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Versions: 00 01 02 03 04 RFC 2858

Network Working Group                                   Tony Bates
Internet Draft                                       Cisco Systems
Expiration Date:  February 1999                       Ravi Chandra
                                                     Cisco Systems
                                                         Dave Katz
                                                  Juniper Networks
                                                     Yakov Rekhter
                                                     Cisco Systems


                   Multiprotocol Extensions for BGP-4

              draft-ietf-idr-bgp4-multiprotocol-v2-01.txt


1. Status of this Memo

   This document is an Internet-Draft.  Internet-Drafts are working
   documents of the Internet Engineering Task Force (IETF), its areas,
   and its working groups.  Note that other groups may also distribute
   working documents as Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as ``work in progress.''

   To view the entire list of current Internet-Drafts, please check the
   "1id-abstracts.txt" listing contained in the Internet-Drafts Shadow
   Directories on ftp.is.co.za (Africa), ftp.nordu.net (Northern
   Europe), ftp.nis.garr.it (Southern Europe), munnari.oz.au (Pacific
   Rim), ftp.ietf.org (US East Coast), or ftp.isi.edu (US West Coast).


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.









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       Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-01.txt   August  1998


       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
          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.











       Bates, Chandra, Katz, Rekhter                                   [Page 2]


       Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-01.txt   August  1998


       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 is encoded as shown below:


             +---------------------------------------------------------+
             | 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)       |
             +---------------------------------------------------------+





       Bates, Chandra, Katz, Rekhter                                   [Page 3]


       Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-01.txt   August  1998


          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

             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.






       Bates, Chandra, Katz, Rekhter                                   [Page 4]


       Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-01.txt   August  1998


             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,
          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





       Bates, Chandra, Katz, Rekhter                                   [Page 5]


       Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-01.txt   August  1998


          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.

          An UPDATE message that carries no NLRI, other than the one encoded in
          the MP_REACH_NLRI attribute, should not carry the NEXT_HOP attribute.
          If such a message contains the NEXT_HOP attribute, the BGP speaker
          that receives the message should ignore this attribute.


       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 is encoded as shown below:

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



          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.






       Bates, Chandra, Katz, Rekhter                                   [Page 6]


       Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-01.txt   August  1998


             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).

             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.










       Bates, Chandra, Katz, Rekhter                                   [Page 7]


       Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-01.txt   August  1998


       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. Use of BGP Capability Negotiation

          A BGP speaker that uses Multiprotocol Extensions should use the
          Capability Negotiation 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:


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


             The use and meaning of this field is as follow:





       Bates, Chandra, Katz, Rekhter                                   [Page 8]


       Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-01.txt   August  1998


                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, Sub-AFI> tuples includes them
          as multiple Capabilities in the Capabilities Optional Parameter.

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


       9. Security Considerations

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


       10. Acknowledgements

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


       11. References

          [BGP-CAP] "Capabilities Negotiation with BGP-4", R. Chandra, J.
          Scudder, draft-ietf-idr-bgp4-cap-neg-01.txt, April 1988

          [BGP-4] "A Border Gateway Protocol 4 (BGP-4)", Y. Rekhter & T. Li,
          RFC1771, March 1995

          [IPv4] "Internet Protocol", J. Postel, September 1981

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









       Bates, Chandra, Katz, Rekhter                                   [Page 9]


       Internet Draftdraft-ietf-idr-bgp4-multiprotocol-v2-01.txt   August  1998


       12. 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


























       Bates, Chandra, Katz, Rekhter                                  [Page 10]


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