rfc1403.txt

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   various external AS without waiting for BGP updates from other BGP
   speakers within the same autonomous system about these routes.

   Additionally, in the specific instance of an AS intermixing routers
   running EGP and BGP as exterior gateway routing protocols and using
   OSPF as an IGP, then within the autonomous system, it may not be
   necessary to run BGP with 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-



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RFC 1403                  BGP OSPF Interaction              January 1993


   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.

     ArbitraryTag
          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.

          local_AS
               The term `local_AS' refers to the AS number of the local
               OSPF routing domain.

          next_hop_AS
               `next_hop_AS' refers to the AS number of an external BGP
               peer.

     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                          |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+



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RFC 1403                  BGP OSPF Interaction              January 1993


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

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

      o    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



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RFC 1403                  BGP OSPF Interaction              January 1993


           carried via BGP to other ASBRs within the same AS.  Such
           routes must not be exported from OSPF into BGP.


   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, RFC1388], it
           SHOULD be possible to associate a tag and/or an AS number
           with every interface running RIP on the ASBR.

























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RFC 1403                  BGP OSPF Interaction              January 1993


   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 MUST 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

      Automatic=0, LocalInfo=Arbitrary_Value =>
                                 ORIGIN=<INCOMPLETE>, AS_PATH=<local_AS>

   4.4.  Automatically generated tags

      4.4.1.  Routes with incomplete path information, PathLength = 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

         Automatic=1, Completeness=0, PathLength=00, AS=0 =>
                                        ORIGIN=<EGP>, AS_PATH=<local_AS>










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RFC 1403                  BGP OSPF Interaction              January 1993


      4.4.2  Routes with incomplete path information, PathLength = 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 path information.  The neighbour AS is carried in
         the routing information.

         The OSPF tag to BGP attribute mappings for these routes MUST be

         Automatic=1, Completeness=0, PathLength=01, AS=<next_hop_AS>
                        => ORIGIN=<EGP>, AS_PATH=<local_AS, next_hop_AS>

         This setting SHOULD be used for importing EGP routes into the
         OSPF routing domain.  This setting MAY also be used when
         importing BGP routes whose ORIGIN=<EGP> and
         AS_PATH=<next_hop_AS>;  if the BGP learned route has no other
         transitive attributes, then its propagation via BGP to ASBRs
         internal to the AS MAY be suppressed.

      4.4.3.  Routes with incomplete path information, PathLength >= 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|1|0|     ArbitraryTag      |       AutonomousSystem        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         These are routes imported from routing protocols with truncated
         path information.

         The OSPF tag to BGP attribute mappings for these routes MUST be

         Automatic=1, Completeness=0, PathLength=10, AS=don't care

         These are imported by a border router, which is running BGP to
         a stub domain, and not running BGP to other ASBRs in the same
         AS.  This causes a truncation of the AS_PATH.  These routes
         MUST not be re-exported into BGP at another ASBR.








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RFC 1403                  BGP OSPF Interaction              January 1993


      4.4.4.  Routes with complete path information, PathLength = 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|1|0|0|     ArbitraryTag      |       AutonomousSystem        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         These are routes imported from routing protocols with either
         complete path information or are known to be complete through
         means other than that carried by the routing protocol.

         The OSPF tag to BGP attribute mappings for these routes MUST be

         Automatic=1, Completeness=1, PathLength=00, AS=0
                                     => ORIGIN=<EGP>, AS_PATH=<local_AS>