rfc2283.txt

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   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   Information, Unfeasible Routes Length, Withdrawn Routes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      | Withdrawn Routes (variable)                             |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      Withdrawn Routes: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         being withdrawn from service.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: [e-mail]


   Ravi Chandra
   Cisco Systems, Inc.
   170 West Tasman Drive
   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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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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