Multiprotocol Extensions for BGP-4
RFC 2283

Network Working Group                                           T. Bates
Request for Comments: 2283                                 Cisco Systems
Category: Standards Track                                     R. Chandra
                                                           Cisco Systems
                                                                 D. Katz
                                                        Juniper Networks
                                                              Y. Rekhter
                                                           Cisco Systems
                                                           February 1998

                   Multiprotocol Extensions for BGP-4

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 (1998).  All Rights Reserved.

2. Abstract

   Currently BGP-4 [BGP-4] is capable of carrying routing information
   only for IPv4 [IPv4]. This document defines extensions to BGP-4 to
   enable it to carry routing information for multiple Network Layer
   protocols (e.g., IPv6, IPX, etc...). The extensions are backward
   compatible - a router that supports the extensions can interoperate
   with a router that doesn't support the extensions.

3. Overview

   The only three pieces of information carried by BGP-4 that are IPv4
   specific are (a) the NEXT_HOP attribute (expressed as an IPv4
   address), (b) AGGREGATOR (contains an IPv4 address), and (c) NLRI
   (expressed as IPv4 address prefixes). This document assumes that any
   BGP speaker (including the one that supports multiprotocol
   capabilities defined in this document) has to have an IPv4 address
   (which will be used, among other things, in the AGGREGATOR
   attribute). Therefore, to enable BGP-4 to support routing for
   multiple Network Layer protocols the only two things that have to be
   added to BGP-4 are (a) the ability to associate a particular Network
   Layer protocol with the next hop information, and (b) the ability to
   associated a particular Network Layer protocol with NLRI. To identify

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   individual Network Layer protocols this document uses Address Family,
   as defined in [RFC1700].

   One could further observe that the next hop information (the
   information provided by the NEXT_HOP attribute) is meaningful (and
   necessary) only in conjunction with the advertisements of reachable
   destinations - in conjunction with the advertisements of unreachable
   destinations (withdrawing routes from service) the next hop
   information is meaningless. This suggests that the advertisement of
   reachable destinations should be grouped with the advertisement of
   the next hop to be used for these destinations, and that the
   advertisement of reachable destinations should be segregated from the
   advertisement of unreachable destinations.

   To provide backward compatibility, as well as to simplify
   introduction of the multiprotocol capabilities into BGP-4 this
   document uses two new attributes, Multiprotocol Reachable NLRI
   (MP_REACH_NLRI), and Multiprotocol Unreachable NLRI
   (MP_UNREACH_NLRI). The first one (MP_REACH_NLRI) is used to carry the
   set of reachable destinations together with the next hop information
   to be used for forwarding to these destinations. The second one
   (MP_UNREACH_NLRI) is used to carry the set of unreachable
   destinations.  Both of these attributes are optional and non-
   transitive.  This way a BGP speaker that doesn't support the
   multiprotocol capabilities will just ignore the information carried
   in these attributes, and will not pass it to other BGP speakers.

4. Multiprotocol Reachable NLRI - MP_REACH_NLRI (Type Code 14):

   This is an optional non-transitive attribute that can be used for the
   following purposes:

      (a) to advertise a feasible route to a peer

      (b) to permit a router to advertise the Network Layer address of
      the router that should be used as the next hop to the destinations
      listed in the Network Layer Reachability Information field of the
      MP_NLRI attribute.

      (c) to allow a given router to report some or all of the
      Subnetwork Points of Attachment (SNPAs) that exist within the
      local system

   The attribute contains one or more triples <Address Family
   Information, Next Hop Information, Network Layer Reachability
   Information>, where each triple is encoded as shown below:

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      +---------------------------------------------------------+
      | Address Family Identifier (2 octets)                    |
      +---------------------------------------------------------+
      | Subsequent Address Family Identifier (1 octet)          |
      +---------------------------------------------------------+
      | Length of Next Hop Network Address (1 octet)            |
      +---------------------------------------------------------+
      | Network Address of Next Hop (variable)                  |
      +---------------------------------------------------------+
      | Number of SNPAs (1 octet)                               |
      +---------------------------------------------------------+
      | Length of first SNPA(1 octet)                           |
      +---------------------------------------------------------+
      | First SNPA (variable)                                   |
      +---------------------------------------------------------+
      | Length of second SNPA (1 octet)                         |
      +---------------------------------------------------------+
      | Second SNPA (variable)                                  |
      +---------------------------------------------------------+
      | ...                                                     |
      +---------------------------------------------------------+
      | Length of Last SNPA (1 octet)                           |
      +---------------------------------------------------------+
      | Last SNPA (variable)                                    |
      +---------------------------------------------------------+
      | Network Layer Reachability Information (variable)       |
      +---------------------------------------------------------+

   The use and meaning of these fields are as follows:

      Address Family Identifier:

         This field carries the identity of the Network Layer protocol
         associated with the Network Address that follows. Presently
         defined values for this field are specified in RFC1700 (see the
         Address Family Numbers section).

      Subsequent Address Family Identifier:

         This field provides additional information about the type of
         the Network Layer Reachability Information carried in the
         attribute.

      Length of Next Hop Network Address:

         A 1 octet field whose value expresses the length of the
         "Network Address of Next Hop" field as measured in octets

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      Network Address of Next Hop:

         A variable length field that contains the Network Address of
         the next router on the path to the destination system

      Number of SNPAs:

         A 1 octet field which contains the number of distinct SNPAs to
         be listed in the following fields.  The value 0 may be used to
         indicate that no SNPAs are listed in this attribute.

      Length of Nth SNPA:

         A 1 octet field whose value expresses the length of the "Nth
         SNPA of Next Hop" field as measured in semi-octets

      Nth SNPA of Next Hop:

         A variable length field that contains an SNPA of the router
         whose Network Address is contained in the "Network Address of
         Next Hop" field.  The field length is an integral number of
         octets in length, namely the rounded-up integer value of one
         half the SNPA length expressed in semi-octets; if the SNPA
         contains an odd number of semi-octets, a value in this field
         will be padded with a trailing all-zero semi-octet.

      Network Layer Reachability Information:

         A variable length field that lists NLRI for the feasible routes
         that are being advertised in this attribute. When the
         Subsequent Address Family Identifier field is set to one of the
         values defined in this document, each NLRI is encoded as
         specified in the "NLRI encoding" section of this document.

   The next hop information carried in the MP_REACH_NLRI path attribute
   defines the Network Layer address of the border router that should be
   used as the next hop to the destinations listed in the MP_NLRI
   attribute in the UPDATE message.  When advertising a MP_REACH_NLRI
   attribute to an external peer, a router may use one of its own
   interface addresses in the next hop component of the attribute,
   provided the external peer to which the route is being advertised
   shares a common subnet with the next hop address.  This is known as a
   "first party" next hop.  A BGP speaker can advertise to an external
   peer an interface of any internal peer router in the next hop
   component, provided the external peer to which the route is being
   advertised shares a common subnet with the next hop address.  This is
   known as a "third party" next hop information.  A BGP speaker can
   advertise any external peer router in the next hop component,

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   provided that the Network Layer address of this border router was
   learned from an external peer, and the external peer to which the
   route is being advertised shares a common subnet with the next hop
   address.  This is a second form of "third party" next hop
   information.

   Normally the next hop information is chosen such that the shortest
   available path will be taken.  A BGP speaker must be able to support
   disabling advertisement of third party next hop information to handle
   imperfectly bridged media or for reasons of policy.

   A BGP speaker must never advertise an address of a peer to that peer
   as a next hop, for a route that the speaker is originating.  A BGP
   speaker must never install a route with itself as the next hop.

   When a BGP speaker advertises the route to an internal peer, the
   advertising speaker should not modify the next hop information
   associated with the route.  When a BGP speaker receives the route via
   an internal link, it may forward packets to the next hop address if
   the address contained in the attribute is on a common subnet with the
   local and remote BGP speakers.

   An UPDATE message that carries the MP_REACH_NLRI must also carry the
   ORIGIN and the AS_PATH attributes (both in EBGP and in IBGP
   exchanges).  Moreover, in IBGP exchanges such a message must also
   carry the LOCAL_PREF attribute. If such a message is received from an
   external peer, the local system shall check whether the leftmost AS
   in the AS_PATH attribute is equal to the autonomous system number of
   the peer than sent the message. If that is not the case, the local
   system shall send the NOTIFICATION message with Error Code UPDATE
   Message Error, and the Error Subcode set to Malformed AS_PATH.

5. Multiprotocol Unreachable NLRI - MP_UNREACH_NLRI (Type Code 15):

   This is an optional non-transitive attribute that can be used for the
   purpose of withdrawing multiple unfeasible routes from service.

   The attribute contains one or more triples <Address Family
   Information, Unfeasible Routes Length, Withdrawn Routes>, where each
   triple is encoded as shown below:

      +---------------------------------------------------------+
      | Address Family Identifier (2 octets)                    |
      +---------------------------------------------------------+
      | Subsequent Address Family Identifier (1 octet)          |
      +---------------------------------------------------------+
      | Withdrawn Routes (variable)                             |
      +---------------------------------------------------------+

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   The use and the meaning of these fields are as follows:

      Address Family Identifier:

         This field carries the identity of the Network Layer protocol
         associated with the NLRI that follows. Presently defined values
         for this field are specified in RFC1700 (see the Address Family
         Numbers section).

      Subsequent Address Family Identifier:

         This field provides additional information about the type of
         the Network Layer Reachability Information carried in the
         attribute.

      Withdrawn Routes:

         A variable length field that lists NLRI for the routes that are
         being withdrawn from service. When the Subsequent Address
         Family Identifier field is set to one of the values defined in
         this document, each NLRI is encoded as specified in the "NLRI
         encoding" section of this document.

   An UPDATE message that contains the MP_UNREACH_NLRI is not required
   to carry any other path attributes.

6. NLRI encoding

   The Network Layer Reachability information is encoded as one or more
   2-tuples of the form <length, prefix>, whose fields are described
   below:

      +---------------------------+
      |   Length (1 octet)        |
      +---------------------------+
      |   Prefix (variable)       |
      +---------------------------+

   The use and the meaning of these fields are as follows:

      a) Length:

         The Length field indicates the length in bits of the address
         prefix. A length of zero indicates a prefix that matches all
         (as specified by the address family) addresses (with prefix,
         itself, of zero octets).

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      b) Prefix:

         The Prefix field contains address prefixes followed by enough
         trailing bits to make the end of the field fall on an octet
         boundary.  Note that the value of trailing bits is irrelevant.

7. Subsequent Address Family Identifier

   This document defines the following values for the Subsequent Address
   Family Identifier field carried in the MP_REACH_NLRI and
   MP_UNREACH_NLRI attributes:

      1 - Network Layer Reachability Information used for unicast
          forwarding

      2 - Network Layer Reachability Information used for multicast
          forwarding

      3 - Network Layer Reachability Information used for both unicast
          and multicast forwarding

   This document reserves values 128-255 for vendor-specific
   applications.

   This document reserves value 0.

   Subsequent Address Family Identifiers (other than those reserved for
   vendor specific use) are assigned only by the IETF consensus process
   and IESG approval.

8. Security Considerations

   This extension to BGP does not change the underlying security issues.

9. Acknowledgements

   The authors would like to thank members of the IDR Working Group for
   their review and comments.

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10. References

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

   [IPv4]    Postel, J., "Internet Protocol", STD 5, RFC 791,
             September 1981.

   [RFC1700] Reynolds, J., and J. Postel, "Assigned Numbers," STD 2,
             RFC 1700, October 1994. (see also
             http://www.iana.org/iana/assignments.html)

11. Author Information

   Tony Bates
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   EMail: tbates@cisco.com

   Ravi Chandra
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   EMail: rchandra@cisco.com

   Dave Katz
   Juniper Networks, Inc.
   3260 Jay St.
   Santa Clara, CA 95054

   EMail: dkatz@jnx.com

   Yakov Rekhter
   Cisco Systems, Inc.
   170 West Tasman Drive
   San Jose, CA 95134

   EMail: yakov@cisco.com

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12.  Full Copyright Statement

   Copyright (C) The Internet Society (1998).  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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