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RFC 7794

Proposed STD
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IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability


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Internet Engineering Task Force (IETF)                  L. Ginsberg, Ed.
Request for Comments: 7794                                 Cisco Systems
Category: Standards Track                                    B. Decraene
ISSN: 2070-1721                                                   Orange
                                                              S. Previdi
                                                           Cisco Systems
                                                                   X. Xu
                                                             U. Chunduri
                                                              March 2016

    IS-IS Prefix Attributes for Extended IPv4 and IPv6 Reachability


   This document introduces new sub-TLVs to support advertisement of
   IPv4 and IPv6 prefix attribute flags and the source router ID of the
   router that originated a prefix advertisement.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at

Page 2 
Copyright Notice

   Copyright (c) 2016 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . .   3
     1.1.  Requirements Language . . . . . . . . . . . . . . . . . .   3
   2.  New Sub-TLVs for Extended Reachability TLVs . . . . . . . . .   3
     2.1.  IPv4/IPv6 Extended Reachability Attribute Flags . . . . .   4
     2.2.  IPv4/IPv6 Source Router ID  . . . . . . . . . . . . . . .   5
     2.3.  Advertising Router IDs  . . . . . . . . . . . . . . . . .   6
   3.  IANA Considerations . . . . . . . . . . . . . . . . . . . . .   6
   4.  Security Considerations . . . . . . . . . . . . . . . . . . .   7
   5.  References  . . . . . . . . . . . . . . . . . . . . . . . . .   7
     5.1.  Normative References  . . . . . . . . . . . . . . . . . .   7
     5.2.  Informative References  . . . . . . . . . . . . . . . . .   8
   Contributors  . . . . . . . . . . . . . . . . . . . . . . . . . .   8
   Authors' Addresses  . . . . . . . . . . . . . . . . . . . . . . .   9

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1.  Introduction

   IS-IS is a link-state routing protocol defined in [ISO10589] and
   [RFC1195].  Extensions in support of advertising new forms of
   IPv4/IPv6 prefix reachability are defined in [RFC5305], [RFC5308],
   and [RFC5120].

   There are existing use cases in which knowing additional attributes
   of a prefix is useful.

   It is useful to know whether or not an advertised prefix is directly
   connected to the advertising router.  In the case of Segment Routing
   as described in [SR], knowing whether or not a prefix is directly
   connected determines what action should be taken as regards
   processing of labels associated with an incoming packet.

   It is useful to know what addresses can be used as addresses of the
   node in support of services (e.g., Remote Loop Free Alternate (RLFA)

   Current formats of the Extended Reachability TLVs for both IPv4 and
   IPv6 are fixed and do not allow the introduction of additional flags
   without backwards compatibility issues.  Therefore, this document
   defines a new sub-TLV that supports the advertisement of attribute
   flags associated with prefix advertisements.

   In cases where multiple node addresses are advertised by a given
   router, it is also useful to be able to associate all of these
   addresses with a single Router ID even when prefixes are advertised
   outside of the area in which they originated.  Therefore, a new sub-
   TLV is introduced to advertise the Router ID of the originator of a
   prefix advertisement.

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].

2.  New Sub-TLVs for Extended Reachability TLVs

   The following new sub-TLVs are introduced:

   o  Prefix Attribute Flags

   o  IPv4 Source Router ID

   o  IPv6 Source Router ID

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   All sub-TLVs are applicable to TLVs 135, 235, 236, and 237.

2.1.  IPv4/IPv6 Extended Reachability Attribute Flags

   This sub-TLV supports the advertisement of additional flags
   associated with a given prefix advertisement.  The behavior of each
   flag when a prefix advertisement is leaked from one level to another
   (upwards or downwards) is explicitly defined below.

   All flags are applicable to TLVs 135, 235, 236, and 237, unless
   otherwise stated.

     Prefix Attribute Flags
     Type:   4
     Length: Number of octets of the Value field.

          (Length * 8) bits.

       0 1 2 3 4 5 6 7...
      |X|R|N|          ...

   Bits are defined/sent starting with Bit 0 defined below.  Additional
   bit definitions that may be defined in the future SHOULD be assigned
   in ascending bit order so as to minimize the number of bits that will
   need to be transmitted.

   Undefined bits MUST be transmitted as 0 and MUST be ignored on

   Bits that are NOT transmitted MUST be treated as if they are set to 0
   on receipt.

   X-Flag:  External Prefix Flag (Bit 0)
      Set if the prefix has been redistributed from another protocol.
      This includes the case where multiple virtual routers are
      supported and the source of the redistributed prefix is another
      IS-IS instance.

      The flag MUST be preserved when leaked between levels.

      In TLVs 236 and 237, this flag SHOULD always be sent as 0 and MUST
      be ignored on receipt.  This is because there is an existing X
      flag defined in the fixed format of these TLVs as specified in
      [RFC5308] and [RFC5120].

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   R-Flag:  Re-advertisement Flag (Bit 1)
      Set when the prefix has been leaked from one level to another
      (upwards or downwards).

   N-flag:  Node Flag (Bit 2)
      Set when the prefix identifies the advertising router, i.e., the
      prefix is a host prefix advertising a globally reachable address
      typically associated with a loopback address.

      The advertising router MAY choose to NOT set this flag even when
      the above conditions are met.

      If the flag is set and the prefix length is NOT a host prefix (/32
      for IPV4, /128 for IPv6), then the flag MUST be ignored.  The flag
      MUST be preserved when leaked between levels.

2.2.  IPv4/IPv6 Source Router ID

   When a reachability advertisement is leaked from one level to
   another, the source of the original advertisement is unknown.  In
   cases where the advertisement is an identifier for the advertising
   router (e.g., with the N-flag set in the Prefix Attribute Flags sub-
   TLV as described in Section 2.1), it may be useful for other routers
   to know the source of the advertisement.  The sub-TLVs defined below
   provide that information.

   Note that the Router ID advertised is always the Router ID of the
   IS-IS instance that originated the advertisement.  This would be true
   even if the prefix had been learned from another protocol (i.e., with
   the X-flag set as defined in Section 2.1).

     IPv4 Source Router ID
     Type:   11
     Length: 4
     Value:  IPv4 Router ID of the source of the advertisement

   Inclusion of this TLV is optional and MAY occur in TLVs 135, 235,
   236, or 237.  When included, the value MUST be identical to the value
   advertised in the Traffic Engineering router ID (TLV 134) defined in

   If present the sub-TLV MUST be included when the prefix advertisement
   is leaked to another level.

     IPv6 Source Router ID
     Type:   12
     Length: 16
     Value: IPv6 Router ID of the source of the advertisement

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   Inclusion of this TLV is optional and MAY occur in TLVs 135, 235,
   236, or 237.  When included, the value MUST be identical to the value
   advertised in the IPv6 TE Router ID (TLV 140) defined in [RFC6119].

   If present, the sub-TLV MUST be included when the prefix
   advertisement is leaked to another level.

2.3.  Advertising Router IDs

   [RFC5305] and [RFC6119] define the advertisement of router IDs for
   IPv4 and IPv6, respectively.  Although both documents discuss the use
   of router ID in the context of Traffic Engineering (TE), the
   advertisement of router IDs is explicitly allowed for purposes other
   than TE.  The use of router IDs to identify the source of a prefix
   advertisement as defined in Section 2.2 is one such use case.
   Therefore, whenever an IPv4 or IPv6 Source Router ID sub-TLV (as
   defined in Section 2.2) is used, the originating router SHOULD also
   advertise the corresponding address-family-specific router ID TLV.

3.  IANA Considerations

   This document adds the following new sub-TLVs to the registry of sub-
   TLVs for TLVs 135, 235, 236, and 237.

      Value: 4
      Name: Prefix Attribute Flags

      Value: 11
      Name: IPv4 Source Router ID

      Value: 12
      Name: IPv6 Source Router ID

   This document also introduces a new registry for bit values in the
   Prefix Attribute Flags sub-TLV.  The registration policy is Expert
   Review as defined in [RFC5226].  This registry is part of the "IS-IS
   TLV Codepoints" registry.  The name of the registry is "Bit Values
   for Prefix Attribute Flags Sub-TLV".  The defined values are:

        Bit #   Name
        -----   ------------------------------
        0       External Prefix Flag (X-flag)
        1       Re-advertisement Flag (R-flag)
        2       Node Flag (N-flag)

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4.  Security Considerations

   Security concerns for IS-IS are addressed in [RFC5304] and [RFC5310].

   Advertisement of the additional information defined in this document
   introduces no new security concerns.

5.  References

5.1.  Normative References

   [ISO10589] International Organization for Standardization,
              "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)",
              ISO/IEC 10589:2002, Second Edition, Nov. 2002.

   [RFC1195]  Callon, R., "Use of OSI IS-IS for routing in TCP/IP and
              dual environments", RFC 1195, DOI 10.17487/RFC1195,
              December 1990, <>.

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119,
              DOI 10.17487/RFC2119, March 1997,

   [RFC5120]  Przygienda, T., Shen, N., and N. Sheth, "M-ISIS: Multi
              Topology (MT) Routing in Intermediate System to
              Intermediate Systems (IS-ISs)", RFC 5120,
              DOI 10.17487/RFC5120, February 2008,

   [RFC5226]  Narten, T. and H. Alvestrand, "Guidelines for Writing an
              IANA Considerations Section in RFCs", BCP 26, RFC 5226,
              DOI 10.17487/RFC5226, May 2008,

   [RFC5304]  Li, T. and R. Atkinson, "IS-IS Cryptographic
              Authentication", RFC 5304, DOI 10.17487/RFC5304, October
              2008, <>.

   [RFC5305]  Li, T. and H. Smit, "IS-IS Extensions for Traffic
              Engineering", RFC 5305, DOI 10.17487/RFC5305, October
              2008, <>.

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   [RFC5308]  Hopps, C., "Routing IPv6 with IS-IS", RFC 5308,
              DOI 10.17487/RFC5308, October 2008,

   [RFC5310]  Bhatia, M., Manral, V., Li, T., Atkinson, R., White, R.,
              and M. Fanto, "IS-IS Generic Cryptographic
              Authentication", RFC 5310, DOI 10.17487/RFC5310, February
              2009, <>.

   [RFC6119]  Harrison, J., Berger, J., and M. Bartlett, "IPv6 Traffic
              Engineering in IS-IS", RFC 6119, DOI 10.17487/RFC6119,
              February 2011, <>.

5.2.  Informative References

   [SR]       Previdi, S., Ed., Filsfils, C., Bashandy, A., Gredler, H.,
              Litkowski, S., Decraene, B., and J. Tantsura, "IS-IS
              Extensions for Segment Routing", Work in Progress,
              draft-ietf-isis-segment-routing-extensions-06, December


   The following people gave a substantial contribution to the content
   of this document:

   Clarence Filsfils
   Cisco Systems


   Stephane Litkowski
   Orange Business Service


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Authors' Addresses

   Les Ginsberg (editor)
   Cisco Systems
   510 McCarthy Blvd.
   Milpitas, CA  95035
   United States


   Bruno Decraene
   38 rue du General Leclerc
   Issy Moulineaux cedex 9  92794


   Stefano Previdi
   Cisco Systems
   Via Del Serafico 200
   Rome  0144


   Xiaohu Xu


   Uma Chunduri