Routing IPv6 with IS-IS
RFC 5308

Document Type RFC - Proposed Standard (October 2008; No errata)
Updated by RFC 7775
Author Christian Hopps 
Last updated 2015-10-14
Stream IETF
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IESG IESG state RFC 5308 (Proposed Standard)
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Responsible AD Ross Callon
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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

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

Hopps                       Standards Track                     [Page 1]
RFC 5308                Routing IPv6 with IS-IS             October 2008

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

   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.

Hopps                       Standards Track                     [Page 2]
RFC 5308                Routing IPv6 with IS-IS             October 2008

   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:
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