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Versions: 00 01 02 03 04 05 06 07 RFC 5308 Draft is active
In: Proposed Standard
Internet Engineering Task Force                       Christian E. Hopps
INTERNET-DRAFT                                              Cisco System
Expires April 2006                                       21 October 2005

                        Routing IPv6 with IS-IS

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   This draft specifies a method for exchanging IPv6 routing information
   using the IS-IS routing protocol.  The described method utilizes 2

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   new TLVs, a reachability TLV and an interface address TLV to
   distribute the necessary IPv6 information throughout a routing
   domain.  Using this method one can route IPv6 along with IPv4 and OSI
   using a single intra-domain routing protocol.

1.  Terms

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

2.  Overview

   IS-IS [0] is an extendible intra-domain routing protocol.  Each
   router in the routing domain issues an LSP that contains information
   pertaining to that router.  The LSP contains typed variable length
   data often referred to as TLVs (type-length-values).  We extend the
   protocol with 2 new TLVs to carry information required to perform
   IPv6 routing.

   In [1] a method is described to route both OSI and IPv4. We utilize
   this same method with some minor changes to allow for IPv6.  To do so
   we must define 2 new TLVs, namely "IPv6 Reachability" and "IPv6
   Interface Address" and a new IPv6 protocol identifier.  In our new
   TLVs we utilize the extended metrics and up/down semantics of [2].

3.  IPv6 Reachability TLV

   The "IPv6 Reachability" TLV is TLV type 236 (0xEC).

   [1] defines 2 Reachability TLVs, "IP Internal Reachability
   Information" and "IP External Reachability Information".  We provide
   the equivalent IPv6 data with the "IPv6 Reachability" TLV and an
   "external" bit.

   The "IPv6 Reachability" TLV describes network reachability through
   the specification of a routing prefix, metric information, a bit to
   indicate if the prefix is being advertised down from a higher level,
   a bit to indicate if the prefix is being distributed from another
   routing protocol and OPTIONALLY the existence of sub-TLVs to allow
   for later extension.  This data is represented by the following

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   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
   |  Type = 236   |    Length     |          Metric ..            |
   |          .. Metric            |U|X|S| Reserve |  Prefix Len   |
   |  Prefix ...
   |Sub-TLV Len(*) | Sub-TLVs(*) ...
   * - if present

   U - up/down bit
   X - external original bit
   S - subtlv present bit

   This structure MAY appear any number of times (including none) within
   the TLV.

   As is described in [2], "the up/down bit is set to 0 when a prefix is
   first injected into IS-IS.  If a prefix is redistributed from a
   higher level to a lower level (e.g., level two to level one), the bit
   SHALL be set to 1 to indicate that the prefix has travelled down the
   hierarchy.  If a prefix is redistributed from an area to another area
   at the same level then the up/down bit SHALL be set to 1."

   If the prefix was distributed into IS-IS from another routing
   protocol the external bit SHALL be set to 1.  This information is
   useful when distributing prefixes from IS-IS to other protocols.

   If the sub-TLV bit is set to 0 then the octets of sub-TLVs are not
   present.  Otherwise the bit is 1 and the octet following the prefix
   will contain the length of the sub-TLV portion of the structure.

   The prefix is "packed" in the data structure.  That is, only the
   required number of octets of prefix are present.  This number can be
   computed from the prefix length octet as follows:

        prefix octets = integer of ((prefix length + 7) / 8)

   Just as in [2], if a prefix is advertised with a metric larger than
   MAX_V6_PATH_METRIC (0xFE000000), this prefix MUST not be considered
   during the normal SPF computation.  This will allow advertisement of
   a prefix for purposes other than building the normal IPv6 routing

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   If sub-TLVs are present they have the same form as normal TLVs as
   shown below.

   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
   |      Type     |    Length     |         Value(*) ..
   * - if present

   Length indicates how many octets of value are present and can be 0.

4.  IPv6 Interface Address TLV

   The "IPv6 Interface Address" TLV is TLV type 232 (0xE8).

   This TLV maps directly to [1]'s "IP Interface Address" TLV.  We
   necessarily modify the contents to be 0-15 16 octet IPv6 interface
   addresses instead of 0-63 4 octet IPv4 interface address.

   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
   |  Type = 232   |    Length     |   Interface Address 1(*) ..   |
   |                  .. Interface Address 1(*) ..                 |
   |                  .. Interface Address 1(*) ..                 |
   |                  .. Interface Address 1(*) ..                 |
   |   Interface Address 1(*) ..   |   Interface Address 2(*) ..
   * - if present

   We further restrict the semantics of this TLV depending on where it
   is advertised.  For Hello PDUs the "Interfaces Address" TLV MUST
   contain only the link-local IPv6 addresses assigned to the interface
   which is sending the Hello.  For LSPs the "Interfaces Address" TLVs
   MUST contain only the non-link-local IPv6 addresses assigned to the

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5.  IPv6 NLPID

   The value of the IPv6 NLPID is 142 (0x8E).

   As with [1] and IPv4, if the IS supports IPv6 routing using IS-IS, it
   MUST advertise this in the "NLPID" TLV by adding the IPv6 NLPID.

6.  Operation

   We utilize the same changes to [1] as made in [2] for the processing
   of prefix information.  These changes are both related to the SPF

   Since the metric space has been extended we need to redefine the
   MAX_PATH_METRIC (1023) from the original specification in [1].  This
   new value MAX_V6_PATH_METRIC is the same as in [2] (0xFE000000).  If
   during the SPF a path metric would exceed MAX_V6_PATH_METRIC it SHALL
   be considered to be MAX_V6_PATH_METRIC.

   The order of preference between paths for a given prefix MUST be
   modified to consider the up/down bit.  The new order of preference is
   as follows (from best to worst).

        1. Level 1 up prefix
        2. Level 2 up prefix
        3. Level 2 down prefix
        4. Level 1 down prefix

   If multiple paths have the same best preference then selection occurs
   based on metric.  Any remaining multiple paths SHOULD be considered
   for equal-cost multi-path routing if the router supports this,
   otherwise the router can select any one of the multiple paths.

7.  IANA Considerations

   IANA is requested to update the IS-IS codepoint registry
   (http://www.iana.org/assignments/isis-tlv-codepoints) so that TLV
   codes 232 and 236 refer to this document's RFC number.

   Note to the RFC editor: this paragraph and the above paragraph may be
   removed or edited on publication as an RFC.

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8.  Security Considerations

   This document raises no new security considerations.

9.  Normative References

[0]  "Intermediate System to Intermediate System Intra-Domain Routeing
     Exchange Protocol for use in Conjunction with the Protocol for
     Providing the Connectionless-mode Network Service (ISO 8473)", ISO
     10589, 1992.

[1]  Callon, R., "Use of OSI IS-IS for Routing in TCP/IP and Dual
     Environments", RFC 1195, December 1990.

[2]  Smit, H., and T. Li, "IS-IS extensions for Traffic Engineering",
     Work in Progress, August 2005.

10.  Author's Address

     Christian E. Hopps
     Cisco Systems
     170 W. Tasman Dr.
     San Jose, CA  95134
     Phone: +1 525 1684
     Email: chopps@cisco.com

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