rfc2283.txt

著名的RFC文档,其中有一些文档是已经翻译成中文的的.

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