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Versions: 00 01 02 rfc3786                                              
Network Working Group                                      Amir Hermelin
Internet Draft                                  Charlotte's Web Networks
Expiration Date: September 2002
                                                         Stefano Previdi
                                                              Mike Shand
                                                           Cisco Systems

    Extending the Number of IS-IS LSP Fragments Beyond the 256 Limit



   This document is an Internet-Draft and is in full conformance with
   all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as Internet-

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   The list of current Internet-Drafts can be accessed at

   The list of Internet-Draft Shadow Directories can be accessed at

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [BCP14].


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

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1.  Introduction

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

   A number of factors can contribute to exceeding this limit:
     -  Introduction of new TLVs and sub-TLVs to be included in LSPs.
     -  The use of LSPs to propagate various types of information (such
     as traffic-engineering information).
     -  The growing size of route tables and AS topologies.
     -  Finer granularity routing, and the ability to inject external
     routes into areas [DOMAIN-WIDE].
     -  Other emerging technologies, such as optical, IPv6, etc.

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

1.1  Definitions of Commonly Used Terms

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

     Originating System
        A router running the IS-IS protocol.

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

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

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

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

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

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

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

     Extension-capable IS
        An IS implementing this extension.

1.2  Operation Modes

   Two administrative operation modes are provided:

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

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

   These modes may be configured per level.  There is no restriction
   that an L1/L2 IS operates in the same mode, for both L1 and L2.

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

1.3  Overview

   Using additional system-ids assigned by the administrator, the

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

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

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

2.0  IS Alias ID TLV (IS-A)

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

   The IS Alias ID TLV is type 24, and contains a new data structure,
   consisting of:
     7 octets of system Id and pseudonode number
     1 octet of length of sub-TLVs
     0-247 octets of sub-TLVs,
        where each sub-TLV consists of a sequence of
           1 octet of sub-type
           1 octet of length
           0-245 octets of value

   Without sub-TLVs, this structure consumes 8 octets per LSP set.  This
   TLV MUST be included in fragment 0 of every LSP set belonging to an
   Originating System.

3.0  Generating LSPs

3.1  Both Operation Modes

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

   |  Originating System                         |
   |  system-id   = S                            |
   |  is-alias-id = S                            |
   |  (  extended system-ids = S',  S''  )       |

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

   Figure 1. Advertising binding between all of a system's LSPs (both

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

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

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

3.1.1  The Attached Bits

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

3.1.2  The Partition Repair Bit

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

3.1.3  ES Neighbors TLV

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

3.2  Operation Mode 1 Additions

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

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

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

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   |  Originating System                         |
   |  system-id   = S                            |
   |  is-alias-id = S                            |
   |  (  extended system-ids = S',  S''  )       |
          |    /\                    |    /\
   cost=0 |    |cost=max-1    cost=0 |    |cost=max-1
          |    |                     |    |
          \/   |                     \/   |
   +-------------------+     +-------------------+
   |  Virtual System   |     |  Virtual System   |
   |  system-id   = S' |     |  system-id   = S''|
   |  is-alias-id = S  |     |  is-alias-id = S  |
   +-------------------+     +-------------------+

   Figure 2. Advertising connections to Virtual Systems under Operation
   Mode 1

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

   When an extended LSP belonging to extended system-id S' is first
   created, the original LSP MUST specify S' as a neighbor, with metric
   set to zero.  This in order to satisfy the two-way connectivity check
   on other routers, as well as to consider the cost of reaching the
   Virtual System S' the same as the cost of reaching its Originating
   System.  Furthermore, the extended LSP MUST specify the original
   system-id as a neighbor, with metric set to (MaxLinkMetric - 1).
   Where relevant, this adjacency SHOULD be considered as point-to-

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

3.2.1  IS Neighbors TLV

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

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   not the extended system, and hence would not satisfy the bi-
   directionality check in the SPF computation.

4.  Purging Extended LSP Fragments

   ISO 10589 [ISIS-ISO] section note 21 suggests that an
   implementation keeps the number of LSP fragments within a certain
   limit based on the optimal (minimal) number of fragments needed.
   Section also recommends that an IS purge its empty LSPs to
   conserve resources.  These recommendations hold for the extended LSP
   fragments as well.  However, an extended LSP fragment zero should not
   be purged until all of the fragments in its set (i.e. belonging to a
   particular extended system-id), are empty as well.  This is to ensure
   implementations consider the fragments in their SPF computations, as
   specified in section 7.2.5.

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

5.  Modifications to LSP handling in SPF

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

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

   If LSP fragment 0 of the original LSP set is missing, all of the LSPs
   generated by that Originating System (extended as well) MUST NOT be
   considered in the SPF.  That is, the large logical LSP isn't
   considered in the SPF.  If an LSP fragment 0 of an extended LSP set
   is missing, only that LSP set MUST NOT be considered in the SPF.

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

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6.  Forming Adjacencies

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

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

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

   It is possible to transition a live network, using the following
     - Upgrade the routers, one-by-one, to run this extension in
     Operation Mode 1. The other non-extension-capable routers will
     interoperate correctly.
     - When all routers are extension-capable, configure them one-by-one
     to run in Operation Mode 2.  All extension-capable routers
     interoperate correctly, regardless of what mode they're run in.

8.  Security Considerations

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

9.  Acknowledgments

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

10.  References

   [ISIS-ISO] ISO 10589, "Intermediate System to Intermediate System
   Intra-Domain Routeing Exchange Protocol for use in Conjunction with

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   the Protocol for Providing the Connectionless-mode Network Service
   (ISO 8473)"

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

   [ES-IS] ISO 9542, "End System to Intermediate System Routeing
   Exchange Protocol for Use in Conjunction with the Protocol for
   Providing the Connectionless-Mode Network Service (ISO 8473)", March

   [DOMAIN-WIDE] RFC 2966, "Domain-wide Prefix Distribution with Two-
   Level IS-IS", T. Li, T. Przygienda, H. Smit, October 2000

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

   [ISIS-CODES] Work in progress, "Reserved TLV Codepoints in ISIS", T.

11.  Authors' Address

   Amir Hermelin                        Email: amir@cwnt.com
   Charlotte's Web Networks, Inc.       Phone: +972 4 9592203
   2 Ha'mada St.                        Fax:   +972 4 9593325
   POB 650
   Yokneam, 20692

   Mike Shand,                          Email: mshand@cisco.com
   Cisco Systems,                       Phone: +44 020 8824 8690
   4, The Square,
   Stockley Park,
   UB11 1BN,

   Stefano Previdi                      email: sprevidi@cisco.com
   Cisco Systems, Inc.                  Phone: +32 2 7046590
   De Kleetlaan 6A
   1831 Diegem

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