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Versions: 00 01 02 RFC 3786

Network Working Group                                      Amir Hermelin
Internet Draft                                  Charlotte's Web Networks
Expiration Date: February 2003
                                                         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.

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   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 increasing number of destinations and AS topologies.
     -  Finer granularity routing, and the ability to inject external
     routes into areas [DOMAIN-WIDE].
     -  Other emerging technologies, such as optical, IPv6, etc.

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

1.1  Keywords

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

1.2  Definitions of Commonly Used Terms

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

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

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

     Additional system-id

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

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

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

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

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

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

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

1.3  Operation Modes

   Two administrative operation modes are provided:

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

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

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

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

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

1.4  Overview

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

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

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

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

2.0  IS Alias ID TLV (IS-A)

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

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

       x CODE   - 24.

       x LENGTH - total length of the value field.

       x VALUE  -

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

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

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

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

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     A series of tuples with the following format:

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

     Type of the sub-TLV

     Total length of the value field

     Type-specific TLV payload.

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

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

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

3.0  Generating LSPs

3.1  Both Operation Modes

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

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

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

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

   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 Additional
   system-ids configured for the router, in their LSP ID.

   Extended LSPs fragment zero should be regarded in the same special
   manner as specified in 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.  See section 5 for details.

3.1.1  The Attached Bits

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

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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 Additional system-id.  Furthermore, OSI-capable routers
   should accept packets destined for this Additional system-id.

3.1.4  Overload Bit

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

3.1.5  Other Fields and TLVs

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

3.2  Operation Mode 1 Additions

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

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

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

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   extended LSPs is the same as the cost of reaching information
   advertised in the new extended LSPs, with an additional hop.

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

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

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

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

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

3.2.1  IS Neighbors TLV (Mode 1 Only)

   An Extended LSP must specify only the Originating System as a
   neighbor, with the metric set to (MaxLinkMetric - 1).  Where

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   relevant, this adjacency should be considered as point-to-point.
   Other neighbors MUST NOT be specified in an Extended LSP, because
   those other neighbors would only specify the Originating System and
   not the Virtual System, and hence would not satisfy the bi-
   directionality check in the SPF computation.

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

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

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

4.  Purging Extended LSP Fragments

   ISO 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 Additional system-id), are empty as well.  This is to
   ensure implementations consider the fragments in their SPF
   computations, as specified in section 7.2.5.

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

5.  Modifications to LSP handling in SPF

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

   When considering LSPs of an extension-capable IS (identified by the
   inclusion of the IS Alias ID TLV), the original and extended LSPs are
   combined to form one large logical LSP.  If the LSP belongs to an IS
   running Operational Mode 1, there might be adjacencies between the

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

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

6.  Forming Adjacencies

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

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

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

   If a live network contains routers exceeding the 256 fragment limit,
   and for some reason the upgrade has to be done incrementally, it is
   possible to transition the network, using the following steps:
     - Upgrade the routers, one-by-one, to run this extension in
     Operation Mode 1. The other non-extension-capable routers will

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     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.

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

8.  Security Considerations

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

9.  Acknowledgments

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

10.  References

10.1  Normative References

   [ISIS-ISO] ISO 10589, "Intermediate System to Intermediate System
   Intra-Domain Routeing Exchange Protocol for use in Conjunction with
   the Protocol for Providing the Connectionless-mode Network Service
   (ISO 8473)"

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

   [ISIS-TE] Work in progress, "IS-IS extensions for Traffic
   Engineering", T. Li, H. Smit

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

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10.2  Informative References

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

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

11.  Authors' Addresses

   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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