rfc1364.txt

来自「RFC 的详细文档！」· 文本 代码 · 共 787 行 · 第 1/3 页

   More precisely, RT3 must use the AS_PATH of the route announced by
   the ASBR, whose BGP Identifier is the same as the OSPF routerID
   corresponding to its route for network X.

      ii.  It will be convenient for the network administrator looking at
           an ASBR to correlate different BGP and OSPF routes based on
           the identifier.

4.  Setting OSPF tags, BGP ORIGIN and AS_PATH attributes

   The OSPF external route tag is a "32-bit field attached to each
   external route . . . It may be used to communicate information
   between AS boundary routers; the precise nature of such information
   is outside the scope of [the] specification." [RFC1247]



Varadhan                                                        [Page 5]

RFC 1364                  BGP OSPF Interaction            September 1992


   OSPF imports information from various routing protocols at all its
   ASBRs.  In some instances, it is possible to use protocols other than
   EGP or BGP across autonomous systems.  It is important, in BGP, to
   differentiate between routes that are external to the OSPF routing
   domain but must be considered internal to the AS, as opposed to
   routes that are external to the AS.

   Routes that are internal to the AS and that may or may not be
   external to the OSPF routing domain will not come to the various BGP
   speakers via IBGP.  Therefore, ASBRs running BGP must have knowledge
   of this class of routes so that they can advertise these routes to
   the various external AS without waiting for IBGP updates about these
   routes.

   Additionally, in the specific instance of an AS intermixing routers
   running EGP and BGP as external gateway routing protocols, using OSPF
   as an IGP, the network administrator does not have to configure IBGP
   on every ASBR running EGP and not running BGP, if this information
   can be carried in the OSPF tag field.

   We use the external route tag field in OSPF to intelligently set the
   ORIGIN and AS_PATH attributes in BGP.  Both the ORIGIN and AS_PATH
   attributes are well-known, mandatory attributes in BGP.  The exact
   mechanism for setting the tags is defined below.

   The tag is broken up into sub-fields shown below.  The various sub-
   fields specify the characteristics of the route imported into the
   OSPF routing domain.

   The high bit of the OSPF tag is known as the "Automatic" bit.  When
   this bit is set to 1, the following sub-fields apply:

       0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |a|c|p l|     ArbitraryTag      |       AutonomousSystem        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   a    is 1 bit called the Automatic bit, indicating that the
        Completeness and PathLength bits have been generated
        automatically by a router.  The meaning of this characteristic
        and its setting are defined below.

   c    is 1 bit of Completeness information.  The meaning of this
        characteristic and its settings are defined below.

   pl   are 2 bits of PathLength information.  The meaning of this
        characteristic and its setting are defined below.



Varadhan                                                        [Page 6]

RFC 1364                  BGP OSPF Interaction            September 1992


   ArbitraryTag (or "at")
        is 12 bits of tag information, which defaults to 0 but can be
        configured to anything else.

   AutonomousSystem (or "as")
        is 16 bits, indicating the AS number corresponding to the
        route, 0 if the route is to be considered as part of the local
        AS.

   When the Automatic bit is set to 0, the following sub-fields apply:

       0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
      |a|                          LocalInfo                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   a    is 1 bit called the Automatic bit, set to 0.

   LocalInfo (or "li")
        is 31 bits of an arbitrary value, manually configured by the
        network administrator.

   The format of the tag for various values of the characteristics bits
   is defined below.

4.1.  Semantics of the characteristics bits

   The Completeness and PathLength characteristics bits define the
   characteristic of the route imported into OSPF from other ASBRs in
   the autonomous system.  This setting is then used to set the ORIGIN
   and NEXT_HOP attributes when re-exporting these routes to an external
   BGP speaker.

   o    The "a" bit or the Automatic characteristic bit is set when
        the Completeness and PathLength characteristics bits are
        automatically set by a border router.

        For backward compatibility, the Automatic bit must default to
        0 and the network administrator must have a mechanism to
        enable automatic tag generation.  Nothing must be inferred
        about the characteristics of the OSPF route from the tag
        bits, unless the tag has been automatically generated.

   o    The "c" bit of the Completeness characteristic bit is set
        when the source of the incoming route is known precisely, for
        instance, from an IGP within the local autonomous system or
        EGP at one of the autonomous system's boundaries.  It refers



Varadhan                                                        [Page 7]

RFC 1364                  BGP OSPF Interaction            September 1992


        to the status of the path information carried by the routing
        protocol.

   o    The "pl" or the PathLength characteristic sub-field is set
        depending on the length of the AS_PATH that the protocol
        could have carried when importing the route into the OSPF
        routing domain.  The length bits will indicate whether the
        AS_PATH attribute for the length is zero, one, or greater
        than one.

        Routes imported from an IGP will usually have an AS_PATH of
        length of 0, routes imported from an EGP will have an AS_PATH
        of length 1, BGP and routing protocols that support complete
        path information, either as AS_PATHs or routing domain paths,
        will indicate a path greater than 1.

        The OSPF tag is not wide enough to carry path information
        about routes that have an associated PathLength greater than
        one.  Path information about these routes will have to be
        carried via IBGP.  Such routes must not be exported from OSPF
        into BGP.

   For brevity in the following sections, the keywords O and P refer to
   the BGP ORIGIN and AS_PATH attributes respectively.  Likewise, we use
   the abbreviations , "l" and "nh" for the local_AS and next_hop_AS
   respectively in the following sections.

4.2.  Configuration parameters for setting the OSPF tag

   o    There must be a mechanism to enable automatic generation of
        the tag characteristic bits.

   o    Configuration of an ASBR running OSPF must include the
        capability to associate a tag value, for the ArbitraryTag, or
        LocalInfo sub-field of the OSPF tag, with each instance of a
        routing protocol.

   o    Configuration of an ASBR running OSPF must include the
        capability to associate an AS number with each instance of a
        routing protocol.

        Associating an AS number with an instance of an IGP is
        equivalent to flagging those set of routes imported from the
        IGP to be external routes outside the local autonomous
        system.

        Specifically, when the IGP is RIP [RFC1058], it should be
        possible to associate a tag and/or an AS number with every



Varadhan                                                        [Page 8]

RFC 1364                  BGP OSPF Interaction            September 1992


        interface running RIP on the ASBR.

4.3.  Manually configured tags

    0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
    0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   |0|                          LocalInfo                          |
   +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

   This tag setting corresponds to the administrator manually setting
   the tag bits.  Nothing shall be inferred about the characteristics of
   the route corresponding to this tag setting.

   For backward compatibility with existing implementations of OSPF
   currently deployed in the field, this must be the default setting for
   importing routes into the OSPF routing domain.  There must be a
   mechanism to enable automatic tag generation for imported routes.

   The OSPF tag to BGP attribute mappings for these routes must be
   a=0, li=Arbitrary_Value  =>  O=<INCOMPLETE>, P=<l>

4.4.  Automatically generated tags

   4.4.1.  Routes with incomplete path information, pl = 0.

     0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
    |1|0|0|0|     ArbitraryTag      |       AutonomousSystem        |
    +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      These are routes imported from routing protocols with incomplete
      path information and cannot or may not carry the neighbour AS or
      AS path as part of the routing information.

      The OSPF tag to BGP attribute mappings for these routes must be
                   a=1,c=0,pl=00,as=0 => O=<EGP>, P=<l>

   4.4.2  Routes with incomplete path information, pl = 1.

        0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
       |1|0|0|1|     ArbitraryTag      |       AutonomousSystem        |
       +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      These are routes imported from routing protocols with incomplete



Varadhan                                                        [Page 9]

RFC 1364                  BGP OSPF Interaction            September 1992


      path information and carry the neighbour AS as part of the routing
      information.

      The OSPF tag to BGP attribute mappings for these routes must be
                a=1,c=0,pl=01,as=nh => O=<EGP>, P=<l, nh>

      This setting should be used for importing EGP routes into the OSPF
      routing domain.  This setting can also be used when importing BGP
      routes whose origin=<EGP> and AS_PATH=<nh>; if the BGP learned
      route has no other transitive attributes, then its propogation via
      IBGP can be suppressed.

   4.4.3.  Routes with incomplete path information, pl >= 1.

        0 0 0 0 0 0 0 0 0 0 1 1 1 1 1 1 1 1 1 1 2 2 2 2 2 2 2 2 2 2 3 3
        0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1