rfc2858.txt

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      Withdrawn Routes: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         being withdrawn from service.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.

4. 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)       |
      +---------------------------+




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   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).

      b) Prefix:

         The Prefix field contains an address prefix 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.

5. 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

6. Error Handling

   If a BGP speaker receives from a neighbor an Update message that
   contains the MP_REACH_NLRI or MP_UNREACH_NLRI attribute, and the
   speaker determines that the attribute is incorrect, the speaker must
   delete all the BGP routes received from that neighbor whose AFI/SAFI
   is the same as the one carried in the incorrect MP_REACH_NLRI or
   MP_UNREACH_NLRI attribute. For the duration of the BGP session over
   which the Update message was received, the speaker then should ignore
   all the subsequent routes with that AFI/SAFI received over that
   session.

   In addition, the speaker may terminate the BGP session over which the
   Update message was received. The session should be terminated with
   the Notification message code/subcode indicating "Update Message
   Error"/"Optional Attribute Error".





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7. Use of BGP Capability Advertisement

   A BGP speaker that uses Multiprotocol Extensions should use the
   Capability Advertisement procedures [BGP-CAP] to determine whether
   the speaker could use Multiprotocol Extensions with a particular
   peer.

   The fields in the Capabilities Optional Parameter are set as follows.
   The Capability Code field is set to 1 (which indicates Multiprotocol
   Extensions capabilities). The Capability Length field is set to 4.
   The Capability Value field is defined as:

      The use and meaning of this field is as follow:

                        0       7      15      23      31
                        +-------+-------+-------+-------+
                        |      AFI      | Res.  | SAFI  |
                        +-------+-------+-------+-------+

         AFI  - Address Family Identifier (16 bit), encoded the same way
                as in the Multiprotocol Extensions

         Res. - Reserved (8 bit) field. Should be set to 0 by the sender
                and ignored by the receiver.

         SAFI - Subsequent Address Family Identifier (8 bit), encoded
                the same way as in the Multiprotocol Extensions.

   A speaker that supports multiple <AFI, SAFI> tuples includes them as
   multiple Capabilities in the Capabilities Optional Parameter.

   To have a bi-directional exchange of routing information for a
   particular <AFI, SAFI> between a pair of BGP speakers, each such
   speaker must advertise to the other (via the Capability Advertisement
   mechanism) the capability to support that particular <AFI, SAFI>
   routes.

8. IANA Considerations

   As specified in this document, the MPL_REACH_NLRI and MP_UNREACH_NLRI
   attributes contain the Subsequence Address Family Identifier (SAFI)
   field. The SAFI name space is defined in Section 9. The IANA will
   maintain and register values for the SAFI namespace as follows.  SAFI
   value 0 is reserved. SAFI values 1, 2, and 3 are assigned in this
   document.  SAFI values 4 through 63 are to be assigned by IANA using
   the "IETF Consensus" policy defined in RFC 2434. SAFI values 64
   through 127 are to be assigned by IANA, using the "First Come First




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   Served" policy defined in RFC 2434. SAFI values 128 through 255 are
   for "private use", and values in this range are not to be assigned by
   IANA.

9. Comparison with RFC 2283

   This document restricts the MP_REACH_NLRI attribute to carry only a
   single instance of <AFI, SAFI, Next Hop Information, ...>.

   This document restricts the MP_UNREACH_NLRI attribute to carry only a
   single instance of <AFI, SAFI, ...>.

   This document clarifies handling of an UPDATE message that carries no
   NLRI, other than the one encoded in the MP_REACH_NLRI attribute.

   This document clarifies error handling in the presence of
   MP_REACH_NLRI or MP_UNREACH_NLRI attributes.

   This document specifies the use of BGP Capabilities Advertisements in
   conjunction with Multi-protocol extensions.

   Finally, this document includes the "IANA Consideration" Section.

10. Security Considerations

   This extension to BGP does not change the underlying security issues
   inherent in the existing BGP [Heffernan].

11. Acknowledgements

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

12. References

   [BGP-CAP]    Chandra, R. and J. Scudder, "Capabilities Advertisement
                with BGP-4", RFC 2842, May 2000.

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

   [Heffernan]  Heffernan, A., "Protection of BGP Sessions via the TCP
                MD5 Signature Option", RFC 2385, August 1998.

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





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

13. Authors' Addresses

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

   EMail: [e-mail]


   Ravi Chandra
   Redback Networks Inc.
   350, Holger Way
   San Jose, CA 95134


   EMail: [e-mail]

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

   EMail: [e-mail]


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

   EMail: [e-mail]















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14.  Full Copyright Statement

   Copyright (C) The Internet Society (2000).  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.

Acknowledgement

   Funding for the RFC Editor function is currently provided by the
   Internet Society.



















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