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RFC 5308

Routing IPv6 with IS-IS

Pages: 7
Proposed Standard
Updated by:  7775

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Network Working Group                                           C. Hopps
Request for Comments: 5308                                 Cisco Systems
Category: Standards Track                                   October 2008

                        Routing IPv6 with IS-IS

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.


This document specifies a method for exchanging IPv6 routing information using the IS-IS routing protocol. The described method utilizes two 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. Overview

IS-IS is an extendible intra-domain routing protocol. Each router in the routing domain issues an Link State Protocol Data Unit (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 two new TLVs to carry information required to perform IPv6 routing. In [RFC1195], 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 two 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 [RFC5305].

1.1. Requirements Language

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in RFC 2119 [RFC2119].
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2. IPv6 Reachability TLV

The "IPv6 Reachability" TLV is TLV type 236 (0xEC). [RFC1195] defines two 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 structure: 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 The above IPv6 Reachability TLV MAY appear any number of times (including none) within an LSP. Link-local prefixes MUST NOT be advertised using this TLV. As is described in [RFC5305]: "The up/down bit SHALL be set to 0 when a prefix is first injected into IS-IS. If a prefix is advertised from a higher level to a lower level (e.g. level 2 to level 1), the bit SHALL be set to 1, indicating that the prefix has traveled down the hierarchy. Prefixes that have the up/down bit set to 1 may only be advertised down the hierarchy, i.e., to lower levels". 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.
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   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 [RFC5305], if a prefix is advertised with a metric larger
   than MAX_V6_PATH_METRIC (0xFE000000), this prefix MUST not be
   considered during the normal Shortest Path First (SPF) computation.
   This will allow advertisement of a prefix for purposes other than
   building the normal IPv6 routing table.

   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.

3. IPv6 Interface Address TLV

The "IPv6 Interface Address" TLV is TLV type 232 (0xE8). TLV 232 maps directly to "IP Interface Address" TLV in [RFC1195] . We necessarily modify the contents to be 0-15 16-octet IPv6 interface addresses instead of 0-63 4-octet IPv4 interface addresses.
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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 = 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 "Interface Address" TLV MUST
   contain only the link-local IPv6 addresses assigned to the interface
   that is sending the Hello.  For LSPs, the "Interface Address" TLVs
   MUST contain only the non-link-local IPv6 addresses assigned to the


The value of the IPv6 Network Layer Protocol ID (NLPID) is 142 (0x8E). As with [RFC1195] and IPv4, if the IS supports IPv6 routing using IS-IS, it MUST advertise this in the "NLPID" TLV by adding the IPv6 NLPID.

5. Operation

We utilize the same changes to [RFC1195] as made in [RFC5305] for the processing of prefix information. These changes are both related to the SPF calculation. Since the metric space has been extended, we need to redefine the MAX_PATH_METRIC (1023) from the original specification in [RFC1195]. This new value MAX_V6_PATH_METRIC is the same as in [RFC5305] (0xFE000000). If, during the SPF, a path metric would exceed MAX_V6_PATH_METRIC, it SHALL be considered to be MAX_V6_PATH_METRIC.
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   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.

6. IANA Considerations

IANA has updated the IS-IS codepoint registry so that TLV codes 232 and 236 refer to this RFC. IANA has also created the following new codepoint registry for Sub- TLVs of TLV 236. The range of values for Type is 0-255. Allocations within the registry require documentation of the use and requires approval by the Designated Expert assigned by the IESG [RFC5226]. All codepoints are currently unassigned.

7. Security Considerations

This document raises no new security considerations. Security considerations for the IS-IS protocol are covered in [ISO10589] and in [RFC5304].

8. References

8.1. Normative References

[ISO10589] ISO, "Intermediate System to Intermediate System intra- domain routeing information exchange protocol for use in conjunction with the protocol for providing the connectionless-mode network service (ISO 8473)", International Standard 10589:2002, Second Edition, 2002. [RFC1195] Callon, R., "Use of OSI IS-IS for routing in TCP/IP and dual environments", RFC 1195, December 1990.
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   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              May 2008.

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, October 2008.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, October 2008.

Author's Address

Christian E. Hopps Cisco Systems 170 W. Tasman Dr. San Jose, California 95134 USA EMail:
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