rfc1745.txt

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           obsoletes [RFC1247].

           An implementation conforming to the older [RFC1247] MUST, in
           this case, drop the more specific route, i.e. the route
           corresponding to the longer prefix in the reachability
           information.

      6.   MULTI_EXIT_DISC is not used to import BGP/IDRP information
           into OSPF, as it is not applicable.

3.  BGP/IDRP Identifier and OSPF router ID

   The BGP/IDRP identifier MUST be the same as the OSPF router id at all
   times that the router is up.

   Note that [RFC1654] requires that the BGP identifier be an address
   assigned to the BGP speaker.

   In the case of IDRP, the IDRP protocol does not explicitly carry the
   identity of the IDRP speaker.  An implicit notion of the identity of
   the IDRP speaker can be obtained by examining the source address in
   the IP packets carrying the IDRP information.  Therefore, all IDRP
   speakers participating in the OSPF protocol MUST bind the IDRP
   identifier to be the address of the OSPF router id.

   This characteristic makes it convenient for the network administrator
   looking at an ASBR to correlate different BGP/IDRP and OSPF
   information based on the identifier.  There is another more important
   reason for this characteristic.








Varadhan, Hares & Rekhter                                       [Page 7]

RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


   Consider the scenario in which 3 ASBRs, RT1, RT2, and RT3, belong to
   the same autonomous system.

                                     +-----+
                                     | RT3 |
                                     +-----+
                                        |

                          Autonomous System running OSPF

                                 /               \

                             +-----+          +-----+
                             | RT1 |          | RT2 |
                             +-----+          +-----+

   Both RT1 and RT2 can reach an external destination X and import this
   information into the OSPF routing domain.  RT3 is advertising this
   information about destination X to other external BGP/IDRP speakers.
   The following rule specifies how RT3 can generate the correct
   advertisement.

   RT3 MUST determine which ASBR(s) it is using to reach destination X
   by matching the OSPF router ID for its route to destination with the
   BGP identifier of the ASBR(s), or the IP source address of the IDRP
   protocol packet from the ASBR(s).

     o    If RT3 has equal cost routes to X through RT1 and RT2, then,
          RT3 MUST merge the PATH through RT1 and RT2 into a SET.

     o    Otherwise, RT3 MAY merge the PATH through RT1 and RT2.

     It MAY then generate the corresponding network layer reachability
     information for further advertisement to external BGP/IDRP peers.

4.  Setting OSPF tags, ORIGIN and 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" [RFC1583].

   We use the external route tag field in OSPF to intelligently set the
   ORIGIN and PATH attributes in BGP/IDRP.  These attributes are well-
   known, mandatory attributes in BGP/IDRP.  The exact mechanism for
   setting the tags is defined in sections 4.2 and 4.3.  Every
   combination of tag bits is described in two parts:




Varadhan, Hares & Rekhter                                       [Page 8]

RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


     import  This describes when an ASBR imports an AS external LSA into
             the OSPF domain with the given tag setting.

     export  This indicates how the BGP/IDRP path attribues should be
             formatted when an ASBR, having a given type 1 or type 2
             OSPF external route in its routing table, decides to export
             according to the considerations in section 2.1.

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

     The high bit of the OSPF tag is known as the "Automatic" bit.
     Setting this bit indicates that the tag has been generated
     automatically by an ASBR.

     When the network administrator configures the tag, this bit MUST be
     0.  This setting is the default tag setting, and is described in
     section 4.2.

     When the tag is automatically generated, this bit is set to 1.  The
     other bits are defined below:

      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|c|p l|     ArbitraryTag      |       AutonomousSystem        |
     +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

     c    1 bit of Completeness information, set when the ORIGIN of the
          route is either <EGP> or <IGP>.

     pl   2 bits of PathLength information;  this field is set depending
          on the length of the PATH that the protocol could have carried
          when importing the reachability information into the OSPF
          routing domain.

     ArbitraryTag
          12 bits of tag information, defaults to 0 but can be
          configured to anything else.

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






Varadhan, Hares & Rekhter                                       [Page 9]

RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


          local_AS:     The AS number of the local OSPF routing domain.

          next_hop_AS:  The AS number of an external BGP peer.

   4.1.  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 domain.

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

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

   4.2.  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                          |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+


      import  This tag setting corresponds to the administrator manually
              setting the OSPF tag bits.

      export  The route SHOULD be exported into BGP with the attributes
              ORIGIN=<EGP>, PATH=<local_AS>.

      Nothing MUST inferred about the characteristics of the set of
      reachable destinations 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 destinations into the OSPF routing domain.  There
      MUST be a mechanism to enable automatic tag generation for
      imported destinations.





Varadhan, Hares & Rekhter                                      [Page 10]

RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


   4.3.  Automatically generated tags

      4.3.1. Tag = <Automatic = 1, Complete = 0, PathLength = 00>

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


         import  These are reachable destinations imported from routing
                 protocols with incomplete path information and cannot
                 or may not carry the neighbour AS or AS path (and hence
                 the IDRP RD_PATH) as part of the routing information.

                 This setting SHOULD be used to import reachable
                 destinations from an IGP that the network administrator
                 has configured as external routes, without specifying
                 the next_hop_AS.

         export  The route SHOULD be exported into BGP/IDRP with the
                 attributes ORIGIN=<EGP>, PATH=<Local_AS>.

      4.3.2. Tag = <Automatic = 1, Complete = 0, PathLength = 01>

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

         import  These are reachable destinations imported from routing
                 protocols with incomplete path information.  The
                 neighbour AS (and therefore IDRP RD) is carried in the
                 routing information.

                 This setting SHOULD be used for importing reachable
                 destinations from EGP into the OSPF routing domain.
                 This setting MAY also be used when importing reachable
                 destinations from BGP/IDRP whose ORIGIN=<EGP> and
                 PATH=<next_hop_AS>; if the BGP/IDRP learned route has
                 no other transitive attributes, then its propagation
                 via BGP/IDRP to ASBRs internal to the autonomous system
                 MAY be suppressed.

         export  The route SHOULD be exported into BGP/IDRP with
                 ORIGIN=<EGP> and PATH=<local_AS, next_hop_AS>.



Varadhan, Hares & Rekhter                                      [Page 11]

RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


      4.3.3. Tag = <Automatic = 1, Complete = 0, PathLength = 10>

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

         import  These are reachable destinations imported from routing
                 protocols with truncated path information.

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

         export  The route MUST never be exported into BGP/IDRP.



      4.3.4. Tag = <Automatic = 1, Complete = 1, PathLength = 00>

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

         import  These are reachable destinations 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.

                 This setting SHOULD be used for importing reachable
                 destinations into OSPF from an IGP.

         export  The route SHOULD be exported to BGP/IDRP with
                 ORIGIN=<IGP>, PATH=<Local_AS>.

      4.3.5. Tag = <Automatic = 1, Complete = 1, PathLength = 01>

       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|1|     ArbitraryTag      |       AutonomousSystem        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+




Varadhan, Hares & Rekhter                                      [Page 12]

RFC 1745          BGP4/IDRP for IP - OSPF Interaction      December 1994


         import  These are reachable destinations 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 routing protocol
                 also has additional information about the next hop AS
                 or RD, the destination was learned from.

                 This setting SHOULD be used when the administrator
                 explicitly associates an AS number with an instance of
                 an IGP.  This setting MAY also be used when importing
                 reachable destinations from BGP/IDRP whose ORIGIN=<IGP>
                 and PATH=<next_hop_AS>; if the BGP/IDRP learned route
                 has no other transitive attributes, then its
                 propagation via BGP/IDRP to other ASBRs internal to the
                 autonomous system MAY be suppressed.

         export  OSPF routes with this tag setting SHOULD be exported
                 with the BGP/IDRP attributes, ORIGIN=<IGP>,
                 PATH=<local_AS, next_hop_AS>.

      4.3.6. Tag = <Automatic = 1, Complete = 1, PathLength = 10>

       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|1|0|     ArbitraryTag      |       AutonomousSystem        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

         import  These are reachable destinations imported from routing
                 protocols with complete path information and carry the
                 AS path information as part of the routing information.

                 These destinations MUST not be exported into BGP/IDRP
                 because these are destinations that are already