set and the
      V6-bit is clear, IPv6 datagrams are not forwarded but diagrams
      belonging to another protocol family may be forwarded.

   o  TheOSPF packet header now includes an "Instance ID" which allows
      multiple OSPF protocol instances to be run on a single link (see
      Section 2.4).

2.8.  LSA format changes

   All addressing semantics have been removed from the LSA header, and
   from Router-LSAs and Network-LSAs. These two LSAs now describe the
   routing domain's topology in a network-protocol-independent manner.
   New LSAs have been added to distribute IPv6 address information, and
   data required for next hop resolution.  The names of some of IPv4's
   LSAs have been changed to be more consistent with each other.

   In detail, changes in LSA format consist of the following:

   o  The Options field has been removed from the LSA header, expanded
      to 24 bits, and moved into the body of Router-LSAs, Network-LSAs,
      Inter-Area-Router-LSAs and Link-LSAs. See Section A.2 for details.

   o  The LSA Type field has been expanded (into the former Options
      space) to 16 bits, with the upper three bits encoding flooding
      scope and the handling of unknown LSA types (see Section 2.9).

   o  Addresses in LSAs are now expressed as [prefix, prefix length]
      instead of [address, mask] (see Section A.4.1). The default route
      is expressed as a prefix with length 0.

   o  The Router and Network LSAs now have no address information, and
      are network-protocol-independent.

   o  Router interface information may be spread across multiple Router
      LSAs. Receivers must concatenate all the Router-LSAs originated by
      a given router when running the SPF calculation.

   o  A new LSA called the Link-LSA has been introduced. The LSAs have
      local-link flooding scope; they are never flooded beyond the link
      that they are associated with. Link-LSAs have three purposes: 1)
      they provide the router's link-local address to all other routers
      attached to the link, 2) they inform other routers attached to the
      link of a list of IPv6 prefixes to associate with the link and 3)
      they allow the router to assert a collection of Options bits to
      associate with the Network-LSA that will be originated for the
      link.  See Section A.4.8 for details.

      In IPv4, the router-LSA carries a router's IPv4 interface
      addresses, the IPv4 equivalent of link-local addresses.  These are
      only used when calculating next hops during the OSPF routing
      calculation (see Section 16.1.1 of [Ref1]), so they do not need to
      be flooded past the local link; hence using link-LSAs to
      distribute these addresses is more efficient. Note that link-local
      addresses cannot be learned through the reception of Hellos in all
      cases: on NBMA links next hop routers do not necessarily exchange
      hellos, but rather learn of each other's existence by way of the
      Designated Router.

   o  The Options field in the Network LSA is set to the logical OR of
      the Options that each router on the link advertises in its Link-
      LSA.

   o  Type-3 summary-LSAs have been renamed "Inter-Area-Prefix-LSAs".
      Type-4 summary LSAs have been renamed "Inter-Area-Router-LSAs".

   o  The Link State ID in Inter-Area-Prefix-LSAs, Inter-Area-Router-
      LSAs and AS-external-LSAs has lost its addressing semantics, and
      now serves solely to identify individual pieces of the Link State
      Database. All addresses or Router IDs that were formerly expressed
      by the Link State ID are now carried in the LSA bodies.

   o  Network-LSAs and Link-LSAs are the only LSAs whose Link State ID
      carries additional meaning. For these LSAs, the Link State ID is
      always the Interface ID of the originating router on the link
      being described. For this reason, Network-LSAs and Link-LSAs are
      now the only LSAs whose size cannot be limited: a Network-LSA must
      list all routers connected to the link, and a Link-LSA must list
      all of a router's addresses on the link.

   o  A new LSA called the Intra-Area-Prefix-LSA has been introduced.
      This LSA carries all IPv6 prefix information that in IPv4 is
      included in Router-LSAs and Network-LSAs.  See Section A.4.9 for
      details.

   o  Inclusion of a forwarding address in AS-external-LSAs is now
      optional, as is the inclusion of an external route tag (see
      [Ref5]). In addition, AS-external-LSAs can now reference another
      LSA, for inclusion of additional route attributes that are outside
      the scope of the OSPF protocol itself. For example, this can be
      used to attach BGP path attributes to external routes as proposed
      in [Ref10].

2.9.  Handling unknown LSA types

   Handling of unknown LSA types has been made more flexible so that,
   based on LS type, unknown LSA types are either treated as having
   link-local flooding scope, or are stored and flooded as if they were
   understood (desirable for things like the proposed External-
   Attributes-LSA in [Ref10]). This behavior is explicitly coded in the
   LSA Handling bit of the link state header's LS type field (see
   Section A.4.2.1).

   The IPv4 OSPF behavior of simply discarding unknown types is
   unsupported due to the desire to mix router capabilities on a single
   link. Discarding unknown types causes problems when the Designated
   Router supports fewer options than the other routers on the link.

2.10.  Stub area support

   In OSPF for IPv4, stub areas were designed to minimize link-state
   database and routing table sizes for the areas' internal routers.
   This allows routers with minimal resources to participate in even
   very large OSPF routing domains.

   In OSPF for IPv6, the concept of stub areas is retained. In IPv6, of
   the mandatory LSA types, stub areas carry only router-LSAs, network-
   LSAs, Inter-Area-Prefix-LSAs, Link-LSAs, and Intra-Area-Prefix-LSAs.
   This is the IPv6 equivalent of the LSA types carried in IPv4 stub
   areas: router-LSAs, network-LSAs and type 3 summary-LSAs.

   However, unlike in IPv4, IPv6 allows LSAs with unrecognized LS types
   to be labeled "Store and flood the LSA, as if type understood" (see
   the U-bit in Section A.4.2.1). Uncontrolled introduction of such LSAs
   could cause a stub area's link-state database to grow larger than its
   component routers' capacities.

   To guard against this, the following rule regarding stub areas has
   been established: an LSA whose LS type is unrecognized may only be
   flooded into/throughout a stub area if both a) the LSA has area or
   link-local flooding scope and b) the LSA has U-bit set to 0. See
   Section 3.5 for details.

2.11.  Identifying neighbors by Router ID

   In OSPF for IPv6, neighboring routers on a given link are always
   identified by their OSPF Router ID. This contrasts with the IPv4
   behavior where neighbors on point-to-point networks and virtual links
   are identified by their Router IDs, and neighbors on broadcast, NBMA
   and Point-to-MultiPoint links are identified by their IPv4 interface
   addresses.

   This change affects the reception of OSPF packets (see Section 8.2 of
   [Ref1]), the lookup of neighbors (Section 10 of [Ref1]) and the
   reception of Hello Packets (Section 10.5 of [Ref1]).

   The Router ID of 0.0.0.0 is reserved, and should not be used.

3.  Implementation details

   When going from IPv4 to IPv6, the basic OSPF mechanisms remain
   unchanged from those documented in [Ref1]. These mechanisms are
   briefly outlined in Section 4 of [Ref1]. Both IPv6 and IPv4 have a
   link-state database composed of LSAs and synchronized between
   adjacent routers. Initial synchronization is performed through the
   Database Exchange process, through the exchange of Database
   Description, Link State Request and Link State Update packets.
   Thereafter database synchronization is maintained via flooding,
   utilizing Link State Update and Link State Acknowledgment packets.
   Both IPv6 and IPv4 use OSPF Hello Packets to discover and maintain
   neighbor relationships, and to elect Designated Routers and Backup
   Designated Routers on broadcast and NBMA links. The decision as to
   which neighbor relationships become adjacencies, along with the basic
   ideas behind inter-area routing, importing external information in
   AS-external-LSAs and the various routing calculations are also the
   same.

   In particular, the following IPv4 OSPF functionality described in
   [Ref1] remains completely unchanged for IPv6:

   o  Both IPv4 and IPv6 use OSPF packet types described in Section 4.3
      of [Ref1], namely: Hello, Database Description, Link State
      Request, Link State Update and Link State Acknowledgment packets.
      While in some cases (e.g., Hello packets) their format has changed
      somewhat, the functions of the various packet types remains the
      same.

   o  The system requirements for an OSPF implementation remain
      unchanged, although OSPF for IPv6 requires an IPv6 protocol stack
      (from the network layer on down) since it runs directly over the
      IPv6 network layer.

   o  The discovery and maintenance of neighbor relationships, and the
      selection and establishment of adjacencies remain the same. This
      includes election of the Designated Router and Backup Designated
      Router on broadcast and NBMA links. These mechanisms are described
      in Sections 7, 7.1, 7.2, 7.3, 7.4 and 7.5 of [Ref1].

   o  The link types (or equivalently, interface types) supported by
      OSPF remain unchanged, namely: point-to-point, broadcast, NBMA,
      Point-to-MultiPoint and virtual links.

   o  The interface state machine, including the list of OSPF interface
      states and events, and the Designated Router and Backup Designated
      Router election algorithm, remain unchanged.  These are described
      in Sections 9.1, 9.2, 9.3 and 9.4 of [Ref1].

   o  The neighbor state machine, including the list of OSPF neighbor
      states and events, remain unchanged. These are described in
      Sections 10.1, 10.2, 10.3 and 10.4 of [Ref1].

   o  Aging of the link-state database, as well as flushing LSAs from
      the routing domain through the premature aging process, remains
      unchanged from the description in Sections 14 and 14.1 of [Ref1].

   However, some OSPF protocol mechanisms have changed, as outlined in
   Section 2 above. These changes are explained in detail in the
   following subsections, making references to the appropriate sections
   of [Ref1].

   The following subsections provide a recipe for turning an IPv4 OSPF
   implementation into an IPv6 OSPF implementation.

3.1.  Protocol data structures

   The major OSPF data structures are the same for both IPv4 and IPv6:
   areas, interfaces, neighbors, the link-state database and the routing
   table. The top-level data structures for IPv6 remain those listed in
   Section 5 of [Ref1], with the following modifications:

   o  All LSAs with known LS type and AS flooding scope appear in the
      top-level data structure, instead of belonging to a specific area
      or link. AS-external-LSAs are the only LSAs defined by this
      specification which have AS flooding scope.  LSAs with unknown LS
      type, U-bit set to 1 (flood even when unrecognized) and AS
      flooding scope also appear in the top-level data structure.

3.1.1.  The Area Data structure

   The IPv6 area data structure contains all elements defined for IPv4
   areas in Section 6 of [Ref1]. In addition, all LSAs of known type
   which have area flooding scope are contained in the IPv6 area data
   structure. This always includes the following LSA types: router-LSAs,
   network-LSAs, inter-area-prefix-LSAs, inter-area-router-LSAs and
   intra-area-prefix-LSAs. LSAs with unknown LS type, U-bit set to 1
   (flood even when unrecognized) and area scope also appear in the area
   data structure. IPv6 routers implementing MOSPF add group-
   membership-LSAs to the area data structure. Type-7-LSAs belong to an
   NSSA area's data structure.

3.1.2.  The Interface Data structure

   In OSPF for IPv6, an interface connects a router to a link.  The IPv6
   interface structure modifies the IPv4 interface structure (as defined
   in Section 9 of [Ref1]) as follows:

   Interface ID
      Every interface is assigned an Interface ID, which uniquely
      identifies the interface with the router. For example, some
      implementations may be able to use the MIB-II IfIndex ([Ref3]) as
      Interface ID. The Interface ID appears in Hello packets sent out
      the interface, the link-local-LSA originated by router for the
      attached link, and the router-LSA originated by the router-LSA for
      the associated area. It will also serve as the Link State ID for
      the network-LSA that the router will originate for the link if the
      router is elected Designated Router.

   Instance ID
      Every interface is assigned an Instance ID. This should default to
      0, and is only necessary to assign differently on those links that
      will contain multiple separate communities of OSPF Routers. For
      example, suppose that there are two communities of routers on a
      given ethernet segment that you wish to keep separate.

      The first community is given an Instance ID of 0, by assigning 0
      as the Instance ID of all its routers' interfaces to the ethernet.
      An Instance ID of 1 is assigned to the other routers' interfaces
      to the ethernet. The OSPF transmit and receive processing (see
      Section 3.2) will then keep the two communities separate.

   List of LSAs with link-local scope
      All LSAs with link-local scope and which were originated/flooded
      on the link belong to the interface structure which connects to
      the link. This includes the collection of the link's link-LSAs.

   List of LSAs with unknown LS type
      All LSAs with unknown LS type and U-bit set to 0 (if unrecognized,
      treat the LSA as if it had link-local flooding scope) are kept in
      the data structure for the interface that received the LSA.

   IP interface address
      For IPv6, the IPv6 address appearing in the source of OSPF packets
      sent out the interface is almost always a link-local address. The
      one exception is for virtual links, which must use one of the
      router's own site-local or global IPv6 addresses as IP interface
      address.

   List of link prefixes
      A list of IPv6 prefixes can be configured for the attached link.
      These will be advertised by the router in link-LSAs, so that they
      can be advertised by the link's Designated Router in intra-area-
      prefix-LSAs.

   In OSPF for IPv6, each router interface has a single metric,
   representing the cost of sending packets out the interface.  In
   addition, OSPF for IPv6 relies on the IP Authentication Header (see
   [Ref19]) and the IP Encapsulating Security Payload (see [Ref20]) to
   ensure integrity and authentication/confidentiality of routing
   exchanges.  For that reason, AuType and Authentication key are not
   associated with IPv6 OSPF interfaces.

   Interface states, events, and the interface state machine remain
   unchanged from IPv4, and are documented in Sections 9.1, 9.2 and 9.3
   of [Ref1] respectively. The Designated Router and Backup Designated
   Router election algorithm also remains unchanged from the IPv4
   election in Section 9.4 of [Ref1].

3.1.3.  The Neighbor Data Structure

   The neighbor structure performs the same function in both IPv6 and
   IPv4. Namely, it collects all information required to form an
   adjacency between two routers, if an adjacency becomes necessary.
   Each neighbor structure is bound to a single OSPF interface. The
   differences between the IPv6 neighbor structure and the neighbor
   structure defined for IPv4 in Section 10 of [Ref1] are:

   Neighbor's Interface ID
      The Interface ID that the neighbor advertises in its Hello Packets
      must be recorded in the neighbor structure. The router will
      include the neighbor's Interface ID in the router's router-LSA
      when either a) advertising a point-to-point link to the neighbor
      or b) advertising a link to a network where the neighbor has
      become Designated Router.

   Neighbor IP address
      Except on virtual links, the neighbor's IP address will be an IPv6
      link-local address.

   Neighbor's Designated Router
      The neighbor's choice of Designated Router is now encoded as a
      Router ID, instead of as an IP address.

   Neighbor's Backup Designated Router
      The neighbor's choice of Designated Router is now encoded as a
      Router ID, instead of as an IP address.

   Neighbor states, events, and the neighbor state machine remain
   unchanged from IPv4, and are documented in Sections 10.1, 10.2 and
   10.3 of [Ref1] respectively. The decision as to which adjacencies to
   form also remains unchanged from the IPv4 logic documented in Section
   10.4 of [Ref1].

3.2.  Protocol Packet Processing

   OSPF for IPv6 runs directly over IPv6's network layer. As such, it is
   encapsulated in one or more IPv6 headers, with the Next Header field
   of the immediately encapsulating IPv6 header set to the value 89.

   As for IPv4, in IPv6 OSPF routing protocol packets are sent along
   adjacencies only (with the exception of Hello packets, which are used
   to discover the adjacencies). OSPF packet types and functions are the
   same in both IPv4 and IPv4, encoded by the

   Type field of the standard OSPF packet header.

3.2.1.  Sending protocol packets

   When an IPv6 router sends an OSPF routing protocol packet, it fills
   in the fields of the standard OSPF for IPv6 packet header (see
   Section A.3.1) as follows:

   Version #
      Set to 3, the version number of the protocol as documented in this
      specification.

   Type
      The type of OSPF packet, such as Link state Update or Hello
      Packet.

   Packet length
      The length of the entire OSPF packet in bytes, including the
      standard OSPF packet header.

   Router ID
      The identity of the router itself (who is originating the packet).

   Area ID
      The OSPF area that the packet is being sent into.

   Instance ID
      The OSPF Instance ID associated with the interface that the packet
      is being sent out of.

   Checksum
      The standard IPv6 16-bit one's complement checksum, covering the
      entire OSPF packet and prepended IPv6 pseudo-header (see Section
      A.3.1).

   Selection of OSPF routing protocol packets' IPv6 source and
   destination addresses is performed identically to the IPv4 logic in
   Section 8.1 of [Ref1]. The IPv6 destination address is chosen from
   among the addresses AllSPFRouters, AllDRouters and the Neighbor IP
   address associated with the other end of the adjacency (which in
   IPv6, for all links except virtual links, is an IPv6 link-local
   address).

   The sending of Link State Request Packets and Link State
   Acknowledgment Packets remains unchanged from the IPv4 procedures
   documented in Sections 10.9 and 13.5 of [Ref1] respectively. Sending
   Hello Packets is documented in Section 3.2.1.1, and the sending of
   Database Description Packets in Section 3.2.1.2. The sending of Link
   State Update Packets is documented in Section 3.5.2.

3.2.1.1.  Sending Hello packets

   IPv6 changes the way OSPF Hello packets are sent in the following
   ways (compare to Section 9.5 of [Ref1]):

   o  Before the Hello Packet is sent out an interface, the interface's
      Interface ID must be copied into the Hello Packet.

   o  The Hello Packet no longer contains an IP network mask, as OSPF
      for IPv6 runs per-link instead of per-subnet.

   o  The choice of Designated Router and Backup Designated Router are
      now indicated within Hellos by their Router IDs, instead of by
      their IP interface addresses.      Advertising the Designated
      Router (or Backup Designated Router) as 0.0.0.0 indicates that the
      Designated Router (or Backup Designated Router) has not yet been
      chosen.

   o  The Options field within Hello packets has moved around, getting
      larger in the process. More options bits are now possible. Those
      that must be set correctly in Hello packets are: The E-bit is set
      if and only if the interface attaches to a non-stub area, the N-
      bit is set if and only if the interface attaches to an NSSA area
      (see [Ref9]), and the DC- bit is set if and only if the router
      wishes to suppress the sending of future Hellos over the interface
      (see [Ref11]). Unrecognized bits in the Hello Packet's Options
      field should be cleared.

   Sending Hello packets on NBMA networks proceeds for IPv6 in exactly
   the same way as for IPv4, as documented in Section 9.5.1 of [Ref1].

3.2.1.2.  Sending Database Description Packets

   The sending of Database Description packets differs from Section 10.8
   of [Ref1] in the following ways:

   o  The Options field within Database Description packets has moved
      around, getting larger in the process. More options bits are now
      possible. Those that must be set correctly in Database Description
      packets are: The MC-bit is set if and only if the router is
      forwarding multicast datagrams according to the MOSPF
      specification in [Ref7], and the DC-bit is set if and only if the
      router wishes to suppress the sending of Hellos over the interface
      (see [Ref11]).  Unrecognized bits in the Database Description
      Packet's Options field should be cleared.

3.2.2.  Receiving protocol packets

   Whenever an OSPF protocol packet is received by the router it is
   marked with the interface it was received on.  For routers that have
   virtual links configured, it may not be immediately obvious which
   interface to associate the packet with.  For example, consider the
   Router RT11 depicted in Figure 6 of [Ref1].  If RT11 receives an OSPF
   protocol packet on its interface to Network N8, it may want to
   associate the packet with the interface to Area 2, or with the
   virtual link to Router RT10 (which is part of the backbone).      In
   the following, we assume that the packet is initially associated with
   the non-virtual link.

   In order for the packet to be passed to OSPF for processing, the
   following tests must be performed on the encapsulating IPv6 headers:

   o  The packet's IP destination address must be one of the IPv6
      unicast addresses associated with the receiving interface (this
      includes link-local addresses), or one of the IP multicast
      addresses AllSPFRouters or AllDRouters.

   o  The Next Header field of the immediately encapsulating IPv6 header
      must specify the OSPF protocol (89).

   o  Any encapsulating IP Authentication Headers (see [Ref19]) and the
      IP Encapsulating Security Payloads (see [Ref20]) must be processed
      and/or verified to ensure integrity and
      authentication/confidentiality of OSPF routing exchanges.

   o  Locally originated packets should not be passed on to OSPF.  That
      is, the source IPv6 address should be examined to make sure this
      is not a multicast packet that the router itself generated.

   After processing the encapsulating IPv6 headers, the OSPF packet
   header is processed.  The fields specified in the header must match
   those configured for the receiving interface.  If they do not, the
   packet should be discarded:

   o  The version number field must specify protocol version 3.

   o  The standard IPv6 16-bit one's complement checksum, covering the
      entire OSPF packet and prepended IPv6 pseudo-header, must be
      verified (see Section A.3.1).

   o  The Area ID found in the OSPF header must be verified.  If both of
      the following cases fail, the packet should be discarded.  The
      Area ID specified in the header must either:

      (1)   Match the Area ID of the receiving interface. In
            this case, unlike for IPv4, the IPv6 source
            address is not restricted to lie on the same IP
            subnet as the receiving interface. IPv6 OSPF runs
            per-link, instead of per-IP-subnet.

      (2)   Indicate the backbone.  In this case, the packet
            has been sent over a virtual link.  The receiving
            router must be an area border router, and the
            Router ID specified in the packet (the source
            router) must be the other end of a configured
            virtual link.  The receiving interface must also
            attach to the virtual link's configured Transit
            area.  If all of these checks succeed, the packet
            is accepted and is from now on associated with
            the virtual link (and the backbone area).

   o  The Instance ID specified in the OSPF header must match the
      receiving interface's Instance ID.

   o  Packets whose IP destination is AllDRouters should only be
      accepted if the state of the receiving interface is DR or Backup
      (see Section 9.1).

   After header processing, the packet is further processed according to
   its OSPF packet type.  OSPF packet types and functions are the same
   for both IPv4 and IPv6.

   If the packet type is Hello, it should then be further processed by
   the Hello Protocol.  All other packet types are sent/received only on
   adjacencies.  This means that the packet must have been sent by one
   of the router's active neighbors. The neighbor is identified by the
   Router ID appearing the the received packet's OSPF header. Packets
   not matching any active neighbor are discarded.

   The receive processing of Database Description Packets, Link State
   Request Packets and Link State Acknowledgment Packets remains
   unchanged from the IPv4 procedures documented in Sections 10.6, 10.7
   and 13.7 of [Ref1] respectively. The receiving of Hello Packets is
   documented in Section 3.2.2.1, and the receiving of Link State Update
   Packets is documented in Section 3.5.1.

3.2.2.1.  Receiving Hello Packets

   The receive processing of Hello Packets differs from Section 10.5 of
   [Ref1] in the following ways:

   o  On all link types (e.g., broadcast, NBMA, point-to- point, etc),
      neighbors are identified solely by their OSPF Router ID. For all
      link types except virtual links, the Neighbor IP address is set to
      the IPv6 source address in the IPv6 header of the received OSPF
      Hello packet.

   o There is no longer a Network Mask field in the Hello Packet.

   o  The neighbor's choice of Designated Router and Backup Designated
      Router is now encoded as an OSPF Router ID instead of an IP
      interface address.

3.3.  The Routing table Structure

   The routing table used by OSPF for IPv4 is defined in Section 11 of
   [Ref1]. For IPv6 there are analogous routing table entries: there are
   routing table entries for IPv6 address prefixes, and also for AS
   boundary routers. The latter routing table entries are only used to
   hold intermediate results during the routing table build process (see
   Section 3.8).

   Also, to hold the intermediate results during the shortest-path
   calculation for each area, there is a separate routing table for each
   area holding the following entries:

   o  An entry for each router in the area. Routers are identified by
      their OSPF router ID. These routing table entries hold the set of
      shortest paths through a given area to a given router, which in
      turn allows calculation of paths to the IPv6 prefixes advertised
      by that router in Intra-area-prefix-LSAs. If the router is also an
      area-border router, these entries are also used to calculate paths
      for inter-area address prefixes. If in addition the router is the
      other endpoint of a virtual link, the routing table entry
      describes the cost and viability of the virtual link.

   o  An entry for each transit link in the area. Transit links have
      associated network-LSAs. Both the transit link and the network-LSA
      are identified by a combination of the Designated Router's
      Interface ID on the link and the Designated Router's OSPF Router
      ID. These routing table entries allow later calculation of paths
      to IP prefixes advertised for the transit link in intra-area-
      prefix-LSAs.

   The fields in the IPv4 OSPF routing table (see Section 11 of [Ref1])
   remain valid for IPv6: Optional capabilities (routers only), path
   type, cost, type 2 cost, link state origin, and for each of the equal
   cost paths to the destination, the next hop and advertising router.

   For IPv6, the link-state origin field in the routing table entry is
   the router-LSA or network-LSA that has directly or indirectly
   produced the routing table entry. For example, if the routing table
   entry describes a route to an IPv6 prefix, the link state origin is
   the router-LSA or network-LSA that is listed in the body of the
   intra-area-prefix-LSA that has produced the route (see Section
   A.4.9).

3.3.1.  Routing table lookup

   Routing table lookup (i.e., determining the best matching routing
   table entry during IP forwarding) is the same for IPv6 as for IPv4.

3.4.  Link State Advertisements

   For IPv6, the OSPF LSA header has changed slightly, with the LS type
   field expanding and the Options field being moved into the body of
   appropriate LSAs. Also, the formats of some LSAs have changed
   somewhat (namely router-LSAs, network-LSAs and AS-external-LSAs),
   while the names of other LSAs have been changed (type 3 and 4
   summary-LSAs are now inter-area-prefix-LSAs and inter-area-router-

   LSAs respectively) and additional LSAs have been added (Link-LSAs and
   Intra-Area-Prefix-LSAs). Type of Service (TOS) has been removed from
   the OSPFv2 specification [Ref1], and is not encoded within OSPF for
   IPv6's LSAs.

   These changes will be described in detail in the following
   subsections.

3.4.1.  The LSA Header

   In both IPv4 and IPv6, all OSPF LSAs begin with a standard 20 byte
   LSA header. However, the contents of this 20 byte header have changed
   in IPv6. The LS age, Advertising Router, LS Sequence Number, LS
   checksum and length fields within the LSA header remain unchanged, as
   documented in Sections 12.1.1, 12.1.5, 12.1.6, 12.1.7 and A.4.1 of
   [Ref1] respectively.  However, the following fields have changed for
   IPv6:

   Options
      The Options field has been removed from the standard 20 byte LSA
      header, and into the body of router-LSAs, network-LSAs, inter-
      area-router-LSAs and link-LSAs. The size of the Options field has
      increased from 8 to 24 bits, and some of the bit definitions have
      changed (see Section A.2). In addition a separate PrefixOptions
      field, 8 bits in length, is attached to each prefix advertised
      within the body of an LSA.

   LS type
      The size of the LS type field has increased from 8 to 16 bits,
      with the top two bits encoding flooding scope and the next bit
      encoding the handling of unknown LS types.  See Section A.4.2.1
      for the current coding of the LS type field.

   Link State ID
      Link State ID remains at 32 bits in length, but except for
      network-LSAs and link-LSAs, Link State ID has shed any addressing
      semantics. For example, an IPv6 router originating multiple AS-
      external-LSAs could start by assigning the first a Link State ID
      of 0.0.0.1, the second a Link State ID of 0.0.0.2, and so on.
      Instead of the IPv4 behavior of encoding the network number within
      the AS-external-LSA's Link State ID, the IPv6 Link State ID simply
      serves as a way to differentiate multiple LSAs originated by the
      same router.

      For network-LSAs, the Link State ID is set to the Designated
      Router's Interface ID on the link. When a router originates a
      Link-LSA for a given link, its Link State ID is set equal to the
      router's Interface ID on the link.

3.4.2.  The link-state database

   In IPv6, as in IPv4, individual LSAs are identified by a combination
   of their LS type, Link State ID and Advertising Router fields. Given
   two instances of an LSA, the most recent instance is determined by
   examining the LSAs' LS Sequence Number, using LS checksum and LS age
   as tiebreakers (see Section 13.1 of [Ref1]).

   In IPv6, the link-state database is split across three separate data
   structures. LSAs with AS flooding scope are contained within the
   top-level OSPF data structure (see Section 3.1) as long as either
   their LS type is known or their U-bit is 1 (flood even when
   unrecognized); this includes the AS-external-LSAs. LSAs with area
   flooding scope are contained within the appropriate area structure
   (see Section 3.1.1) as long as either their LS type is known or their
   U-bit is 1 (flood even when unrecognized); this includes router-LSAs,
   network-LSAs, inter-area-prefix-LSAs, inter-area-router-LSAs, and
   intra-area-prefix-LSAs. LSAs with unknown LS type and U-bit set to 0
   and/or link-local flooding scope are contained within the appropriate
   interface structure (see Section 3.1.2); this includes link-LSAs.

   To lookup or install an LSA in the database, you first examine the LS
   type and the LSA's context (i.e., to which area or link does the LSA
   belong). This information allows you to find the correct list of
   LSAs, all of the same LS type, where you then search based on the
   LSA's Link State ID and Advertising Router.

3.4.3.  Originating LSAs

   The process of reoriginating an LSA in IPv6 is the same as in IPv4:
   the LSA's LS sequence number is incremented, its LS age is set to 0,
   its LS checksum is calculated, and the LSA is added to the link state
   database and flooded out the appropriate interfaces.

   To the list of events causing LSAs to be reoriginated, which for IPv4
   is given in Section 12.4 of [Ref1], the following events and/or
   actions are added for IPv6:

   o  The state of one of the router's interfaces changes. The router
      may need to (re)originate or flush its Link-LSA and one or more
      router-LSAs and/or intra-area-prefix-LSAs.

   o  The identity of a link's Designated Router changes. The router may
      need to (re)originate or flush the link's network-LSA and one or
      more router-LSAs and/or intra-area-prefix-LSAs.

   o  A neighbor transitions to/from "Full" state.  The router may need
      to (re)originate or flush the link's network-LSA and one or more
      router-LSAs and/or intra-area-prefix-LSAs.

   o  The Interface ID of a neighbor changes. This may cause a new
      instance of a router-LSA to be originated for the associated area,
      and the reorigination of one or more intra-area-prefix-LSAs.

   o  A new prefix is added to an attached link, or a prefix is deleted
      (both through configuration). This causes the router to
      reoriginate its link-LSA for the link, or, if it is the only
      router attached to the link, causes the router to reoriginate an
      intra-area-prefix-LSA.

   o  A new link-LSA is received, causing the link's collection of
      prefixes to change. If the router is Designated Router for the
      link, it originates a new intra-area-prefix-LSA.

   Detailed construction of the seven required IPv6 LSA types is
   supplied by the following subsections. In order to display example
   LSAs, the network map in Figure 15 of [Ref1] has been reworked to
   show IPv6 addressing, resulting in Figure 1. The OSPF cost of each
   interface is has been displayed in Figure 1. The assignment of IPv6
   prefixes to network links is shown in Table 1. A single area address
   range has been configured for Area 1, so that outside of Area 1 all
   of its prefixes are covered by a single route to 5f00:0000:c001::/48.
   The OSPF interface IDs and the link-local addresses for the router
   interfaces in Figure 1 are given in Table 2.

       ..........................................
       .                                  Area 1.
       .     +                                  .
       .     |                                  .
       .     | 3+---+1                          .
       .  N1 |--|RT1|-----+                     .
       .     |  +---+      \                    .
       .     |              \  ______           .
       .     +               \/       \      1+---+
       .                     *    N3   *------|RT4|------
       .     +               /\_______/       +---+
       .     |              /     |             .
       .     | 3+---+1     /      |             .
       .  N2 |--|RT2|-----+      1|             .
       .     |  +---+           +---+           .
       .     |                  |RT3|----------------
       .     +                  +---+           .
       .                          |2            .
       .                          |             .
       .                   +------------+       .
       .                          N4            .
       ..........................................

       Figure 1: Area 1 with IP addresses shown

              Network   IPv6 prefix
              -----------------------------------
              N1        5f00:0000:c001:0200::/56
              N2        5f00:0000:c001:0300::/56
              N3        5f00:0000:c001:0100::/56
              N4        5f00:0000:c001:0400::/56

       Table 1: IPv6 link prefixes for sample network

            Router   interface   Interface ID   link-local address
            -------------------------------------------------------
            RT1      to N1       1              fe80:0001::RT1
                     to N3       2              fe80:0002::RT1
            RT2      to N2       1              fe80:0001::RT2
                     to N3       2              fe80:0002::RT2
            RT3      to N3       1              fe80:0001::RT3
                     to N4       2              fe80:0002::RT3
            RT4      to N3       1              fe80:0001::RT4

       Table 2: OSPF Interface IDs and link-local addresses

3.4.3.1.  Router-LSAs

   The LS type of a router-LSA is set to the value 0x2001.  Router-LSAs
   have area flooding scope. A router may originate one or more router-
   LSAs for a given area. Each router-LSA contains an integral number of
   interface descriptions; taken together, the collection of router-LSAs
   originated by the router for an area describes the collected states
   of all the router's interfaces to the area. When multiple router-LSAs
   are used, they are distinguished by their Link State ID fields.

   The Options field in the router-LSA should be coded as follows. The
   V6-bit should be set. The E-bit should be clear if and only if the
   attached area is an OSPF stub area. The MC-bit should be set if and
   only if the router is running MOSPF (see [Ref8]). The N-bit should be
   set if and only if the attached area is an OSPF NSSA area.  The R-bit
   should be set. The DC-bit should be set if and only if the router can
   correctly process the DoNotAge bit when it appears in the LS age
   field of LSAs (see [Ref11]). All unrecognized bits in the Options
   field should be cleared

   To the left of the Options field, the router capability bits V, E and
   B should be coded according to Section 12.4.1 of [Ref1]. Bit W should
   be coded according to [Ref8].

   Each of the router's interfaces to the area are then described by
   appending "link descriptions" to the router-LSA. Each link
   description is 16 bytes long, consisting of 5 fields: (link) Type,
   Metric, Interface ID, Neighbor Interface ID and Neighbor Router ID
   (see Section A.4.3). Interfaces in state "Down" or "Loopback" are not
   described (although looped back interfaces can contribute prefixes to
   Intra-Area-Prefix-LSAs). Nor are interfaces without any full
   adjacencies described. All other interfaces to the area add zero, one
   or more link descriptions, the number and content of which depend on
   the interface type. Within each link description, the Metric field is
   always set the interface's output cost and the Interface ID field is
   set to the interface's OSPF Interface ID.

   Point-to-point interfaces
      If the neighboring router is fully adjacent, add a Type 1 link
      description (point-to-point). The Neighbor Interface ID field is
      set to the Interface ID advertised by the neighbor in its Hello
      packets, and the Neighbor Router ID field is set to the neighbor's
      Router ID.

   Broadcast and NBMA interfaces
      If the router is fully adjacent to the link's Designated Router,
      or if the router itself is Designated Router and is fully adjacent
      to at least one other router, add a single Type 2 link description
      (transit network). The Neighbor Interface ID field is set to the
      Interface ID advertised by the Designated Router in its Hello
      packets, and the Neighbor Router ID field is set to the Designated
      Router's Router ID.

   Virtual links
      If the neighboring router is fully adjacent, add a Type 4 link
      description (virtual). The Neighbor Interface ID field is set to
      the Interface ID advertised by the neighbor in its Hello packets,
      and the Neighbor Router ID field is set to the neighbor's Router
      ID. Note that the output cost of a virtual link is calculated
      during the routing table calculation (see Section 3.7).

   Point-to-MultiPoint interfaces
      For each fully adjacent neighbor associated with the interface,
      add a separate Type 1 link description (point-to-point) with
      Neighbor Interface ID field set to the Interface ID advertised by
      the neighbor in its Hello packets, and Neighbor Router ID field
      set to the neighbor's Router ID.

   As an example, consider the router-LSA that router RT3 would
   originate for Area 1 in Figure 1. Only a single interface must be
   described, namely that which connects to the transit network N3. It
   assumes that RT4 has been elected Designated Router of Network N3.

     ; RT3's router-LSA for Area 1

     LS age = 0                     ;newly (re)originated
     LS type = 0x2001               ;router-LSA
     Link State ID = 0              ;first fragment
     Advertising Router = 192.1.1.3 ;RT3's Router ID
     bit E = 0                      ;not an AS boundary router
     bit B = 1                      ;area border router
     Options = (V6-bit|E-bit|R-bit)
         Type = 2                     ;connects to N3
         Metric = 1            ;cost to N3
         Interface ID = 1             ;RT3's Interface ID on N3
         Neighbor Interface ID = 1    ;RT4's Interface ID on N3
         Neighbor Router ID = 192.1.1.4 ; RT4's Router ID

   If for example another router was added to Network N4, RT3 would have
   to advertise a second link description for its connection to (the now
   transit) network N4. This could be accomplished by reoriginating the
   above router-LSA, this time with two link descriptions. Or, a

   separate router-LSA could be originated with a separate Link State ID
   (e.g., using a Link State ID of 1) to describe the connection to N4.

   Host routes no longer appear in the router-LSA, but are instead
   included in intra-area-prefix-LSAs.

3.4.3.2.  Network-LSAs

   The LS type of a network-LSA is set to the value 0x2002.  Network-
   LSAs have area flooding scope. A network-LSA is originated for every
   broadcast or NBMA link having two or more attached routers, by the
   link's Designated Router. The network-LSA lists all routers attached
   to the link.

   The procedure for originating network-LSAs in IPv6 is the same as the
   IPv4 procedure documented in Section 12.4.2 of [Ref1], with the
   following exceptions:

   o  An IPv6 network-LSA's Link State ID is set to the Interface ID of
      the Designated Router on the link.

   o  IPv6 network-LSAs do not contain a Network Mask. All addressing
      information formerly contained in the IPv4 network-LSA has now
      been consigned to intra-Area-Prefix-LSAs.

   o  The Options field in the network-LSA is set to the logical OR of
      the Options fields contained within the link's associated link-
      LSAs.  In this way, the network link exhibits a capability when at
      least one of the link's routers requests that the capability be
      asserted.

   As an example, assuming that Router RT4 has been elected Designated
   Router of Network N3 in Figure 1, the following network-LSA is
   originated:

     ; Network-LSA for Network N3

     LS age = 0                     ;newly (re)originated
     LS type = 0x2002               ;network-LSA
     Link State ID = 1              ;RT4's Interface ID on N3
     Advertising Router = 192.1.1.4 ;RT4's Router ID
     Options = (V6-bit|E-bit|R-bit)
            Attached Router = 192.1.1.4    ;Router ID
            Attached Router = 192.1.1.1    ;Router ID
            Attached Router = 192.1.1.2    ;Router ID
            Attached Router = 192.1.1.3    ;Router ID

3.4.3.3.  Inter-Area-Prefix-LSAs

   The LS type of an inter-area-prefix-LSA is set to the value 0x2003.
   Inter-area-prefix-LSAs have area flooding scope. In IPv4, inter-
   area-prefix-LSAs were called type 3 summary-LSAs. Each inter-area-
   prefix-LSA describes a prefix external to the area, yet internal to
   the Autonomous System.

   The procedure for originating inter-area-prefix-LSAs in IPv6 is the
   same as the IPv4 procedure documented in Sections 12.4.3 and 12.4.3.1
   of [Ref1], with the following exceptions:

   o  The Link State ID of an inter-area-prefix-LSA has lost all of its
      addressing semantics, and instead simply serves to distinguish
      multiple inter-area-prefix-LSAs that are originated by the same
      router.

   o  The prefix is described by the PrefixLength, PrefixOptions and
      Address Prefix fields embedded within the LSA body. Network Mask
      is no longer specified.

   o  The NU-bit in the PrefixOptions field should be clear. The coding
      of the MC-bit depends upon whether, and if so how, MOSPF is
      operating in the routing domain (see [Ref8]).

   o  Link-local addresses must never be advertised in inter-area-
      prefix-LSAs.

      As an example, the following shows the inter-area-prefix-LSA that
      Router RT4 originates into the OSPF backbone area, condensing all
      of Area 1's prefixes into the single prefix 5f00:0000:c001::/48.
      The cost is set to 4, which is the maximum cost to all of the
      prefix' individual components. The prefix is padded out to an even
      number of 32-bit words, so that it consumes 64-bits of space
      instead of 48 bits.

        ; Inter-area-prefix-LSA for Area 1 addresses
        ; originated by Router RT4 into the backbone

        LS age = 0                  ;newly (re)originated
        LS type = 0x2003            ;inter-area-prefix-LSA
        Advertising Router = 192.1.1.4       ;RT4's ID
        Metric = 4                  ;maximum to components
        PrefixLength = 48
        PrefixOptions = 0
        Address Prefix = 5f00:0000:c001 ;padded to 64-bits

3.4.3.4.  Inter-Area-Router-LSAs

      The LS type of an inter-area-router-LSA is set to the value
      0x2004. Inter-area-router-LSAs have area flooding scope. In IPv4,
      inter-area-router-LSAs were called type 4 summary-LSAs. Each
      inter-area-router-LSA describes a path to a destination OSPF
      router (an ASBR) that is external to the area, yet internal to the
      Autonomous System.

      The procedure for originating inter-area-router-LSAs in IPv6 is
      the same as the IPv4 procedure documented in Section 12.4.3 of
      [Ref1], with the following exceptions:

   o  The Link State ID of an inter-area-router-LSA is no longer the
      destination router's OSPF Router ID, but instead simply serves to
      distinguish multiple inter-area-router-LSAs that are originated by
      the same router. The destination router's Router ID is now found
      in the body of the LSA.

   o  The Options field in an inter-area-router-LSA should be set equal
      to the Options field contained in the destination router's own
      router-LSA. The Options field thus describes the capabilities
      supported by the destination router.

   As an example, consider the OSPF Autonomous System depicted in Figure
   6 of [Ref1]. Router RT4 would originate into Area 1 the following
   inter-area-router-LSA for destination router RT7.

     ; inter-area-router-LSA for AS boundary router RT7
     ; originated by Router RT4 into Area 1

     LS age = 0                  ;newly (re)originated
     LS type = 0x2004            ;inter-area-router-LSA
     Advertising Router = 192.1.1.4  ;RT4's ID
     Options = (V6-bit|E-bit|R-bit)  ;RT7's capabilities
     Metric = 14                     ;cost to RT7
     Destination Router ID = Router RT7's ID

3.4.3.5.  AS-external-LSAs

   The LS type of an AS-external-LSA is set to the value 0x4005. AS-
   external-LSAs have AS flooding scope. Each AS-external-LSA describes
   a path to a prefix external to the Autonomous System.

   The procedure for originating AS-external-LSAs in IPv6 is the same as
   the IPv4 procedure documented in Section 12.4.4 of [Ref1], with the
   following exceptions:

   o  The Link State ID of an AS-external-LSA has lost all of its
      addressing semantics, and instead simply serves to distinguish
      multiple AS-external-LSAs that are originated by the same router.

   o  The prefix is described by the PrefixLength, PrefixOptions and
      Address Prefix fields embedded within the LSA body. Network Mask
      is no longer specified.

   o  The NU-bit in the PrefixOptions field should be clear. The coding
      of the MC-bit depends upon whether, and if so how, MOSPF is
      operating in the routing domain (see [Ref8]).

   o  Link-local addresses can never be advertised in AS-external-LSAs.

   o  The forwarding address is present in the AS-external-LSA if and
      only if the AS-external-LSA's bit F is set.

   o  The external route tag is present in the AS-external-LSA if and
      only if the AS-external-LSA's bit T is set.

   o  The capability for an AS-external-LSA to reference another LSA has
      been included, by inclusion of the Referenced LS Type field and
      the optional Referenced Link State ID field (the latter present if
      and only if Referenced LS Type is non-zero). This capability is
      for future use; for now Referenced LS Type should be set to 0 and
      received non-zero values for this field should be ignored.

   As an example, consider the OSPF Autonomous System depicted in Figure
   6 of [Ref1]. Assume that RT7 has learned its route to N12 via BGP,
   and that it wishes to advertise a Type 2 metric into the AS.  Further
   assume the the IPv6 prefix for N12 is the value 5f00:0000:0a00::/40.
   RT7 would then originate the following AS-external-LSA for the
   external network N12.  Note that within the AS-external-LSA, N12's
   prefix occupies 64 bits of space, to maintain 32-bit alignment.

     ; AS-external-LSA for Network N12,
     ; originated by Router RT7

     LS age = 0                  ;newly (re)originated
     LS type = 0x4005            ;AS-external-LSA
     Link State ID = 123         ;or something else
     Advertising Router = Router RT7's ID
     bit E = 1                   ;Type 2 metric
     bit F = 0                   ;no forwarding address
     bit T = 1                   ;external route tag included
     Metric = 2
     PrefixLength = 40
     PrefixOptions = 0

     Referenced LS Type = 0      ;no Referenced Link State ID
     Address Prefix = 5f00:0000:0a00 ;padded to 64-bits
     External Route Tag = as per BGP/OSPF interaction

3.4.3.6.  Link-LSAs

   The LS type of a Link-LSA is set to the value 0x0008.  Link-LSAs have
   link-local flooding scope. A router originates a separate Link-LSA
   for each attached link that supports 2 or more (including the
   originating router itself) routers.

   Link-LSAs have three purposes: 1) they provide the router's link-
   local address to all other routers attached to the link and 2) they
   inform other routers attached to the link of a list of IPv6 prefixes
   to associate with the link and 3) they allow the router to assert a
   collection of Options bits in the Network-LSA that will be originated
   for the link.

   A Link-LSA for a given Link L is built in the following fashion:

   o  The Link State ID is set to the router's Interface ID on Link L.

   o  The Router Priority of the router's interface to Link L is
      inserted into the Link-LSA.

   o  The Link-LSA's Options field is set to those bits that the router
      wishes set in Link L's Network LSA.

   o  The router inserts its link-local address on Link L into the
      Link-LSA. This information will be used when the other routers on
      Link L do their next hop calculations (see Section 3.8.1.1).

   o  Each IPv6 address prefix that has been configured into the router
      for Link L is added to the Link-LSA, by specifying values for
      PrefixLength, PrefixOptions, and Address Prefix fields.

   After building a Link-LSA for a given link, the router installs the
   link-LSA into the associate interface data structure and floods the
   Link-LSA onto the link. All other routers on the link will receive
   the Link-LSA, but it will go no further.

   As an example, consider the Link-LSA that RT3 will build for N3 in
   Figure 1. Suppose that the prefix 5f00:0000:c001:0100::/56 has been
   configured within RT3 for N3. This will give rise to the following
   Link-LSA, which RT3 will flood onto N3, but nowhere else. Note that
   not all routers on N3 need be configured with the prefix; those not
   configured will learn the prefix when receiving RT3's Link-LSA.

     ; RT3's Link-LSA for N3

     LS age = 0                  ;newly (re)originated
     LS type = 0x0008            ;Link-LSA
     Link State ID = 1           ;RT3's Interface ID on N3
     Advertising Router = 192.1.1.3 ;RT3's Router ID
     Rtr Pri = 1                 ;RT3's N3 Router Priority
     Options = (V6-bit|E-bit|R-bit)
     Link-local Interface Address = fe80:0001::RT3
     # prefixes = 1
     PrefixLength = 56
     PrefixOptions = 0
     Address Prefix = 5f00:0000:c001:0100 ;pad to 64-bits

3.4.3.7.  Intra-Area-Prefix-LSAs

   The LS type of an intra-area-prefix-LSA is set to the value 0x2009.
   Intra-area-prefix-LSAs have area flooding scope. An intra-area-
   prefix-LSA has one of two functions. It associates a list of IPv6
   address prefixes with a transit network link by referencing a
   network- LSA, or associates a list of IPv6 address prefixes with a
   router by referencing a router-LSA. A stub link's prefixes are
   associated with its attached router.

   A router may originate multiple intra-area-prefix-LSAs for a given
   area, distinguished by their Link State ID fields. Each intra-area-
   prefix-LSA contains an integral number of prefix descriptions.

   A link's Designated Router originates one or more intra-area-prefix-
   LSAs to advertise the link's prefixes throughout the area. For a link
   L, L's Designated Router builds an intra-area-prefix-LSA in the
   following fashion:

   o  In order to indicate that the prefixes are to be associated with
      the Link L, the fields Referenced LS type, Referenced Link State
      ID, and Referenced

      Advertising Router are set to the corresponding fields in Link L's
      network-LSA (namely LS type, Link State ID, and Advertising Router
      respectively). This means that Referenced LS Type is set to
      0x2002, Referenced Link State ID is set to the Designated Router's
      Interface ID on Link L, and Referenced Advertising Router is set
      to the Designated Router's Router ID.

   o  Each Link-LSA associated with Link L is examined (these are in the
      Designated Router's interface structure for Link L). If the Link-
      LSA's Advertising Router is fully adjacent to the Designated
      Router, the list of prefixes in the Link-LSA is copied into the

      intra-area-prefix-LSA that is being built.  Prefixes having the
      NU-bit and/or LA-bit set in their Options field should not be
      copied, nor should link-local addresses be copied.  Each prefix is
      described by the PrefixLength, PrefixOptions, and Address Prefix
      fields. Multiple prefixes having the same PrefixLength and Address
      Prefix are considered to be duplicates; in this case their Prefix
      Options fields should be merged by logically OR'ing the fields
      together, and a single resulting prefix should be copied into the
      intra-area-prefix-LSA. The Metric field for all prefixes is set to
      0.

   o  The "# prefixes" field is set to the number of prefixes that the
      router has copied into the LSA. If necessary, the list of prefixes
      can be spread across multiple intra-area-prefix-LSAs in order to
      keep the LSA size small.

      A router builds an intra-area-prefix-LSA to advertise its own
      prefixes, and those of its attached stub links.  A Router RTX
      would build its intra-area-prefix-LSA in the following fashion:

   o  In order to indicate that the prefixes are to be associated with
      the Router RTX itself, RTX sets Referenced LS type to 0x2001,
      Referenced Link State ID to 0, and Referenced Advertising Router
      to RTX's own Router ID.

   o  Router RTX examines its list of interfaces to the area. If the
      interface is in state Down, its prefixes are not included. If the
      interface has been reported in RTX's router-LSA as a Type 2 link
      description (link to transit network), its prefixes are not
      included (they will be included in the intra-area-prefix-LSA for
      the link instead). If the interface type is Point-to-MultiPoint,
      or the interface is in state Loopback, or the interface connects
      to a point-to-point link which has not been assigned a prefix,
      then the site-local and global scope IPv6 addresses associated
      with the interface (if any) are copied into the intra-area-
      prefix-LSA, setting the LA-bit in the PrefixOptions field, and
      setting the PrefixLength to 128 and the Metric to 0.  Otherwise,
      the list of site-local and global prefixes configured in RTX for
      the link are copied into the intra-area-prefix-LSA by specifying
      the PrefixLength, PrefixOptions, and Address Prefix fields. The
      Metric field for each of these prefixes is set to the interface's
      output cost.

   o  RTX adds the IPv6 prefixes for any directly attached hosts
      belonging to the area (see Section C.7) to the intra-area-prefix-
      LSA.

   o  If RTX has one or more virtual links configured through the area,
      it includes one of its site-local or global scope IPv6 interface
      addresses in the LSA (if it hasn't already), setting the LA-bit in
      the PrefixOptions field, and setting the PrefixLength to 128 and
      the Metric to 0. This information will be used later in the
      routing calculation so that the two ends of the virtual link can
      discover each other's IPv6 addresses.

   o  The "# prefixes" field is set to the number of prefixes that the
      router has copied into the LSA. If necessary, the list of prefixes
      can be spread across multiple intra-area-prefix-LSAs in order to
      keep the LSA size small.

   For example, the intra-area-prefix-LSA originated by RT4 for Network
   N3 (assuming that RT4 is N3's Designated Router), and the intra-
   area-prefix-LSA originated into Area 1 by Router RT3 for its own
   prefixes, are pictured below.

     ; Intra-area-prefix-LSA
     ; for network link N3

     LS age = 0                  ;newly (re)originated
     LS type = 0x2009            ;Intra-area-prefix-LSA
     Link State ID = 5           ;or something
     Advertising Router = 192.1.1.4 ;RT4's Router ID
     # prefixes = 1
     Referenced LS type = 0x2002 ;network-LSA reference
     Referenced Link State ID = 1
     Referenced Advertising Router = 192.1.1.4
     PrefixLength = 56           ;N3's prefix
     PrefixOptions = 0
     Metric = 0
     Address Prefix = 5f00:0000:c001:0100 ;pad

     ; RT3's Intra-area-prefix-LSA
     ; for its own prefixes

     LS age = 0                  ;newly (re)originated
     LS type = 0x2009            ;Intra-area-prefix-LSA
     Link State ID = 177         ;or something
     Advertising Router = 192.1.1.3 ;RT3's Router ID
     # prefixes = 1
     Referenced LS type = 0x2001 ;router-LSA reference
     Referenced Link State ID = 0
     Referenced Advertising Router = 192.1.1.3
     PrefixLength = 56           ;N4's prefix

     PrefixOptions = 0
     Metric = 2                  ;N4 interface cost
     Address Prefix = 5f00:0000:c001:0400 ;pad

   When network conditions change, it may be necessary for a router to
   move prefixes from one intra-area-prefix-LSA to another. For example,
   if the router is Designated Router for a link but the link has no
   other attached routers, the link's prefixes are advertised in an
   intra-area-prefix-LSA referring to the Designated Router's router-
   LSA.  When additional routers appear on the link, a network-LSA is
   originated for the link and the link's prefixes are moved to an
   intra-area-prefix-LSA referring to the network-LSA.

   Note that in the intra-area-prefix-LSA, the "Referenced Advertising
   Router" is always equal to the router that is originating the intra-
   area-prefix-LSA (i.e., the LSA's Advertising Router). The reason that
   the Referenced Advertising Router field appears is that, even though
   it is currently redundant, it may not be in the future. We may
   sometime want to use the same LSA format to advertise address
   prefixes for other protocol suites. In that event, the Designated
   Router may not be running the other protocol suite, and so another of
   the link's routers may need to send out the prefix-LSA. In that case,
   "Referenced Advertising Router" and "Advertising Router" would be
   different.

3.5.  Flooding

   Most of the flooding algorithm remains unchanged from the IPv4
   flooding mechanisms described in Section 13 of [Ref1]. In particular,
   the processes for determining which LSA instance is newer (Section
   13.1 of [Ref1]), responding to updates of self-originated LSAs
   (Section 13.4 of [Ref1]), sending Link State Acknowledgment packets
   (Section 13.5 of [Ref1]), retransmitting LSAs (Section 13.6 of
   [Ref1]) and receiving Link State Acknowledgment packets (Section 13.7
   of [Ref1]) are exactly the same for IPv6 and IPv4.

   However, the addition of flooding scope and handling options for
   unrecognized LSA types (see Section A.4.2.1) has caused some changes
   in the OSPF flooding algorithm: the reception of Link State Updates
   (Section 13 in [Ref1]) and the sending of Link State Updates (Section
   13.3 of [Ref1]) must take into account the LSA's scope and U-bit
   setting. Also, installation of LSAs into the OSPF database (Section
   13.2 of [Ref1]) causes different events in IPv6, due to the
   reorganization of LSA types and contents in IPv6. These changes are
   described in detail below.

3.5.1.  Receiving Link State Update packets

   The encoding of flooding scope in the LS type and the need to process
   unknown LS types causes modifications to the processing of received
   Link State Update packets. As in IPv4, each LSA in a received Link
   State Update packet is examined. In IPv4, eight steps are executed
   for each LSA, as described in Section 13 of [Ref1]. For IPv6, all the
   steps are the same, except that Steps 2 and 3 are modified as
   follows:

   (2)   Examine the LSA's LS type.  If the LS type is
         unknown, the area has been configured as a stub area,
         and either the LSA's flooding scope is set to "AS
         flooding scope" or the U-bit of the LS type is set to
         1 (flood even when unrecognized), then discard the
         LSA and get the next one from the Link State Update
         Packet. This generalizes the IPv4 behavior where AS-
         external-LSAs are not flooded into/throughout stub
         areas.

   (3)   Else if the flooding scope of the LSA is set to
         "reserved", discard the LSA and get the next one from
         the Link State Update Packet.

   Steps 5b (sending Link State Update packets) and 5d (installing LSAs
   in the link state database) in Section 13 of [Ref1] are also somewhat
   different for IPv6, as described in Sections 3.5.2 and 3.5.3 below.

3.5.2.  Sending Link State Update packets

   The sending of Link State Update packets is described in Section 13.3
   of [Ref1]. For IPv4 and IPv6, the steps for sending a Link State
   Update packet are the same (steps 1 through 5 of Section 13.3 in
   [Ref1]). However, the list of eligible interfaces out which to flood
   the LSA is different.  For IPv6, the eligible interfaces are selected
   based on the following factors:

   o  The LSA's flooding scope.

   o  For LSAs with area or link-local flooding scoping, the particular
      area or interface that the LSA is associated with.

   o  Whether the LSA has a recognized LS type.

   o  The setting of the U-bit in the LS type. If the U-bit is set to 0,
      unrecognized LS types are treated as having link-local scope. If
      set to 1, unrecognized LS types are stored and flooded as if they
      were recognized.

   Choosing the set of eligible interfaces then breaks into the
   following cases:

   Case 1
      The LSA's LS type is recognized. In this case, the set of eligible
      interfaces is set depending on the flooding scope encoded in the
      LS type. If the flooding scope is "AS flooding scope", the
      eligible interfaces are all router interfaces excepting virtual
      links. In addition, AS-external-LSAs are not flooded out
      interfaces connecting to stub areas. If the flooding scope is
      "area flooding scope", the set of eligible interfaces are those
      interfaces connecting to the LSA's associated area. If the
      flooding scope is "link-local flooding scope", then there is a
      single eligible interface, the one connecting to the LSA's
      associated link (which, when the LSA is received in a Link State
      Update packet, is also the interface the LSA was received on).

   Case 2
      The LS type is unrecognized, and the U-bit in the LS Type is set
      to 0 (treat the LSA as if it had link-local flooding scope). In
      this case there is a single eligible interface, namely, the
      interface on which the LSA was received.

   Case 3
      The LS type is unrecognized, and the U-bit in the LS Type is set
      to 1 (store and flood the LSA, as if type understood). In this
      case, select the eligible interfaces based on the encoded flooding
      scope as in Case 1 above. However, in this case interfaces
      attached to stub areas are always excluded.

   A further decision must sometimes be made before adding an LSA to a
   given neighbor's link-state retransmission list (Step 1d in Section
   13.3 of [Ref1]). If the LS type is recognized by the router, but not
   by the neighbor (as can be determined by examining the Options field
   that the neighbor advertised in its Database Description packet) and
   the LSA's U-bit is set to 0, then the LSA should be added to the
   neighbor's link-state retransmission list if and only if that
   neighbor is the Designated Router or Backup Designated Router for the
   attached link. The LS types described in detail by this memo, namely
   router-LSAs (LS type 0x2001), network-LSAs (0x2002), Inter-Area-
   Prefix-LSAs (0x2003), Inter-Area-Router-LSAs (0x2004), AS-External-
   LSAs (0x4005), Link-LSAs (0x0008) and Intra-Area-Prefix-LSAs (0x2009)
   are assumed to be understood by all routers. However, as an example
   the group-membership-LSA (0x2006) is understood only by MOSPF routers
   and since it has its U-bit set to 0, it should only be forwarded to a
   non-MOSPF neighbor (determined by examining the MC-bit in the
   neighbor's Database Description packets' Options field) when the
   neighbor is Designated Router or Backup Designated Router for the

   attached link.

   The previous paragraph solves a problem in IPv4 OSPF extensions such
   as MOSPF, which require that the Designated Router support the
   extension in order to have the new LSA types flooded across broadcast
   and NBMA networks (see Section 10.2 of [Ref8]).

3.5.3.  Installing LSAs in the database

   There are three separate places to store LSAs, depending on their
   flooding scope. LSAs with AS flooding scope are stored in the global
   OSPF data structure (see Section 3.1) as long as their LS type is
   known or their U-bit is 1. LSAs with area flooding scope are stored
   in the appropriate area data structure (see Section 3.1.1) as long as
   their LS type is known or their U-bit is 1. LSAs with link-local
   flooding scope, and those LSAs with unknown LS type and U-bit set to
   0 (treat the LSA as if it had link-local flooding scope) are stored
   in the appropriate interface structure.

   When storing the LSA into the link-state database, a check must be
   made to see whether the LSA's contents have changed.  Changes in
   contents are indicated exactly as in Section 13.2 of [Ref1]. When an
   LSA's contents have been changed, the following parts of the routing
   table must be recalculated, based on the LSA's LS type:

   Router-LSAs, Network-LSAs, Intra-Area-Prefix-LSAs and Link-LSAs
      The entire routing table is recalculated, starting with the
      shortest path calculation for each area (see Section 3.8).

   Inter-Area-Prefix-LSAs and Inter-Area-Router-LSAs
      The best route to the destination described by the LSA must be
      recalculated (see Section 16.5 in [Ref1]).  If this destination is
      an AS boundary router, it may also be necessary to re-examine all
      the AS-external-LSAs.

   AS-external-LSAs
      The best route to the destination described by the AS-external-LSA
      must be recalculated (see Section 16.6 in [Ref1]).

   As in IPv4, any old instance of the LSA must be removed from the
   database when the new LSA is installed.  This old instance must also
   be removed from all neighbors' Link state retransmission lists.

3.6.  Definition of self-originated LSAs

   In IPv6 the definition of a self-originated LSA has been simplified
   from the IPv4 definition appearing in Sections 13.4 and 14.1 of
   [Ref1]. For IPv6, self-originated LSAs are those LSAs whose
   Advertising Router is equal to the router's own Router ID.

3.7.  Virtual links

   OSPF virtual links for IPv4 are described in Section 15 of [Ref1].
   Virtual links are the same in IPv6, with the following exceptions:

   o  LSAs having AS flooding scope are never flooded over virtual
      adjacencies, nor are LSAs with AS flooding scope summarized over
      virtual adjacencies during the Database Exchange process. This is
      a generalization of the IPv4 treatment of AS-external-LSAs.

   o  The IPv6 interface address of a virtual link must be an IPv6
      address having site-local or global scope, instead of the link-
      local addresses used by other interface types. This address is
      used as the IPv6 source for OSPF protocol packets sent over the
      virtual link.

   o  Likewise, the virtual neighbor's IPv6 address is an IPv6 address
      with site-local or global scope. To enable the discovery of a
      virtual neighbor's IPv6 address during the routing calculation,
      the neighbor advertises its virtual link's IPv6 interface address
      in an Intra-Area-Prefix-LSA originated for the virtual link's
      transit area (see Sections 3.4.3.7 and 3.8.1).

   o  Like all other IPv6 OSPF interfaces, virtual links are assigned
      unique (within the router) Interface IDs. These are advertised in
      Hellos sent over the virtual link, and in the router's router-
      LSAs.

3.8.  Routing table calculation

   The IPv6 OSPF routing calculation proceeds along the same lines as
   the IPv4 OSPF routing calculation, following the five steps specified
   by Section 16 of [Ref1]. High level differences between the IPv6 and
   IPv4 calculations include:

   o  Prefix information has been removed from router-LSAs, and now is
      advertised in intra-area-prefix-LSAs. Whenever [Ref1] specifies
      that stub networks within router-LSAs be examined, IPv6 will
      instead examine prefixes within intra-area-prefix-LSAs.

   o  Type 3 and 4 summary-LSAs have been renamed inter-area-prefix-LSAs
      and inter-area-router-LSAs (respectively).

   o  Addressing information is no longer encoded in Link State IDs, and
      must instead be found within the body of LSAs.

   o  In IPv6, a router can originate multiple router-LSAs within a
      single area, distinguished by Link State ID. These router-LSAs
      must be treated as a single aggregate by the area's shortest path
      calculation (see Section 3.8.1).

   For each area, routing table entries have been created for the area's
   routers and transit links, in order to store the results of the
   area's shortest-path tree calculation (see Section 3.8.1). These
   entries are then used when processing intra-area-prefix-LSAs, inter-
   area-prefix-LSAs and inter-area-router-LSAs, as described in Section
   3.8.2.

   Events generated as a result of routing table changes (Section 16.7
   of [Ref1]), and the equal-cost multipath logic (Section 16.8 of
   [Ref1]) are identical for both IPv4 and IPv6.

3.8.1.  Calculating the shortest path tree for an area

   The IPv4 shortest path calculation is contained in Section 16.1 of
   [Ref1].  The graph used by the shortest-path tree calculation is
   identical for both IPv4 and IPv6. The graph's vertices are routers
   and transit links, represented by router-LSAs and network-LSAs
   respectively. A router is identified by its OSPF Router ID, while a
   transit link is identified by its Designated Router's Interface ID
   and OSPF Router ID. Both routers and transit links have associated
   routing table entries within the area (see Section 3.3).

   Section 16.1 of [Ref1] splits up the shortest path calculations into
   two stages. First the Dijkstra calculation is performed, and then the
   stub links are added onto the tree as leaves. The IPv6 calculation
   maintains this split.

   The Dijkstra calculation for IPv6 is identical to that specified for
   IPv4, with the following exceptions (referencing the steps from the
   Dijkstra calculation as described in Section 16.1 of [Ref1]):

   o  The Vertex ID for a router is the OSPF Router ID. The Vertex ID
      for a transit network is a combination of the Interface ID and
      OSPF Router ID of the network's Designated Router.

   o  In Step 2, when a router Vertex V has just been added to the
      shortest path tree, there may be multiple LSAs associated with the
      router. All Router-LSAs with Advertising Router set to V's OSPF
      Router ID must processed as an aggregate, treating them as
      fragments of a single large router-LSA. The Options field and the
      router type bits (bits W, V, E and B) should always be taken from
      "fragment" with the smallest Link State ID.

   o  Step 2a is not needed in IPv6, as there are no longer stub network
      links in router-LSAs.

   o  In Step 2b, if W is a router, there may again be multiple LSAs
      associated with the router. All Router-LSAs with Advertising
      Router set to W's OSPF Router ID must processed as an aggregate,
      treating them as fragments of a single large router-LSA.

   o  In Step 4, there are now per-area routing table entries for each
      of an area's routers, instead of just the area border routers.
      These entries subsume all the functionality of IPv4's area border
      router routing table entries, including the maintenance of virtual
      links.  When the router added to the area routing table in this
      step is the other end of a virtual link, the virtual neighbor's IP
      address is set as follows: The collection of intra-area-prefix-
      LSAs originated by the virtual neighbor is examined, with the
      virtual neighbor's IP address being set to the first prefix
      encountered having the "LA-bit" set.

   o  Routing table entries for transit networks, which are no longer
      associated with IP networks, are also modified in Step 4.

   The next stage of the shortest path calculation proceeds similarly to
   the two steps of the second stage of Section 16.1 in [Ref1]. However,
   instead of examining the stub links within router-LSAs, the list of
   the area's intra-area-prefix-LSAs is examined. A prefix advertisement
   whose "NU-bit" is set should not be included in the routing
   calculation. The cost of any advertised prefix is the sum of the
   prefix' advertised metric plus the cost to the transit vertex (either
   router or transit network) identified by intra-area-prefix-LSA's
   Referenced LS type, Referenced Link State ID and Referenced
   Advertising Router fields. This latter cost is stored in the transit
   vertex' routing table entry for the area.

3.8.1.1.  The next hop calculation

   In IPv6, the calculation of the next hop's IPv6 address (which will
   be a link-local address) proceeds along the same lines as the IPv4
   next hop calculation (see Section 16.1.1 of [Ref1]). The only
   difference is in calculating the next hop IPv6 address for a router

   (call it Router X) which shares a link with the calculating router.
   In this case the calculating router assigns the next hop IPv6 address
   to be the link-local interface address contained in Router X's Link-
   LSA (see Section A.4.8) for the link. This procedure is necessary
   since on some links, such as NBMA links, the two routers need not be
   neighbors, and therefore might not be exchanging OSPF Hellos.

3.8.2.  Calculating the inter-area routes

   Calculation of inter-area routes for IPv6 proceeds along the same
   lines as the IPv4 calculation in Section 16.2 of [Ref1], with the
   following modifications:

   o  The names of the Type 3 summary-LSAs and Type 4 summary-LSAs have
      been changed to inter-area-prefix-LSAs and inter-area-router-LSAs
      respectively.

   o  The Link State ID of the above LSA types no longer encodes the
      network or router described by the LSA.  Instead, an address
      prefix is contained in the body of an inter-area-prefix-LSA, and a
      described router's OSPF Router ID is carried in the body of an
      inter-area- router-LSA.

   o  Prefixes having the "NU-bit" set in their Prefix Options field
      should be ignored by the inter-area route calculation.

   When a single inter-area-prefix-LSA or inter-area-router-LSA has
   changed, the incremental calculations outlined in Section 16.5 of
   [Ref1] can be performed instead of recalculating the entire routing
   table.

3.8.3.  Examining transit areas' summary-LSAs

   Examination of transit areas' summary-LSAs in IPv6 proceeds along the
   same lines as the IPv4 calculation in Section 16.3 of [Ref1],
   modified in the same way as the IPv6 inter-area route calculation in
   Section 3.8.2.

3.8.4.  Calculating AS external routes

   The IPv6 AS external route calculation proceeds along the same lines
   as the IPv4 calculation in Section 16.4 of [Ref1], with the following
   exceptions:

   o  The Link State ID of the AS-external-LSA types no longer encodes
      the network described by the LSA. Instead, an address prefix is
      contained in the body of an AS- external-LSA.

   o  The default route is described by AS-external-LSAs which advertise
      zero length prefixes.

   o  Instead of comparing the AS-external-LSA's Forwarding address
      field to 0.0.0.0 to see whether a forwarding address has been
      used, bit F of the external-LSA is examined. A forwarding address
      is in use if and only if bit F is set.

   o  Prefixes having the "NU-bit" set in their Prefix Options field
      should be ignored by the inter-area route calculation.

   When a single AS-external-LSA has changed, the incremental
   calculations outlined in Section 16.6 of [Ref1] can be performed
   instead of recalculating the entire routing table.

3.9.  Multiple interfaces to a single link

   In OSPF for IPv6, a router may have multiple interfaces to a single
   link. All interfaces are involved in the reception and transmission
   of data traffic, however only a single interface sends and receives
   OSPF control traffic. In more detail:

   o  Each of the multiple interfaces are assigned different Interface
      IDs.  In this way the router can automatically detect when
      multiple interfaces attach to the same link, when receiving Hellos
      from its own Router ID but with an Interface ID other than the
      receiving interface's.

   o  The router turns off the sending and receiving of OSPF packets
      (that is, control traffic) on all but one of the interfaces to the
      link. The choice of interface to send and receive control traffic
      is implementation dependent; as one example, the interface with
      the highest Interface ID could be chosen.  If the router is
      elected DR, it will be this interface's Interface ID that will be
      used as the network-LSA's Link State ID.

   o  All the multiple interfaces to the link will however appear in the
      router-LSA. In addition, a Link-LSA will be generated for each of
      the multiple interfaces. In this way, all interfaces will be
      included in OSPF's routing calculations.

   o  If the interface which is responsible for sending and receiving
      control traffic fails,  another will have to take over, reforming
      all neighbor adjacencies from scratch. This failure can be
      detected by the router itself, when the other interfaces to the
      same link cease to hear the router's own Hellos.

References

   [Ref1]  Moy, J., "OSPF Version 2", STD 54, RFC 2328, April 1998.

   [Ref2]  McKenzie, A., "ISO Transport Protocol specification ISO DP
           8073", RFC 905, April 1984.

   [Ref3]  McCloghrie, K. and F. Kastenholz, "The Interfaces Group MIB
           using SMIv2", RFC 2233, November 1997.

   [Ref4]  Fuller, V., Li, T, Yu, J. and K. Varadhan, "Classless Inter-
           Domain Routing (CIDR): an Address Assignment and Aggregation
           Strategy", RFC 1519, September 1993.

   [Ref5]  Varadhan, K., Hares, S. and Y. Rekhter, "BGP4/IDRP for IP---
           OSPF Interaction", RFC 1745, December 1994

   [Ref6]  Reynolds, J. and J. Postel, "Assigned Numbers", STD 2, RFC
           1700, October 1994.

   [Ref7]  deSouza, O. and M. Rodrigues, "Guidelines for Running OSPF
           Over Frame Relay Networks", RFC 1586, March 1994.

   [Ref8]  Moy, J., "Multicast Extensions to OSPF", RFC 1584, March
           1994.

   [Ref9]  Coltun, R. and V. Fuller, "The OSPF NSSA Option", RFC 1587,
           March 1994.

   [Ref10] Ferguson, D., "The OSPF External Attributes LSA",
           unpublished.

   [Ref11] Moy, J., "Extending OSPF to Support Demand Circuits", RFC
           1793, April 1995.

   [Ref12] Mogul, J. and S. Deering, "Path MTU Discovery", RFC 1191,
           November 1990.

   [Ref13] Rekhter, Y. and T. Li, "A Border Gateway Protocol 4 (BGP-4)",
           RFC 1771, March 1995.

   [Ref14] Deering, S. and R. Hinden, "Internet Protocol, Version 6
           (IPv6) Specification", RFC 2460, December 1998.

   [Ref15] Hinden, R. and S. Deering, "IP Version 6 Addressing
           Architecture", RFC 2373, July 1998.

   [Ref16] Conta, A. and S. Deering, "Internet Control Message Protocol
           (ICMPv6) for the Internet Protocol Version 6 (IPv6)
           Specification" RFC 2463, December 1998.

   [Ref17] Narten, T., Nordmark, E. and W. Simpson, "Neighbor Discovery
           for IP Version 6 (IPv6)", RFC 2461, December 1998.

   [Ref18] McCann, J., Deering, S. and J. Mogul, "Path MTU Discovery for
           IP version 6", RFC 1981, August 1996.

   [Ref19] Kent, S. and R. Atkinson, "IP Authentication Header", RFC
           2402, November 1998.

   [Ref20] Kent, S. and R. Atkinson, "IP Encapsulating Security Payload
           (ESP)", RFC 2406, November 1998.

A. OSPF data formats

   This appendix describes the format of OSPF protocol packets and OSPF
   LSAs.  The OSPF protocol runs directly over the IPv6 network layer.
   Before any data formats are described, the details of the OSPF
   encapsulation are explained.

   Next the OSPF Options field is described.  This field describes
   various capabilities that may or may not be supported by pieces of
   the OSPF routing domain. The OSPF Options field is contained in OSPF
   Hello packets, Database Description packets and in OSPF LSAs.

   OSPF packet formats are detailed in Section A.3.

   A description of OSPF LSAs appears in Section A.4. This section
   describes how IPv6 address prefixes are represented within LSAs,
   details the standard LSA header, and then provides formats for each
   of the specific LSA types.

A.1 Encapsulation of OSPF packets

   OSPF runs directly over the IPv6's network layer.  OSPF packets are
   therefore encapsulated solely by IPv6 and local data-link headers.

   OSPF does not define a way to fragment its protocol packets, and
   depends on IPv6 fragmentation when transmitting packets larger than
   the link MTU. If necessary, the length of OSPF packets can be up to
   65,535 bytes.  The OSPF packet types that are likely to be large
   (Database Description Packets, Link State Request, Link State Update,
   and Link State Acknowledgment packets) can usually be split into
   several separate protocol packets, without loss of functionality.
   This is recommended; IPv6 fragmentation should be avoided whenever
   possible.  Using this reasoning, an attempt should be made to limit
   the sizes of OSPF packets sent over virtual links to 1280 bytes
   unless Path MTU Discovery is being performed [Ref14].

   The other important features of OSPF's IPv6 encapsulation are:

   o   Use of IPv6 multicast. Some OSPF messages are multicast, when
       sent over broadcast networks.  Two distinct IP multicast
       addresses are used.  Packets sent to these multicast addresses
       should never be forwarded; they are meant to travel a single hop
       only. As such, the multicast addresses have been chosen with
       link-local scope, and packets sent to these addresses should have
       their IPv6 Hop Limit set to 1.

   AllSPFRouters
      This multicast address has been assigned the value FF02::5.  All
      routers running OSPF should be prepared to receive packets sent to
      this address.  Hello packets are always sent to this destination.
      Also, certain OSPF protocol packets are sent to this address
      during the flooding procedure.

   AllDRouters
      This multicast address has been assigned the value FF02::6.  Both
      the Designated Router and Backup Designated Router must be
      prepared to receive packets destined to this address.  Certain
      OSPF protocol packets are sent to this address during the flooding
      procedure.

   o  OSPF is IP protocol 89.  This number should be inserted in the
      Next Header field of the encapsulating IPv6 header.

A.2 The Options field

   The 24-bit OSPF Options field is present in OSPF Hello packets,
   Database Description packets and certain LSAs (router-LSAs, network-
   LSAs, inter-area-router-LSAs and link-LSAs). The Options field
   enables OSPF routers to support (or not support) optional
   capabilities, and to communicate their capability level to other OSPF
   routers.  Through this mechanism routers of differing capabilities
   can be mixed within an OSPF routing domain.

   An option mismatch between routers can cause a variety of behaviors,
   depending on the particular option. Some option mismatches prevent
   neighbor relationships from forming (e.g., the E-bit below); these
   mismatches are discovered through the sending and receiving of Hello
   packets. Some option mismatches prevent particular LSA types from
   being flooded across adjacencies (e.g., the MC-bit below); these are
   discovered through the sending and receiving of Database Description
   packets. Some option mismatches prevent routers from being included
   in one or more of the various routing calculations because of their
   reduced functionality (again the MC-bit is an example); these
   mismatches are discovered by examining LSAs.

   Six bits of the OSPF Options field have been assigned. Each bit is
   described briefly below. Routers should reset (i.e.  clear)
   unrecognized bits in the Options field when sending Hello packets or
   Database Description packets and when originating LSAs. Conversely,
   routers encountering unrecognized Option bits in received Hello
   Packets, Database Description packets or LSAs should ignore the
   capability and process the packet/LSA normally.

                             1                     2
         0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8  9  0  1  2  3
        -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+--+--+--+--+
         | | | | | | | | | | | | | | | | | |DC| R| N|MC| E|V6|
        -+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+--+--+--+--+--+--+

                        The Options field

   V6-bit
     If this bit is clear, the router/link should be excluded from IPv6
     routing calculations. See Section 3.8 of this memo.

   E-bit
     This bit describes the way AS-external-LSAs are flooded, as
     described in Sections 3.6, 9.5, 10.8 and 12.1.2 of [Ref1].

   MC-bit
     This bit describes whether IP multicast datagrams are forwarded
     according to the specifications in [Ref7].

   N-bit
     This bit describes the handling of Type-7 LSAs, as specified in
     [Ref8].

   R-bit
     This bit (the `Router' bit) indicates whether the originator is an
     active router.  If the router bit is clear routes which transit the
     advertising node cannot be computed. Clearing the router bit would
     be appropriate for a multi-homed host that wants to participate in
     routing, but does not want to forward non-locally addressed
     packets.

   DC-bit
     This bit describes the router's handling of demand circuits, as
     specified in [Ref10].

A.3 OSPF Packet Formats

   There are five distinct OSPF packet types.  All OSPF packet types
   begin with a standard 16 byte header.  This header is described
   first.  Each packet type is then described in a succeeding section.
   In these sections each packet's division into fields is displayed,
   and then the field definitions are enumerated.

   All OSPF packet types (other than the OSPF Hello packets) deal with
   lists of LSAs.  For example, Link State Update packets implement the
   flooding of LSAs throughout the OSPF routing domain. The format of
   LSAs is described in Section A.4.

   The receive processing of OSPF packets is detailed in Section 3.2.2.
   The sending of OSPF packets is explained in Section 3.2.1.

A.3.1 The OSPF packet header

   Every OSPF packet starts with a standard 16 byte header. Together
   with the encapsulating IPv6 headers, the OSPF header contains all the
   information necessary to determine whether the packet should be
   accepted for further processing.  This determination is described in
   Section 3.2.2 of this memo.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |   Version #   |     Type      |         Packet length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Router ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Area ID                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Checksum             |  Instance ID  |      0        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Version #
       The OSPF version number.  This specification documents version 3
       of the OSPF protocol.

   Type
       The OSPF packet types are as follows. See Sections A.3.2 through
       A.3.6 for details.

         Type   Description
         ---------------------------------
         1      Hello
         2      Database Description
         3      Link State Request
         4      Link State Update
         5      Link State Acknowledgment

   Packet length
       The length of the OSPF protocol packet in bytes.  This length
       includes the standard OSPF header.

   Router ID
       The Router ID of the packet's source.

   Area ID
       A 32 bit number identifying the area that this packet belongs to.
       All OSPF packets are associated with a single area.  Most travel
       a single hop only.  Packets travelling over a virtual link are
       labelled with the backbone Area ID of 0.

   Checksum
       OSPF uses the standard checksum calculation for IPv6
       applications: The 16-bit one's complement of the one's complement
       sum of the entire contents of the packet, starting with the OSPF
       packet header, and prepending a "pseudo-header" of IPv6 header
       fields, as specified in [Ref14, section 8.1]. The "Upper-Layer
       Packet Length" in the pseudo-header is set to value of the OSPF
       packet header's length field.  The Next Header value used in the
       pseudo-header is 89. If the packet's length is not an integral
       number of 16-bit words, the packet is padded with a byte of zero
       before checksumming. Before computing the checksum, the checksum
       field in the OSPF packet header is set to 0.

   Instance ID
       Enables multiple instances of OSPF to be run over a single link.
       Each protocol instance would be assigned a separate Instance ID;
       the Instance ID has local link significance only. Received
       packets whose Instance ID is not equal to the receiving
       interface's Instance ID are discarded.

       0       These fields are reserved.  They must be 0.

A.3.2 The Hello packet

   Hello packets are OSPF packet type 1.  These packets are sent
   periodically on all interfaces (including virtual links) in order to
   establish and maintain neighbor relationships.  In addition, Hello
   Packets are multicast on those links having a multicast or broadcast
   capability, enabling dynamic discovery of neighboring routers.

   All routers connected to a common link must agree on certain
   parameters (HelloInterval and RouterDeadInterval).  These parameters
   are included in Hello packets, so that differences can inhibit the
   forming of neighbor relationships. The Hello packet also contains
   fields used in Designated Router election (Designated Router ID and
   Backup Designated Router ID), and fields used to detect bi-
   directionality (the Router IDs of all neighbors whose Hellos have
   been recently received).

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      3               |       1       |  Packet length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Router ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Area ID                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Checksum            |  Instance ID  |      0         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Interface ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Rtr Pri    |             Options                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        HelloInterval         |        RouterDeadInterval      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Designated Router ID                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                Backup Designated Router ID                    |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Neighbor ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    ...                              |

   Interface ID
       32-bit number uniquely identifying this interface among the
       collection of this router's interfaces. For example, in some
       implementations it may be possible to use the MIB-II IfIndex
       ([Ref3]).

   Rtr Pri
       This router's Router Priority.  Used in (Backup) Designated
       Router election.  If set to 0, the router will be ineligible to
       become (Backup) Designated Router.

   Options
       The optional capabilities supported by the router, as documented
       in Section A.2.

   HelloInterval
       The number of seconds between this router's Hello packets.

   RouterDeadInterval
       The number of seconds before declaring a silent router down.

   Designated Router ID
       The identity of the Designated Router for this network, in the
       view of the sending router.  The Designated Router is identified
       by its Router ID. Set to 0.0.0.0 if there is no Designated
       Router.

   Backup Designated Router ID
       The identity of the Backup Designated Router for this network, in
       the view of the sending router.  The Backup Designated Router is
       identified by its IP Router ID.  Set to 0.0.0.0 if there is no
       Backup Designated Router.

   Neighbor ID
       The Router IDs of each router from whom valid Hello packets have
       been seen recently on the network.  Recently means in the last
       RouterDeadInterval seconds.

A.3.3 The Database Description packet

   Database Description packets are OSPF packet type 2.  These packets
   are exchanged when an adjacency is being initialized.  They describe
   the contents of the link-state database.  Multiple packets may be
   used to describe the database.  For this purpose a poll-response
   procedure is used.      One of the routers is designated to be the
   master, the other the slave.  The master sends Database Description
   packets (polls) which are acknowledged by Database Description
   packets sent by the slave (responses).  The responses are linked to
   the polls via the packets' DD sequence numbers.

    0                  1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      3        |       2       |        Packet length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           Router ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Area ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           Checksum            |  Instance ID  |      0        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |       0       |               Options                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        Interface MTU         |       0        |0|0|0|0|0|I|M|MS
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    DD sequence number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-                     An LSA Header                           -+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    ...                              |

   The format of the Database Description packet is very similar to both
   the Link State Request and Link State Acknowledgment packets.  The
   main part of all three is a list of items, each item describing a
   piece of the link-state database.      The sending of Database
   Description Packets is documented in Section 10.8 of [Ref1].  The
   reception of Database Description packets is documented in Section
   10.6 of [Ref1].

   Options
      The optional capabilities supported by the router, as documented
      in Section A.2.

   Interface MTU
      The size in bytes of the largest IPv6 datagram that can be sent
      out the    associated interface, without fragmentation.  The MTUs
      of common Internet link  types can be found in Table 7-1    of
      [Ref12]. Interface MTU should be set to 0 in Database Description
      packets sent over virtual links.

   I-bit
      The Init bit.  When set to 1, this packet is the first in the
      sequence of Database Description Packets.

   M-bit
      The More bit.  When set to 1, it indicates that more Database
      Description Packets are to follow.

   MS-bit
      The Master/Slave bit.  When set to 1, it indicates that the router
      is the master during the Database Exchange process.  Otherwise,
      the router is the slave.

   DD sequence number
      Used to sequence the collection of Database Description Packets.
      The initial value (indicated by the Init bit being set) should be
      unique.  The DD sequence number then increments until the complete
      database description has been sent.

   The rest of the packet consists of a (possibly partial) list of the
   link-state database's pieces.  Each LSA in the database is described
   by its LSA header.      The LSA header is documented in Section
   A.4.1.  It contains all the information required to uniquely identify
   both the LSA and the LSA's current instance.

A.3.4 The Link State Request packet

   Link State Request packets are OSPF packet type 3.  After exchanging
   Database Description packets with a neighboring router, a router may
   find that parts of its link-state database are out-of-date. The Link
   State Request packet is used to request the pieces of the neighbor's
   database that are more up-to-date.  Multiple Link State Request
   packets may need to be used.

   A router that sends a Link State Request packet has in mind the
   precise instance of the database pieces it is requesting. Each
   instance is defined by its LS sequence number, LS checksum, and LS
   age, although these fields are not specified in the Link State
   Request Packet itself.  The router may receive even more recent
   instances in response.

   The sending of Link State Request packets is documented in Section
   10.9 of [Ref1].  The reception of Link State Request packets is
   documented in Section 10.7 of [Ref1].

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      3           |       3       |     Packet length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Router ID                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             Area ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Checksum               |  Instance ID  |      0      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |              0                  |        LS type              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Link State ID                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Advertising Router                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       ...                     |

   Each LSA requested is specified by its LS type, Link State ID, and
   Advertising Router.  This uniquely identifies the LSA, but not its
   instance.  Link State Request packets are understood to be requests
   for the most recent instance (whatever that might be).

A.3.5 The Link State Update packet

   Link State Update packets are OSPF packet type 4.  These packets
   implement the flooding of LSAs.  Each Link State Update packet
   carries a collection of LSAs one hop further from their origin.
   Several LSAs may be included in a single packet.

   Link State Update packets are multicast on those physical networks
   that support multicast/broadcast.  In order to make the flooding
   procedure reliable, flooded LSAs are acknowledged in Link State
   Acknowledgment packets.  If retransmission of certain LSAs is
   necessary, the retransmitted LSAs are always carried by unicast Link
   State Update packets. For more information on the reliable flooding
   of LSAs, consult Section 3.5.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      3        |       4       |         Packet length         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Router ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Area ID                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Checksum            |  Instance ID  |      0         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                           # LSAs                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-                                                            +-+
   |                            LSAs                               |
   +-                                                            +-+
   |                    ...                              |

   # LSAs
      The number of LSAs included in this update.

   The body of the Link State Update packet consists of a list of LSAs.
   Each LSA begins with a common 20 byte header, described in Section
   A.4.2. Detailed formats of the different types of LSAs are described
   in Section A.4.

A.3.6 The Link State Acknowledgment packet

   Link State Acknowledgment Packets are OSPF packet type 5.  To make
   the flooding of LSAs reliable, flooded LSAs are explicitly
   acknowledged.  This acknowledgment is accomplished through the
   sending and receiving of Link State Acknowledgment packets. The
   sending of Link State Acknowledgement packets is documented in
   Section 13.5 of [Ref1].  The reception of Link State Acknowledgement
   packets is documented in Section 13.7 of [Ref1].

   Multiple LSAs can be acknowledged in a single Link State
   Acknowledgment packet.  Depending on the state of the sending
   interface and the sender of the corresponding Link State Update
   packet, a Link State Acknowledgment packet is sent either to the
   multicast address AllSPFRouters, to the multicast address
   AllDRouters, or as a unicast (see Section 13.5 of [Ref1] for
   details).

   The format of this packet is similar to that of the Data Description
   packet.  The body of both packets is simply a list of LSA headers.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      3              |       5       |  Packet length          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         Router ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                          Area ID                              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |          Checksum            |  Instance ID  |      0         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-                        An LSA Header                        -+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    ...                              |

   Each acknowledged LSA is described by its LSA header.  The LSA header
   is documented in Section A.4.2.  It contains all the information
   required to uniquely identify both the LSA and the LSA's current
   instance.

A.4 LSA formats

   This memo defines seven distinct types of LSAs.  Each LSA begins with
   a standard 20 byte LSA header.  This header is explained in Section
   A.4.2.  Succeeding sections then diagram the separate LSA types.

   Each LSA describes a piece of the OSPF routing domain.  Every router
   originates a router-LSA. A network-LSA is advertised for each link by
   its Designated Router. A router's link-local addresses are advertised
   to its neighbors in link-LSAs. IPv6 prefixes are advertised in
   intra-area-prefix-LSAs, inter-area-prefix-LSAs and AS-external-LSAs.
   Location of specific routers can be advertised across area boundaries
   in inter-area-router-LSAs. All LSAs are then flooded throughout the
   OSPF routing domain.  The flooding algorithm is reliable, ensuring
   that all routers have the same collection of LSAs.  (See Section 3.5
   for more information concerning the flooding algorithm).  This
   collection of LSAs is called the link-state database.

   From the link state database, each router constructs a shortest path
   tree with itself as root.  This yields a routing table (see Section
   11 of [Ref1]).  For the details of the routing table build process,
   see Section 3.8.

A.4.1 IPv6 Prefix Representation

   IPv6 addresses are bit strings of length 128. IPv6 routing
   algorithms, and OSPF for IPv6 in particular, advertise IPv6 address
   prefixes. IPv6 address prefixes are bit strings whose length ranges
   between 0 and 128 bits (inclusive).

   Within OSPF, IPv6 address prefixes are always represented by a
   combination of three fields: PrefixLength, PrefixOptions, and Address
   Prefix. PrefixLength is the length in bits of the prefix.
   PrefixOptions is an 8-bit field describing various capabilities
   associated with the prefix (see Section A.4.2). Address Prefix is an
   encoding of the prefix itself as an even multiple of 32-bit words,
   padding with zero bits as necessary; this encoding consumes
   (PrefixLength + 31) / 32) 32-bit words.

   The default route is represented by a prefix of length 0.

   Examples of IPv6 Prefix representation in OSPF can be found in
   Sections A.4.5, A.4.7, A.4.8 and A.4.9.

A.4.1.1 Prefix Options

   Each prefix is advertised along with an 8-bit field of capabilities.
   These serve as input to the various routing calculations, allowing,
   for example, certain prefixes to be ignored in some cases, or to be
   marked as not readvertisable in others.

                  0  1  2  3  4  5  6  7
                 +--+--+--+--+--+--+--+--+
                 |  |  |  |  | P|MC|LA|NU|
                 +--+--+--+--+--+--+--+--+

                 The Prefix Options field

   NU-bit
      The "no unicast" capability bit. If set, the prefix should be
      excluded from IPv6 unicast calculations, otherwise it should be
      included.

   LA-bit
      The "local address" capability bit. If set, the prefix is actually
      an IPv6 interface address of the advertising router.

   MC-bit
      The "multicast" capability bit. If set, the prefix should be
      included in IPv6 multicast routing calculations, otherwise it
      should be excluded.

   P-bit
      The "propagate" bit. Set on NSSA area prefixes that should be
      readvertised at the NSSA area border.

A.4.2 The LSA header

   All LSAs begin with a common 20 byte header.  This header contains
   enough information to uniquely identify the LSA (LS type, Link State
   ID, and Advertising Router).  Multiple instances of the LSA may exist
   in the routing domain at the same time.  It is then necessary to
   determine which instance is more recent.  This is accomplished by
   examining the LS age, LS sequence number and LS checksum fields that
   are also contained in the LSA header.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           LS age             |           LS type              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Link State ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Advertising Router                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    LS sequence number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        LS checksum           |             length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   LS age
      The time in seconds since the LSA was originated.

   LS type
      The LS type field indicates the function performed by the LSA.
      The high-order three bits of LS type encode generic properties of
      the LSA, while the remainder (called LSA function code) indicate
      the LSA's specific functionality. See Section A.4.2.1 for a
      detailed description of LS type.

   Link State ID
      Together with LS type and Advertising Router, uniquely identifies
      the LSA in the link-state database.

   Advertising Router
      The Router ID of the router that originated the LSA.  For example,
      in network-LSAs this field is equal to the Router ID of the
      network's Designated Router.

   LS sequence number
      Detects old or duplicate LSAs.  Successive instances of an LSA are
      given successive LS sequence numbers.  See Section 12.1.6 in
      [Ref1] for more details.

   LS checksum
      The Fletcher checksum of the complete contents of the LSA,
      including the LSA header but excluding the LS age field. See
      Section 12.1.7 in [Ref1] for more details.

   length
      The length in bytes of the LSA.  This includes the 20 byte LSA
      header.

A.4.2.1 LS type

   The LS type field indicates the function performed by the LSA.  The
   high-order three bits of LS type encode generic properties of the
   LSA, while the remainder (called LSA function code) indicate the
   LSA's specific functionality. The format of the LS type is as
   follows:

           0  1  2  3  4  5  6  7  8  9  0  1  2  3  4  5
         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+
         |U |S2|S1|           LSA Function Code          |
         +--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+--+

   The U bit indicates how the LSA should be handled by a router which
   does not recognize the LSA's function code.  Its values are:

     U-bit   LSA Handling
     -------------------------------------------------------------
     0       Treat the LSA as if it had link-local flooding scope
     1       Store and flood the LSA, as if type understood

   The S1 and S2 bits indicate the flooding scope of the LSA. The values
   are:

   S2  S1   Flooding Scope
   ---------------------------------------------------------------------
   0  0    Link-Local Scoping. Flooded only on link it is originated on.
   0  1    Area Scoping. Flooded to all routers in the originating area
   1  0    AS Scoping. Flooded to all routers in the AS
   1  1    Reserved

   The LSA function codes are defined as follows. The origination and
   processing of these LSA function codes are defined elsewhere in this
   memo, except for the group-membership-LSA (see [Ref7]) and the Type-
   7-LSA (see [Ref8]). Each LSA function code also implies a specific
   setting for the U, S1 and S2 bits, as shown below.

         LSA function code   LS Type   Description
         ----------------------------------------------------
         1                   0x2001    Router-LSA
         2                   0x2002    Network-LSA
         3                   0x2003    Inter-Area-Prefix-LSA
         4                   0x2004    Inter-Area-Router-LSA
         5                   0x4005    AS-External-LSA
         6                   0x2006    Group-membership-LSA
         7                   0x2007    Type-7-LSA
         8                   0x0008    Link-LSA
         9                   0x2009    Intra-Area-Prefix-LSA

A.4.3 Router-LSAs

   Router-LSAs have LS type equal to 0x2001.  Each router in an area
   originates one or more router-LSAs.   The complete collection of
   router-LSAs originated by the router describe the state and cost of
   the router's interfaces to the area. For details concerning the
   construction of router-LSAs, see Section 3.4.3.1. Router-LSAs are
   flooded throughout a single area only.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           LS age             |0|0|1|          1               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Link State ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Advertising Router                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    LS sequence number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        LS checksum           |             length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    0  |W|V|E|B|            Options                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |       0       |          Metric               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Interface ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Neighbor Interface ID                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Neighbor Router ID                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |     Type      |       0       |          Metric               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                      Interface ID                             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                   Neighbor Interface ID                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Neighbor Router ID                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             ...                               |

   A single router may originate one or more Router LSAs, distinguished
   by their Link-State IDs (which are chosen arbitrarily by the
   originating router).  The Options field and V, E and B bits should be
   the same in all Router LSAs from a single originator.  However, in
   the case of a mismatch the values in the LSA with the lowest Link
   State ID take precedence. When more than one Router LSA is received
   from a single router, the links are processed as if concatenated into
   a single LSA.

   bit V
      When set, the router is an endpoint of one or more fully adjacent
      virtual links having the described area as Transit area (V is for
      virtual link endpoint).

   bit E
      When set, the router is an AS boundary router (E is for external).

   bit B
      When set, the router is an area border router (B is for border).

   bit W
      When set, the router is a wild-card multicast receiver.  When
      running MOSPF, these routers receive all multicast datagrams,
      regardless of destination. See Sections 3, 4 and A.2 of [Ref8] for
      details.

   Options
      The optional capabilities supported by the router, as documented
      in Section A.2.

   The following fields are used to describe each router interface.  The
   Type field indicates the kind of interface being described.  It may
   be an interface to a transit network, a point-to-point connection to
   another router or a virtual link.  The values of all the other fields
   describing a router interface depend on the interface's Type field.

   Type
      The kind of interface being described.  One of the following:

          Type   Description
          ---------------------------------------------------
          1      Point-to-point connection to another router
          2      Connection to a transit network
          3      Reserved
          4      Virtual link

   Metric
      The cost of using this router interface, for outbound traffic.

   Interface ID
      The Interface ID assigned to the interface being described. See
      Sections 3.1.2 and C.3.

   Neighbor Interface ID
      The Interface ID the neighbor router (or the attached link's
      Designated Router, for Type 2 interfaces) has been advertising
      in hello packets sent on the attached link.

   Neighbor Router ID
      The Router ID the neighbor router (or the attached link's
      Designated Router, for Type 2 interfaces).

      For Type 2 links, the combination of Neighbor Interface ID and
      Neighbor Router ID allows the network-LSA for the attached link
      to be found in the link-state database.

A.4.4 Network-LSAs

   Network-LSAs have LS type equal to 0x2002.  A network-LSA is
   originated for each broadcast and NBMA link in the area which
   supports two or more routers.  The network-LSA is originated by the
   link's Designated Router.  The LSA describes all routers attached to
   the link, including the Designated Router itself.  The LSA's Link
   State ID field is set to the Interface ID that the Designated Router
   has been advertising in Hello packets on the link.

   The distance from the network to all attached routers is zero.  This
   is why the metric fields need not be specified in the network-LSA.
   For details concerning the construction of network-LSAs, see Section
   3.4.3.2.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           LS age             |0|0|1|          2               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Link State ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Advertising Router                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    LS sequence number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        LS checksum           |             length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      0               |              Options                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Attached Router                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             ...                               |

   Attached Router
      The Router IDs of each of the routers attached to the link.
      Actually, only those routers that are fully adjacent to the
      Designated Router are listed.  The Designated Router includes
      itself in this list.  The number of routers included can be
      deduced from the LSA header's length field.

A.4.5 Inter-Area-Prefix-LSAs

   Inter-Area-Prefix-LSAs have LS type equal to 0x2003.  These LSAs are
   are the IPv6 equivalent of OSPF for IPv4's type 3 summary-LSAs (see
   Section 12.4.3 of [Ref1]).  Originated by area border routers, they
   describe routes to IPv6 address prefixes that belong to other areas.
   A separate Inter-Area-Prefix-LSA is originated for each IPv6 address
   prefix. For details concerning the construction of Inter-Area-
   Prefix-LSAs, see Section 3.4.3.3.

   For stub areas, Inter-Area-Prefix-LSAs can also be used to describe a
   (per-area) default route.  Default summary routes are used in stub
   areas instead of flooding a complete set of external routes.  When
   describing a default summary route, the Inter-Area-Prefix-LSA's
   PrefixLength is set to 0.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           LS age             |0|0|1|          3               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Link State ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Advertising Router                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    LS sequence number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        LS checksum           |             length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      0               |                  Metric                |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | PrefixLength  | PrefixOptions |             (0)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Address Prefix                         |
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Metric
      The cost of this route.  Expressed in the same units as the
      interface costs in the router-LSAs. When the Inter-Area-Prefix-LSA
      is describing a route to a range of addresses (see Section C.2)
      the cost is set to the maximum cost to any reachable component of
      the address range.

   PrefixLength, PrefixOptions and Address Prefix
      Representation of the IPv6 address prefix, as described in Section
      A.4.1.

A.4.6 Inter-Area-Router-LSAs

   Inter-Area-Router-LSAs have LS type equal to 0x2004.  These LSAs are
   are the IPv6 equivalent of OSPF for IPv4's type 4 summary-LSAs (see
   Section 12.4.3 of [Ref1]).  Originated by area border routers, they
   describe routes to routers in other areas.  (To see why it is
   necessary to advertise the location of each ASBR, consult Section
   16.4 in [Ref1].)  Each LSA describes a route to a single router. For
   details concerning the construction of Inter-Area-Router-LSAs, see
   Section 3.4.3.4.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           LS age             |0|0|1|        4                 |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Link State ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Advertising Router                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    LS sequence number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        LS checksum           |             length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      0               |          Options                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |      0               |          Metric                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Destination Router ID                      |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Options
      The optional capabilities supported by the router, as documented
      in Section A.2.

   Metric
      The cost of this route.  Expressed in the same units as the
      interface costs in the router-LSAs.

   Destination Router ID
      The Router ID of the router being described by the LSA.

A.4.7 AS-external-LSAs

   AS-external-LSAs have LS type equal to 0x4005.  These LSAs are
   originated by AS boundary routers, and describe destinations external
   to the AS. Each LSA describes a route to a single IPv6 address
   prefix. For details concerning the construction of AS-external-LSAs,
   see Section 3.4.3.5.

   AS-external-LSAs can be used to describe a default route.  Default
   routes are used when no specific route exists to the destination.
   When describing a default route, the AS-external-LSA's PrefixLength
   is set to 0.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           LS age             |0|1|0|          5               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Link State ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Advertising Router                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    LS sequence number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        LS checksum           |             length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        |E|F|T|                 Metric                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   | PrefixLength  | PrefixOptions |     Referenced LS Type        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Address Prefix                         |
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-                Forwarding Address (Optional)                -+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           External Route Tag (Optional)                       |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Referenced Link State ID (Optional)             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   bit E
      The type of external metric.  If bit E is set, the metric
      specified is a Type 2 external metric.  This means the metric is
      considered larger than any intra-AS path.  If bit E is zero, the
      specified metric is a Type 1 external metric.  This means that it
      is expressed in the same units as the link state metric (i.e., the
      same units as interface cost).

   bit F
      If set, a Forwarding Address has been included in the LSA.

   bit T
      If set, an External Route Tag has been included in the LSA.

   Metric
      The cost of this route.  Interpretation depends on the external
      type indication (bit E above).

   PrefixLength, PrefixOptions and Address Prefix
      Representation of the IPv6 address prefix, as described in Section
      A.4.1.

   Referenced LS type
      If non-zero, an LSA with this LS type is to be associated with
      this LSA (see Referenced Link State ID below).

   Forwarding address
      A fully qualified IPv6 address (128 bits).  Included in the LSA if
      and only if bit F has been set.  If included, Data traffic for the
      advertised destination will be forwarded to this address. Must not
      be set to the IPv6 Unspecified Address (0:0:0:0:0:0:0:0).

   External Route Tag
      A 32-bit field which may be used to communicate additional
      information between AS boundary routers; see [Ref5] for example
      usage. Included in the LSA if and only if bit T has been set.

   Referenced Link State ID Included if and only if Reference LS Type is
      non-zero.  If included, additional information concerning the
      advertised external route can be found in the LSA having LS type
      equal to "Referenced LS Type", Link State ID equal to "Referenced
      Link State ID" and Advertising Router the same as that specified
      in the AS-external-LSA's link state header. This additional
      information is not used by the OSPF protocol itself.  It may be
      used to communicate information between AS boundary routers; the
      precise nature of such information is outside the scope of this
      specification.

   All, none or some of the fields labeled Forwarding address, External
   Route Tag and Referenced Link State ID may be present in the AS-
   external-LSA (as indicated by the setting of bit F, bit T and
   Referenced LS type respectively). However, when present Forwarding
   Address always comes first, with External Route Tag always preceding
   Referenced Link State ID.

A.4.8 Link-LSAs

   Link-LSAs have LS type equal to 0x0008.  A router originates a
   separate Link-LSA for each link it is attached to. These LSAs have
   local-link flooding scope; they are never flooded beyond the link
   that they are associated with. Link-LSAs have three purposes: 1) they
   provide the router's link-local address to all other routers attached
   to the link and 2) they inform other routers attached to the link of
   a list of IPv6 prefixes to associate with the link and 3) they allow
   the router to assert a collection of Options bits to associate with
   the Network-LSA that will be originated for the link.

   A link-LSA's Link State ID is set equal to the originating router's
   Interface ID on the link.

    0                   1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           LS age             |0|0|0|           8              |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Link State ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     Advertising Router                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                     LS sequence number                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        LS checksum           |             length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |    Rtr Pri    |                Options                        |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-                Link-local Interface Address                 -+
   |                                                               |
   +-                                                             -+
   |                                                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                         # prefixes                            |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PrefixLength | PrefixOptions |             (0)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Address Prefix                         |
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PrefixLength | PrefixOptions |             (0)               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                        Address Prefix                         |
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   Rtr Pri
      The Router Priority of the interface attaching the originating
      router to the link.

   Options
      The set of Options bits that the router would like set in the
      Network-LSA that will be originated for the link.

   Link-local Interface Address
      The originating router's link-local interface address on the
      link.

   # prefixes
      The number of IPv6 address prefixes contained in the LSA.

      The rest of the link-LSA contains a list of IPv6 prefixes to be
      associated with the link.

   PrefixLength, PrefixOptions and Address Prefix
      Representation of an IPv6 address prefix, as described in
      Section A.4.1.

A.4.9 Intra-Area-Prefix-LSAs

   Intra-Area-Prefix-LSAs have LS type equal to 0x2009. A router uses
   Intra-Area-Prefix-LSAs to advertise one or more IPv6 address
   prefixes that are associated with a) the router itself, b) an
   attached stub network segment or c) an attached transit network
   segment. In IPv4, a) and b) were accomplished via the router's
   router-LSA, and c) via a network-LSA. However, in OSPF for IPv6 all
   addressing information has been removed from router-LSAs and
   network-LSAs, leading to the introduction of the Intra-Area-Prefix-LSA.

   A router can originate multiple Intra-Area-Prefix-LSAs for each
   router or transit network; each such LSA is distinguished by its
   Link State ID.

    0                  1                   2                   3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |           LS age             |0|0|1|            9             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Link State ID                           |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    Advertising Router                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                    LS sequence number                         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |        LS checksum           |             length             |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |         # prefixes           |     Referenced LS type         |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                  Referenced Link State ID                     |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |               Referenced Advertising Router                   |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PrefixLength | PrefixOptions |          Metric               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Address Prefix                          |
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |  PrefixLength | PrefixOptions |          Metric               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |                       Address Prefix                          |
   |                             ...                               |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   # prefixes
      The number of IPv6 address prefixes contained in the LSA.

      Router
   Referenced LS type, Referenced Link State ID and Referenced
      Advertising
      Identifies the router-LSA or network-LSA with which the IPv6
      address prefixes should be associated. If Referenced LS type is 1,
      the prefixes are associated with a router-LSA, Referenced Link
      State ID should be 0 and Referenced Advertising Router should be
      the originating router's Router ID. If Referenced LS type is 2,
      the prefixes are associated with a network-LSA, Referenced Link
      State ID should be the Interface ID of the link's Designated
      Router and Referenced Advertising Router should be the Designated
      Router's Router ID.

   The rest of the Intra-Area-Prefix-LSA contains a list of IPv6
   prefixes to be associated with the router or transit link, together
   with the cost of each prefix.

   PrefixLength, PrefixOptions and Address Prefix
      Representation of an IPv6 address prefix, as described in Section
      A.4.1.

   Metric
      The cost of this prefix.  Expressed in the same units as the
      interface costs in the router-LSAs.

B. Architectural Constants

   Architectural constants for the OSPF protocol are defined in Appendix
   B of [Ref1]. The only difference for OSPF for IPv6 is that
   DefaultDestination is encoded as a prefix of length 0 (see Section
   A.4.1).

C. Configurable Constants

   The OSPF protocol has quite a few configurable parameters.  These
   parameters are listed below.  They are grouped into general
   functional categories (area parameters, interface parameters, etc.).
   Sample values are given for some of the parameters.

   Some parameter settings need to be consistent among groups of
   routers.  For example, all routers in an area must agree on that
   area's parameters, and all routers attached to a network must agree
   on that network's HelloInterval and RouterDeadInterval.

   Some parameters may be determined by router algorithms outside of
   this specification (e.g., the address of a host connected to the
   router via a SLIP line).  From OSPF's point of view, these items are
   still configurable.

C.1 Global parameters

   In general, a separate copy of the OSPF protocol is run for each
   area.  Because of this, most configuration parameters are defined on
   a per-area basis.  The few global configuration parameters are listed
   below.

   Router ID
      This is a 32-bit number that uniquely identifies the router in the
      Autonomous System. If a router's OSPF Router ID is changed, the
      router's OSPF software should be restarted before the new Router
      ID takes effect. Before restarting in order to change its Router

      ID, the router should flush its self-originated LSAs from the
      routing domain (see Section 14.1 of [Ref1]), or they will persist
      for up to MaxAge minutes.

      Because the size of the Router ID is smaller than an IPv6 address,
      it cannot be set to one of the router's IPv6 addresses (as is
      commonly done for IPv4). Possible Router ID assignment procedures
      for IPv6 include: a) assign the IPv6 Router ID as one of the
      router's IPv4 addresses or b) assign IPv6 Router IDs through some
      local administrative procedure (similar to procedures used by
      manufacturers to assign product serial numbers).

      The Router ID of 0.0.0.0 is reserved, and should not be used.

C.2 Area parameters

   All routers belonging to an area must agree on that area's
   configuration.  Disagreements between two routers will lead to an
   inability for adjacencies to form between them, with a resulting
   hindrance to the flow of routing protocol and data traffic.  The
   following items must be configured for an area:

   Area ID
       This is a 32-bit number that identifies the area.  The Area
       ID of 0 is reserved for the backbone.

   List of address ranges
       Address ranges control the advertisement of routes across
       area boundaries. Each address range consists of the
       following items:

       [IPv6 prefix, prefix length]
               Describes the collection of IPv6 addresses contained in
               the address range.

       Status  Set to either Advertise or DoNotAdvertise.  Routing
               information is condensed at area boundaries.  External to
               the area, at most a single route is advertised (via a
               inter-area-prefix-LSA) for each address range. The route
               is advertised if and only if the address range's Status
               is set to Advertise.  Unadvertised ranges allow the
               existence of certain networks to be intentionally hidden
               from other areas. Status is set to Advertise by default.

   ExternalRoutingCapability
      Whether AS-external-LSAs will be flooded into/throughout the area.
      If AS-external-LSAs are excluded from the area, the area is called
      a "stub".  Internal to stub areas, routing to external
      destinations will be based solely on a default inter-area route.
      The backbone cannot be configured as a stub area. Also, virtual
      links cannot be configured through stub areas. For more
      information, see Section 3.6 of [Ref1].

   StubDefaultCost
      If the area has been configured as a stub area, and the router
      itself is an area border router, then the StubDefaultCost
      indicates the cost of the default inter-area-prefix-LSA that the
      router should advertise into the area. See Section 12.4.3.1 of
      [Ref1] for more information.

C.3 Router interface parameters

   Some of the configurable router interface parameters (such as Area
   ID, HelloInterval and RouterDeadInterval) actually imply properties
   of the attached links, and therefore must be consistent across all
   the routers attached to that link.  The parameters that must be
   configured for a router interface are:

   IPv6 link-local address
      The IPv6 link-local address associated with this interface.  May
      be learned through auto-configuration.

   Area ID
      The OSPF area to which the attached link belongs.

   Instance ID
      The OSPF protocol instance associated with this OSPF interface.
      Defaults to 0.

   Interface ID
      32-bit number uniquely identifying this interface among the
      collection of this router's interfaces. For example, in some
      implementations it may be possible to use the MIB-II IfIndex
      ([Ref3]).

   IPv6 prefixes
      The list of IPv6 prefixes to associate with the link. These will
      be advertised in intra-area-prefix-LSAs.

   Interface output cost(s)
      The cost of sending a packet on the interface, expressed in the
      link state metric.  This is advertised as the link cost for this
      interface in the router's router-LSA. The interface output cost
      must always be greater than 0.

   RxmtInterval
      The number of seconds between LSA retransmissions, for adjacencies
      belonging to this interface.  Also used when retransmitting
      Database Description and Link State Request Packets.  This should
      be well over the expected round-trip delay between any two routers
      on the attached link.  The setting of this value should be
      conservative or needless retransmissions will result.  Sample
      value for a local area network: 5 seconds.

   InfTransDelay
      The estimated number of seconds it takes to transmit a Link State
      Update Packet over this interface.  LSAs contained in the update
      packet must have their age incremented by this amount before
      transmission.  This value should take into account the
      transmission and propagation delays of the interface. It must be
      greater than 0.  Sample value for a local area network: 1 second.

   Router Priority
      An 8-bit unsigned integer. When two routers attached to a network
      both attempt to become Designated Router, the one with the highest
      Router Priority takes precedence. If there is still a tie, the
      router with the highest Router ID takes precedence.  A router
      whose Router Priority is set to 0 is ineligible to become
      Designated Router on the attached link.  Router Priority is only
      configured for interfaces to broadcast and NBMA networks.

   HelloInterval
      The length of time, in seconds, between the Hello Packets that the
      router sends on the interface.  This value is advertised in the
      router's Hello Packets.  It must be the same for all routers
      attached to a common link.  The smaller the HelloInterval, the
      faster topological changes will be detected; however, more OSPF
      routing protocol traffic will ensue.  Sample value for a X.25 PDN:
      30 seconds.  Sample value for a local area network (LAN): 10
      seconds.

   RouterDeadInterval
      After ceasing to hear a router's Hello Packets, the number of
      seconds before its neighbors declare the router down.  This is
      also advertised in the router's Hello Packets in their

      RouterDeadInterval field.  This should be some multiple of the
      HelloInterval (say 4).  This value again must be the same for all
      routers attached to a common link.

C.4 Virtual link parameters

   Virtual links are used to restore/increase connectivity of the
   backbone.  Virtual links may be configured between any pair of area
   border routers having interfaces to a common (non-backbone) area.
   The virtual link appears as an unnumbered point-to-point link in the
   graph for the backbone.  The virtual link must be configured in both
   of the area border routers.

   A virtual link appears in router-LSAs (for the backbone) as if it
   were a separate router interface to the backbone.  As such, it has
   most of the parameters associated with a router interface (see
   Section C.3).  Virtual links do not have link-local addresses, but
   instead use one of the router's global-scope or site-local IPv6
   addresses as the IP source in OSPF protocol packets it sends along
   the virtual link.  Router Priority is not used on virtual links.
   Interface output cost is not configured on virtual links, but is
   dynamically set to be the cost of the intra-area path between the two
   endpoint routers.  The parameter RxmtInterval must be configured, and
   should be well over the expected round-trip delay between the two
   routers.  This may be hard to estimate for a virtual link; it is
   better to err on the side of making it too large.

   A virtual link is defined by the following two configurable
   parameters: the Router ID of the virtual link's other endpoint, and
   the (non-backbone) area through which the virtual link runs (referred
   to as the virtual link's Transit area).  Virtual links cannot be
   configured through stub areas.

C.5 NBMA network parameters

   OSPF treats an NBMA network much like it treats a broadcast network.
   Since there may be many routers attached to the network, a Designated
   Router is selected for the network.  This Designated Router then
   originates a network-LSA, which lists all routers attached to the
   NBMA network.

   However, due to the lack of broadcast capabilities, it may be
   necessary to use configuration parameters in the Designated Router
   selection.  These parameters will only need to be configured in those
   routers that are themselves eligible to become Designated Router
   (i.e., those router's whose Router Priority for the network is non-
   zero), and then only if no automatic procedure for discovering
   neighbors exists:

   List of all other attached routers
      The list of all other routers attached to the NBMA network.  Each
      router is configured with its Router ID and IPv6 link-local
      address on the network.  Also, for each router listed, that
      router's eligibility to become Designated Router must be defined.
      When an interface to a NBMA network comes up, the router sends
      Hello Packets only to those neighbors eligible to become
      Designated Router, until the identity of the Designated Router is
      discovered.

   PollInterval If a neighboring router has become inactive (Hello
      Packets have not been seen for RouterDeadInterval seconds), it may
      still be necessary to send Hello Packets to the dead neighbor.
      These Hello Packets will be sent at the reduced rate PollInterval,
      which should be much larger than HelloInterval.  Sample value for
      a PDN X.25 network: 2 minutes.

C.6 Point-to-MultiPoint network parameters

   On Point-to-MultiPoint networks, it may be necessary to configure the
   set of neighbors that are directly reachable over the Point-to-
   MultiPoint network. Each neighbor is configured with its Router ID
   and IPv6 link-local address on the network.  Designated Routers are
   not elected on Point-to-MultiPoint networks, so the Designated Router
   eligibility of configured neighbors is undefined.

C.7 Host route parameters

   Host prefixes are advertised in intra-area-prefix-LSAs.  They
   indicate either internal router addresses, router interfaces to
   point-to-point networks, looped router interfaces, or IPv6 hosts that
   are directly connected to the router (e.g., via a PPP connection).
   For each host directly connected to the router, the following items
   must be configured:

   Host IPv6 prefix
      The IPv6 prefix belonging to the host.

   Cost of link to host
      The cost of sending a packet to the host, in terms of the link
      state metric. However, since the host probably has only a single
      connection to the internet, the actual configured cost(s) in many
      cases is unimportant (i.e., will have no effect on routing).

   Area ID
      The OSPF area to which the host's prefix belongs.

Security Considerations

   When running over IPv6, OSPF relies on the IP Authentication Header
   (see [Ref19]) and the IP Encapsulating Security Payload (see [Ref20])
   to ensure integrity and authentication/confidentiality of routing
   exchanges.

   Most OSPF implementations will be running on systems that support
   multiple protocols, many of them having independent security
   assumptions and domains.  When IPSEC is used to protect OSPF packets,
   it is important for the implementation to check the IPSEC SA, and
   local SA database to make sure that the packet originates from a
   source THAT IS TRUSTED FOR OSPF PURPOSES.

Authors' Addresses

   Rob Coltun
   Siara Systems
   300 Ferguson Drive
   Mountain View, CA 94043

   Phone: (650) 390-9030
   EMail: rcoltun@siara.com

   Dennis Ferguson
   Juniper Networks
   385 Ravendale Drive
   Mountain View, CA  94043

   Phone: +1 650 526 8004
   EMail: dennis@juniper.com

   John Moy
   Sycamore Networks, Inc.
   10 Elizabeth Drive
   Chelmsford, MA 01824

   Phone: (978) 367-2161
   Fax:   (978) 250-3350
   EMail: jmoy@sycamorenet.com

Full Copyright Statement

   Copyright (C) The Internet Society (1999).  All Rights Reserved.

   This document and translations of it may be copied and furnished to
   others, and derivative works that comment on or otherwise explain it
   or assist in its implementation may be prepared, copied, published
   and distributed, in whole or in part, without restriction of any
   kind, provided that the above copyright notice and this paragraph are
   included on all such copies and derivative works.  However, this
   document itself may not be modified in any way, such as by removing
   the copyright notice or references to the Internet Society or other
   Internet organizations, except as needed for the purpose of
   developing Internet standards in which case the procedures for
   copyrights defined in the Internet Standards process must be
   followed, or as required to translate it into languages other than
   English.

   The limited permissions granted above are perpetual and will not be
   revoked by the Internet Society or its successors or assigns.

   This document and the information contained herein is provided on an
   "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
   TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
   BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
   HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
   MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.

 

User Contributions:

Comment about this RFC, ask questions, or add new information about this topic:

CAPTCHA