Network Working Group                                        P. Marques
Request for Comments: 2545                          cisco Systems, Inc.
Category: Standards Track                                     F. Dupont
                                                                 Inria
                                                            March 1999


 Use of BGP-4 Multiprotocol Extensions for IPv6 Inter-Domain Routing

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.

Copyright Notice

  Copyright (C) The Internet Society (1999).  All Rights Reserved.

Abstract

  BGP-4 Multiprotocol Extensions [BGP-MP] defines the format of two BGP
  attributes (MP_REACH_NLRI and MP_UNREACH_NLRI) that can be used to
  announce and withdraw the announcement of reachability information.
  This document defines how compliant systems should make use of those
  attributes for the purpose of conveying IPv6 routing information.

1. Introduction

  The BGP-4 protocol [BGP-4] in particular, and path vector routing
  protocols in general, are mostly independent of the particular
  Address Family for which the protocol is being used.

  IPv6 falls under the generic category of protocols for which BGP-4 is
  suitable and, unless stated otherwise in this document, the BGP-4
  procedures to apply when using BGP-4 to carry IPv6 reachability
  information are those defined in [BGP-4] and in subsequent documents
  that extend or update the BGP-4 specification.

  In terms of routing information, the most significant difference
  between IPv6 and IPv4 (for which BGP was originally designed) is the
  fact that IPv6 introduces scoped unicast addresses and defines
  particular situations when a particular address scope must be used.
  This document concerns itself essentially with the necessary rules to
  accommodate IPv6 address scope requirements.




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2. IPv6 Address Scopes

  IPv6 defines 3 unicast address scopes [ADDR-ARCH]: global, site-local
  and link-local. Site-local addresses are non-link-local address which
  are valid within the scope of a "site" and cannot be exported beyond
  it. As this document makes no assumption on the characteristics of a
  particular routing realm where BGP-4 is used, it makes no distinction
  between global and site-local addresses and refers to both as
  "global" or "non-link-local". Network administrators must however
  respect address scope restrictions and should be aware that the
  concepts of a BGP-4 routing domain and "site" are orthogonal notions
  and that they may or may not coincide in a given situation.

  Companion IPv6 specifications further define that only link-local
  address can be used when generating ICMP Redirect Messages [ND] and
  as next hop addresses in some routing protocols (eg. RIPng [RIP]).

  This restrictions does imply that an IPv6 router must have a link-
  local next hop address for all directly connected routes (routes for
  which the given router and the next hop router share a common subnet
  prefix).

  Link-local addresses are not, however, well suited to be used as next
  hop attributes in BGP-4 given the rules defined for this attribute in
  the protocol specification [BGP-4].

  For the above reasons, when BGP-4 is used to convey IPv6 reachability
  information it is sometimes necessary to announce a next hop
  attribute that consists of a global address and a link-local address.
  The following section describes the rules that should be followed
  when constructing the Network Address of Next Hop field of an
  MP_REACH_NLRI attribute.

3. Constructing the Next Hop field

  A BGP speaker shall advertise to its peer in the Network Address of
  Next Hop field the global IPv6 address of the next hop, potentially
  followed by the link-local IPv6 address of the next hop.

  The value of the Length of Next Hop Network Address field on a
  MP_REACH_NLRI attribute shall be set to 16, when only a global
  address is present, or 32 if a link-local address is also included in
  the Next Hop field.

  The link-local address shall be included in the Next Hop field if and
  only if the BGP speaker shares a common subnet with the entity
  identified by the global IPv6 address carried in the Network Address
  of Next Hop field and the peer the route is being advertised to.



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  In all other cases a BGP speaker shall advertise to its peer in the
  Network Address field only the global IPv6 address of the next hop
  (the value of the Length of Network Address of Next Hop field shall
  be set to 16).

  As a consequence, a BGP speaker that advertises a route to an
  internal peer may modify the Network Address of Next Hop field by
  removing the link-local IPv6 address of the next hop.

4. Transport

  TCP connections, on top of which BGP-4 messages are exchanged, can be
  established either over IPv4 or IPv6. While BGP-4 itself is
  independent of the particular transport used it derives implicit
  configuration information from the address used to establish the
  peering session.  This information (the network address of a peer) is
  taken in account in the route dissemination procedure. Thus, when
  using TCP over IPv4 as a transport for IPv6 reachability information,
  additional explicit configuration of the peer's network address is
  required.

  Note that the information referred above is distinct from the BGP
  Identifier used in the BGP-4 tie breaking procedure. The BGP
  Identifier is a 32 bit unsigned integer exchanged between two peers
  at session establishment time, within an OPEN message. This is a
  system wide value determined at startup which must be unique in the
  network and should be derived from an IPv4 address regardless of the
  network protocol(s) a particular BGP-4 instance is configured to
  convey at a given moment.

  The use of TCP over IPv6 as transport protocol for IPv6 reachability
  information also has the advantage of providing explicit confirmation
  of IPv6 network reachability between two peers.

5. Security Considerations

  The extensions defined in this document allow BGP to propagate
  reachability information about IPv6 routes. As such, no new security
  issues are raised beyond those that already exist in BGP-4 and its
  use with IPv4.

6. Acknowledgments

  This document derives from the work on BGP-4 Multiprotocol Extensions
  by Tony Bates, Ravi Chandra, Dave Katz and Yakov Rekhter.






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7. References

  [ADDR-ARCH] Hinden, R. and S. Deering, "IP Version 6 Addressing
              Architecture", RFC 2373, July 1998.

  [BGP-4]     Rekhter, Y. and T. Li, "A Border Gateway Protocol 4
              (BGP-4)", RFC 1771, March 1995.

  [BGP-MP]    Bates, T., Chandra, R., Katz, D. and Y. Rekhter,
              "Multiprotocol Extensions for BGP-4", RFC 2283, February
              1998.

  [IPv6]      Deering, S. and R. Hinden, "Internet Protocol, Version 6
              (IPv6) Specification", RFC 2460, December 1998.

  [ND]        Narten, T., Nordmark, E. and W. Simpson, "Neighbor
              Discovery for IP Version 6 (IPv6)", RFC 2461, December
              1998.

  [RIP]       Malkin, G. and R. Minnear, "RIPng for IPv6",
              RFC 2080, January 1997.

8. Author Information

  Pedro R. Marques
  cisco Systems, Inc.
  170 W. Tasman Dr.
  San Jose, CA 95134
  USA

  Phone: +1 408 527-5202
  Fax:   +1 408 526-4952
  EMail: [email protected]


  Francis Dupont
  GIE DYADE
  INRIA Rocquencourt
  Domaine de Voluceau
  BP 105
  78153 Le Chesnay CEDEX
  FRANCE

  Phone: +33 1 39 63 52 13
  Fax:   +33 1 39 63 58 66
  EMail: [email protected]





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9.  Full Copyright Statement

  Copyright (C) The Internet Society (1999).  All Rights Reserved.

  This document and translations of it may be copied and furnished to
  others, and derivative works that comment on or otherwise explain it
  or assist in its implementation may be prepared, copied, published
  and distributed, in whole or in part, without restriction of any
  kind, provided that the above copyright notice and this paragraph are
  included on all such copies and derivative works.  However, this
  document itself may not be modified in any way, such as by removing
  the copyright notice or references to the Internet Society or other
  Internet organizations, except as needed for the purpose of
  developing Internet standards in which case the procedures for
  copyrights defined in the Internet Standards process must be
  followed, or as required to translate it into languages other than
  English.

  The limited permissions granted above are perpetual and will not be
  revoked by the Internet Society or its successors or assigns.

  This document and the information contained herein is provided on an
  "AS IS" basis and THE INTERNET SOCIETY AND THE INTERNET ENGINEERING
  TASK FORCE DISCLAIMS ALL WARRANTIES, EXPRESS OR IMPLIED, INCLUDING
  BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE INFORMATION
  HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED WARRANTIES OF
  MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
























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