BGP4/IDRP-OSPF: BGP4/IDRP for IP---OSPF Interaction

Network Working Group                                       K.  Varadhan
Request for Comments: 1745                                  OARnet & ISI
Category: Standards Track                                       S. Hares
                                                            NSFnet/Merit
                                                              Y. Rekhter
                                  T.J. Watson Research Center, IBM Corp.
                                                           December 1994


                  BGP4/IDRP for IP---OSPF Interaction

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.

Abstract

   This memo defines the various criteria to be used when designing an
   Autonomous System Border Router (ASBR) that will run either BGP4 or
   IDRP for IP with other ASBRs external to the AS and OSPF as its IGP.

Table of Contents

   1.  Introduction .................................................  2
   2.  Reachability Information Exchange ............................  4
   2.1.  Exporting OSPF information into BGP/IDRP  ..................  4
   2.2.  Importing BGP/IDRP information into OSPF ...................  6
   3.  BGP/IDRP Identifier and OSPF router ID .......................  7
   4.  Setting OSPF tags, ORIGIN and PATH attributes ................  8
   4.1.  Configuration parameters for setting the OSPF tag .......... 10
   4.2.  Manually configured tags ................................... 10
   4.3.  Automatically generated tags ............................... 11
   4.3.1. Tag = <Automatic = 1, Complete = 0, PathLength = 00> ...... 11
   4.3.2. Tag = <Automatic = 1, Complete = 0, PathLength = 01> ...... 11
   4.3.3. Tag = <Automatic = 1, Complete = 0, PathLength = 10> ...... 12
   4.3.4. Tag = <Automatic = 1, Complete = 1, PathLength = 00> ...... 12
   4.3.5. Tag = <Automatic = 1, Complete = 1, PathLength = 01> ...... 12
   4.3.6. Tag = <Automatic = 1, Complete = 1, PathLength = 10> ...... 13
   4.4.  Miscellaneous tag settings ................................. 14
   5.  Setting OSPF Forwarding Address and BGP NEXT_HOP attribute ... 14
   6.  Changes from the BGP 3 - OSPF interactions document .......... 15
   7.  Security Considerations ...................................... 16
   8.  Acknowledgements ............................................. 16
   9.  Bibliography ................................................. 16
   10.  Appendix .................................................... 18
   11.  Authors' Present Addresses .................................. 19

1.  Introduction

   This document defines the various criteria to be used when designing
   an Autonomous System Border Router (ASBR) that will run BGP4
   [RFC1654] or IDRP for IP [IDRP] with other ASBRs external to the AS,
   and OSPF [RFC1583] as its IGP.

   All future references of BGP in this document will refer to BGP
   version 4, as defined in [RFC1654].  All reference to IDRP are
   references to the Inter-Domain Routing Protocol (ISO 10747) which has
   been defined by the IDRP for IP document [IDRP] for use in Autonomous
   Systems.

   This document defines how the following fields in OSPF and attributes
   in BGP/IDRP are to be set when interfacing between BGP/IDRP and OSPF
   at an ASBR:

   IDRP came out of the same work as BGP, and may be consider a follow
   on to BGP-3 and BGP-4.  Most fields defined in the interaction
   between BGP and IDRP are named the same.  Where different, the IDRP
   fields are shown separately.

           BGP/IDRP MULTI_EXIT_DISC

           BGP ORIGIN and AS_PATH/AS_SET     vs. OSPF tag
           IDRP EXT_INFO and RD_PATH/RD_SET

           BGP/IDRP NEXT_HOP                 vs. OSPF Forwarding Address

           BGP/IDRP LOCAL_PREF               vs. OSPF cost and type

   IDRP contains RD_PATH and RD_SET fields which serves the same purpose
   as AS_PATH and AS_SET fields for IDRP for IP.  In this document, we
   will use the terms PATH and SET to refer to the BGP AS_PATH and
   AS_SET, or the IDRP RD_PATH and RD_SET fields respectively, depending
   on the context of the protocol being used.

   Both IDRP and BGP provide a mechanism to indicate whether the routing
   information was originated via an IGP, or some other means.  In IDRP,
   if route information is originated by means other than an IGP, then
   the EXT_INFO attribute is present.  Likewise, in BGP, if a route
   information is originated by means other than an IGP, then the ORIGIN
   attribute is set to <EGP> or <INCOMPLETE>.  For the purpose of this
   document, we need to distinguish between the two cases:

        (a)  Route information was originated via an IGP,

        (b)  Route information was originated by some other means.

   The former case is realized in IDRP by not including the EXT_INFO
   attribute, and in BGP by setting the BGP ORIGIN=<IGP>;  The latter
   case is realized by including the EXT_INFO attribute in IDRP, and by
   setting the BGP ORIGIN=<EGP>.  For the rest of the document, we will
   use the BGP ORIGIN=<IGP> to refer to the former scenario, and BGP
   ORIGIN=<EGP> to refer to the latter.

   One other difference between IDRP and BGP remains.  IDRP for IP
   identifies an autonomous system by an identifier of variable length
   that is syntactically identical to an NSAP address prefix, and
   explicitly embeds the autonomous system number [IDRP].  BGP
   identifies an autonomous system just by an autonomous system number.
   Since there is a one-to-one mapping between how an autonomous system
   is identified in IDRP and in BGP, in this document, we shall identify
   an autonomous system by its autonomous system number.

   For a more general treatise on routing and route exchange problems,
   please refer to [ROUTE-LEAKING] and [NEXT-HOP] by Philip Almquist.

   This document uses the two terms "Autonomous System" and "Routing
   Domain".  The definitions for the two are below:

   The term Autonomous System is the same as is used in the BGP RFC
   [RFC1267], given below:

      "The use of the term Autonomous System here stresses the fact
      that, even when multiple IGPs and metrics are used, the
      administration of an AS appears to other ASs to have a single
      coherent interior routing plan and presents a consistent picture
      of what destinations are reachable through it.  From the
      standpoint of exterior routing, an AS can be viewed as monolithic:
      reachability to destinations directly connected to the AS must be
      equivalent from all border gateways of the AS."

   The term Routing Domain was first used in [ROUTE-LEAKING] and is
   given below:

      "A Routing Domain is a collection of routers which coordinate
      their routing knowledge using a single [instance of a] routing
      protocol."

   By definition, a Routing Domain forms a single Autonomous System, but
   an Autonomous System may be composed of a collection of Routing
   Domains.

   BGP, IDRP and OSPF have the concept of a set of reachable
   destinations.  This set is called NLRI or Network Layer Reachability
   Information.  The set can be represented either as an IP address
   prefix, or an address, mask pair.  Note that if the mask is
   contiguous in the latter, then the two representations are
   equivalent.  In this document, we use the term "address/mask pair" in
   the context of OSPF, and "destination" or "set of reachable
   destinations" in the context of BGP or IDRP.

   This document follows the conventions embodied in the Host
   Requirements RFCs [RFC1122, RFC1123], when using the terms "MUST",
   "SHOULD," and "MAY" for the various requirements.

   A minimal implementation of BGP/IDRP OSPF exchange MUST not advertise
   a route containing a set of reachable destinations when none of the
   destinations in the address/mask pair is reachable via OSPF (section
   2.1, bullet 3), MUST merge the PATH into a SET when multiple exit
   points exist within the same autonomous system for the same external
   destination (section 3), MUST set the OSPF tag accurately (section
   4).  This subset is chosen so as to cause minimal havoc to the
   Internet at large.  It is strongly recommended that implementors
   implement more than a minimalistic specification.

2.  Reachability Information Exchange

   This section discusses the constraints that must be met to exchange
   the set of reachable destinations between an external BGP/IDRP peer
   from another AS and internal OSPF address/mask pairs.

   2.1.  Exporting OSPF information into BGP

      1.   The administrator MUST be able to selectively export
           address/mask pairs into BGP/IDRP via an appropriate filter
           mechanism.

           This filter mechanism MUST support such control with the
           granularity of an address/mask pair.

           This filter mechanism will be the primary method of
           aggregation of OSPF internal and type 1 and type 2 external
           routes within the AS into BGP/IDRP.

           Additionally, the administrator MUST be able to filter based
           on the OSPF tag and the various sub-fields of the OSPF tag.
           The settings of the tag and the sub-fields are defined in
           section 4 in more detail.

           o    The default MUST be to export no routes from OSPF into
                BGP/IDRP.  A single configuration parameter MUST permit
                all OSPF inter-area and intra-area address/mask pairs to
                be exported into BGP/IDRP.

                OSPF external address/mask pairs of type 1 and type 2
                MUST never be exported into BGP/IDRP unless they are
                explicitly configured.

      2.   An address/mask pair having a non-contiguous mask MUST not be
           exported to BGP/IDRP.

      3.   When configured to export an address/mask pair from OSPF into
           BGP/IDRP, the ASBR MAY advertise the route containing the set
           of reachable destinations via BGP/IDRP as soon as at least
           one of the destinations in the address/mask pair is
           determined to be reachable via OSPF; it MUST stop advertising
           the route containing the set of reachable destinations when
           none of the destinations in the address/mask pair is
           reachable via OSPF.

      4.   The network administrator MUST be able to statically
           configure the BGP/IDRP attribute MULTI_EXIT_DISC attribute to
           be used for any route.

           o    The default MUST be to omit the MULTI_EXIT_DISC in the
                route advertised via BGP/IDRP.

      5.   An implementation of BGP/IDRP and OSPF on an ASBR MUST have a
           mechanism to set up a minimum amount of time that must elapse
           between the learning of a new address/mask pair via OSPF and
           subsequent advertisement of the address/mask pair via
           BGP/IDRP to the external neighbours.

           o    The default value for this setting MUST be 0, indicating
                that the address/mask pair is to be advertised to the
                neighbour BGP/IDRP peers instantly.

                Note that BGP and IDRP mandate a mechanism to dampen the
                inbound advertisements from adjacent neighbours.  See
                the variable MinRouteAdvertisementInterval in section
                9.2.3.1, [RFC1654] or in section 7.17.3.1, [IS10747].

      6.   LOCAL_PREF is not used when exporting OSPF information into
           BGP/IDRP, as it is not applicable.

   2.2.  Importing BGP/IDRP information into OSPF

      1.   BGP/IDRP implementations SHOULD allow an AS to control
           announcements of BGP/IDRP learned set of reachable
           destinations into OSPF.  Implementations SHOULD support such
           control with the granularity of a single destination.

           Implementations SHOULD also support such control with the
           granularity of an autonomous system, where the autonomous
           system may be either the autonomous system that originated
           the information or the autonomous system that advertised the
           information to the local system (adjacent autonomous system).

           o    The default MUST be to import nothing from BGP/IDRP into
                OSPF.  Administrators must configure every destination
                they wish to import.

                A configuration parameter MAY allow an administrator to
                configure an ASBR to import all the set of reachable
                destinations from BGP/IDRP into the OSPF routing domain.

      2.   The administrator MUST be able to configure the OSPF cost and
           the OSPF metric type of every destination imported into OSPF.
           The OSPF metric type MUST default to type 2. If the
           LOCAL_PREF value is used to construct the OSPF cost, one must
           be extremely careful with such a conversion. In OSPF the
           lower cost is preferred, while in BGP/IDRP the higher value
           of the LOCAL_PREF is preferred.  In addition, the OSPF cost
           ranges between 1 and 2^24, while the LOCAL_PREF value ranges
           between 0 and 2^32.  Note that if ASBRs within a domain are
           configured to correlate BGP/IDRP and OSPF information (as
           described in Section 3), then the route selection by the
           ASBRs is determined solely by the OSPF cost, and the value
           carried by the LOCAL_PREF attribute has no impact on the
           route selection.

      3.   Information learned via BGP/IDRP from peers within the same
           AS MUST not be imported into OSPF.

      4.   The ASBR MUST never generate a default destination into the
           OSPF routing domain unless explicitly configured to do so.

           A default destination is a set of all possible destinations.
           By convention, it is represented as a prefix of 0 length or a
           mask of all zeroes.

           A possible criterion for generating default into an IGP is to
           allow the administrator to specify a set of (set of reachable
           destinations, PATH, default cost, default type) tuples.  If
           the ASBR learns of at least one of the destinations in the
           set of reachable destinations, with the corresponding PATH,
           then it generates a default destination into the OSPF routing
           domain, with the appropriate cost and type.  The lowest cost
           route will then be injected into the OSPF routing domain.

           This is the recommended method for originating default
           destinations in the OSPF routing domain.

      5.   Note that [RFC1247] requires the network number to be used as
           the Link State ID.  This will produce a conflict if the ASBR
           tries to import two destinations, differing only in their
           prefix length.  This problem is fixed in [RFC1583], which
           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.

   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:

     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.

          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.

   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>.

      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        |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
         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
                 imported from BGP/IDRP into the OSPF RD.  Hence, it is
                 assumed that the BGP/IDRP speaker will convey these
                 routes to other BGP/IDRP speakers within the same
                 autonomous system via BGP/IDRP.  An ASBR learning of
                 such a destination MUST wait for the BGP update from
                 its internal neighbours before advertising it to
                 external BGP/IDRP peers.

         export  These routes MUST not be exported into BGP/IDRP.

   4.4.  Miscellaneous tag settings

       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|x|1|1|              Reserved  for  future  use               |
      +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

      The value of PathLength=11 is reserved during automatic tag
      generation.  Routers MUST NOT generate such a tag when importing
      reachable destinations into the OSPF routing domain.  ASBRs must
      ignore tags which indicate a PathLength=11.

5.  Setting OSPF Forwarding Address and BGP/IDRP NEXT_HOP attribute

   Forwarding addresses are used to avoid extra hops between multiple
   routers that share a common network and that speak different routing
   protocols with each other on the common network.

   Both BGP/IDRP and OSPF have equivalents of forwarding addresses.  In
   BGP and IDRP, the NEXT_HOP attribute is a well-known, mandatory
   attribute.  OSPF has a Forwarding address field.  We will discuss how
   these are to be filled in various situations.

   Consider the 4 router situation below:

   RT1 and RT2 are in one autonomous system, RT3 and RT4 are in another.
   RT1 and RT3 are talking BGP/IDRP with each other.  RT3 and RT4 are
   talking OSPF with each other.

            +-----+                 +-----+
            | RT1 |                 | RT2 |
            +-----+                 +-----+
               |                       |            common network
            ---+-----------------------+--------------------------
            <BGP/IDRP> |                       |
                    +-----+     <OSPF>      +-----+
                    | RT3 |                 | RT4 |
                    +-----+                 +-----+


     - Importing a reachable destination into OSPF:
          When importing a destination from BGP/IDRP into OSPF, RT3 MUST
          always fill the OSPF Forwarding Address with the BGP/IDRP
          NEXT_HOP attribute for the destination.

     - Exporting a reachable destination into BGP:
          When exporting set of reachable destinations internal to the
          OSPF routing domain from OSPF to BGP/IDRP, if all the
          destinations in the set of reachable destinations are through
          RT4, then RT3 MAY fill the NEXT_HOP attribute for the set of
          reachable destinations with the address of RT4.  This is to
          avoid requiring packets to take an extra hop through RT3 when
          traversing the AS boundary.  This is similar to the concept of
          indirect neighbour support in EGP [RFC888, RFC827].

6.  Changes from the BGP 3 - OSPF interactions document

     o    The use of the term "route" has attained a more complicated
          structure in BGP 4.  This document follows the constraint in
          the manner shown below:

          -    The term "set of reachable destinations" is called a NLRI
               in [RFC1654].

          -    The term "route" in the BGP context refers to a set of
               reachable destinations, and the associated attributes for
               the set.

          -    The term "route" in the OSPF context refers to the set of
               reachable destinations, and the cost and the type to
               reach destinations.  This is to keep the definitions
               consistent in the document.

     o    The notion of exchanging reachability information between BGP
          4 and OSPF has been updated to handle variable length net mask
          information.

     o    The previous term INTER_AS_METRIC in BGP 3 has now been
          changed to MULTI_EXIT_DISC.

     o    The default metric to be used for importing BGP information
          into the OSPF RD is now the LOCAL_PREF attribute, instead of a
          constant value.

     o    Section 3 which requires an ASBR to match the OSPF tag
          corresponding to a route to the BGP Identifier, can cause
          potential loops if the AS has equal cost multipath routing
          amongst the ASBRs.  This scenario is outlined in the Appendix
          below.  This is fixed in BGP4 by requiring the ASBR seeing
          equal cost multi-path routes to merge the PATHs through the
          various ASBRs into appropriate SETs.

     o    BGP 4 requires that the BGP identifier be an address assigned
          to the BGP speaker.  This is dealt with in section 3.

     o    Section 5 on setting NEXT_HOP attributes and Forwarding
          Address field has been updated to account for variable length
          net information.

     o    This section, 6, has been added.

7.  Security Considerations

   Security issues are not discussed in this memo.

8.  Acknowledgements

   We would like to thank Jeff Honig (Cornell University), John Moy
   (Cascade Communications Corp.), Tony Li (cisco Systems), Rob Coltun
   (Consultant), Dennis Ferguson (ANS, Inc.), Phil Almquist
   (Consultant), Scott Bradner (Harvard University), and Joel Halpern
   (Newbridge Networks Inc.) for their help and suggestions in writing
   this document.  Cengiz Aleattinoglu (USC/ISI) and Steve Hotz
   (USC/ISI) provided fresh insights into the packet looping problem
   described in the appendix.

   We would also like to thank the countless number of people from the
   OSPF and BGP working groups who have offered numerous suggestions and
   comments on the different stages of this document.

   Thanks also to Bob Braden (ISI), whose suggestions on the earlier
   BGP-OSPF document, [RFC1403] were useful even for this one, and have
   been carried through.

   We would also like to thank OARnet, where one of the authors did most
   of his work on this document, before moving to USC to resurrect his
   PhD.

9.  Bibliography

   [RFC827]  Rosen, E., "Exterior Gateway Protocol (EGP)", RFC 827,
             BBN, October 1982.

   [RFC888]  Seamonson, L., and E. Rosen, "`STUB' Exterior Gateway
             Protocol", RFC 888, BBN, January 1984.

   [RFC1058] Hedrick, C, "Routing Information Protocol", RFC 1058,
             Rutgers University, June 1988.

   [RFC1122] Braden, R., Editor, "Requirements for Internet Hosts -
             Communication Layers", STD 3, RFC 1122, USC/Information
             Sciences Institute, October 1989.

   [RFC1123] Braden, R., Editor, "Requirements for Internet Hosts -
             Application and Support", STD 3, RFC 1123,
             USC/Information Sciences Institute, October 1989.

   [RFC1247] Moy, J., "The OSPF Specification Version 2", RFC 1247,
             Proteon, January 1991.

   [RFC1403] Varadhan, K., "BGP OSPF Interaction", RFC 1403,
             OARnet, January 1993.

   [RFC1519] Fuller, V., Li, T., Yu, J., and K. Varadhan, "Supernetting:
             an Address Assignment and Aggregation Strategy", RFC 1519,
             BARRNet, cisco, Merit, OARnet, September 1993.

   [RFC1583] Moy, J., "The OSPF Specification Version 2", RFC 1583,
             (Obsoletes [RFC1247]), Proteon, March 1994.

   [RFC1654] Rekhter, Y., and T. Li, Editors, "A Border Gateway
             Protocol 4 (BGP-4)", RFC 1654, T.J. Watson Research Center,
             IBM Corp., cisco Systems, July 1994.

   [ROUTE-LEAKING] Almquist, P., "Ruminations on Route Leaking",
                   Work in Progress.

   [NEXT-HOP] Almquist, P., "Ruminations on the Next Hop,
              Work in Progress.

   [IDRP] Hares, S., "IDRP for IP", Work in Progress.

   [IS10747] ISO/IEC IS 10747 - Information Processing Systems -
             Telecommunications and Information Exchange between
             Systems - Protocol for Exchange of Inter-domain Routeing
             Information among Intermediate Systems to Support
             Forwarding of ISO 8473 PDUs, 1993.

10.  Appendix

   This is an example of how the two routing protocols, BGP/IDRP and
   OSPF, might both be consistent in their behaviour, and yet packets
   from a source domain, S, to a destination in domain X might be stuck
   in a forwarding loop.

                                       +--------+
                           X ----------| C1     |
                           |           |Domain C|
                           |           | C3  C2 |
                           |           +--------+
                           B             /   \
                            \           /     \
                             \         /      S
                              \       /      /
                               \     /      /
                             +--------+    /
                             | A1  A2 |   /
                             |Domain A|  /
                             |     A3 |-/
                             +--------+

   Given the domains, X, A, B, C and S, let domains A and C be running
   OSPF, and ASBRs A3 and C3 have equal cost multipath routes to A1, A2
   and C1, C2 respectively.  The picture above shows the internal
   structure of domains A and C only.

   During steady state, the following are the route advertisements:

     o    Domain B advertises to A path <B,X>

     o    ASBR A3 in domain A advertises path <A,B,X> to domain C, at
          ASBR C2.

     o    Domain C has two equal cost paths to X: one direct <C,X>, and
          another through A <C,A,B,X>

     o    BR C3 in domain C advertises to A2 path <C,X>

     o    Domain A has two equal cost paths to X: <A,C,X> and <A,B,X>

   Both C1 and C2 inject a route to X within the domain C, and likewise
   A1 and A2 inject a route to X within the domain A.  Since A3 and C3
   see equal cost routes to X through A1, A2 and C1, C2 respectively, a
   stable loop through ASBRs <A3, A2, C3, C2, A3> exists.

   Section 4 specifies that A3 and C3 that advertise a PATH to
   destination X, MUST aggregate all the PATHs through A1 and A2, and C1
   and C2 respectively.  This has the consequence of constraining the
   BGP/IDRP speaker in either domain A or domain C from choosing
   multiple routes to destination X, and importing only one route into
   OSPF.  This breaks the multiple paths seen in one domain.  The exact
   domain in which the multiple paths are broken is nondeterministic.

11.  Authors' Present Addresses

   Kannan Varadhan
   USC/Information Sciences Institute
   4676 Admiralty Way
   Marina Del Rey, CA 90292-6695

   Phone: +1 310 822 1511 x 402
   EMail: kannan@isi.edu


   Susan Hares
   Merit, Inc.
   1071 Beal Avenue,
   Ann Arbor, MI 48109

   Phone: +1 313 936 2095
   EMail: skh@merit.edu


   Yakov Rekhter
   T.J. Watson Research Center, IBM Corporation
   P.O. Box 704,
   Yorktown Heights, NY 10598.

   Phone: +1 914 784 7361
   EMail: yakov@watson.ibm.com