RFC1403

Network Working Group K. Varadhan Request for Comments: 1403 OARnet Obsoletes: 1364 January 1993

                      BGP OSPF Interaction

Status of this Memo This RFC specifies an IAB standards track protocol for the Internet community, and requests discussion and suggestions for improvements. Please refer to the current edition of the "IAB Official Protocol Standards" 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 Routers (ASBR) that will run BGP with other ASBRs external to the AS and OSPF as its IGP. This is a republication of RFC 1364 to correct some editorial problems. Table of Contents

1.  Introduction ....................................................  2
2.  Route Exchange ..................................................  3
2.1.  Exporting OSPF routes into BGP ................................  3
2.2.  Importing BGP routes into OSPF ................................  4
3.  BGP Identifier and OSPF router ID ...............................  5
4.  Setting OSPF tags, BGP ORIGIN and AS_PATH attributes ............  6
4.1.  Semantics of the characteristics bits .........................  8
4.2.  Configuration parameters for setting the OSPF tag .............  9
4.3.  Manually configured tags ...................................... 10
4.4.  Automatically generated tags .................................. 10
4.4.1.  Routes with incomplete path information, PathLength = 0 ..... 10
4.4.2.  Routes with incomplete path information, PathLength = 1 ..... 11
4.4.3.  Routes with incomplete path information, PathLength >= 1 .... 11
4.4.4.  Routes with complete path information, PathLength = 0 ....... 12
4.4.5.  Routes with complete path information, PathLength = 1 ....... 12
4.4.6.  Routes with complete path information, PathLength >= 1 ...... 13
4.5.  Miscellaneous tag settings .................................... 13
4.6.  Summary of the TagType field setting .......................... 14
5.  Setting OSPF Forwarding Address and BGP NEXT_HOP attribute ...... 14
6.  Security Considerations ......................................... 15
7.  Acknowledgements ................................................ 15
8.  Bibliography .................................................... 16
9.  Author's Address ................................................ 17

Introduction

This document defines the various criteria to be used when designing an Autonomous System Border Routers (ASBR) that will run BGP [RFC1267] with other ASBRs external to the AS, and OSPF [RFC1247] as its IGP. This document defines how the following fields in OSPF and attributes in BGP are to be set when interfacing between BGP and OSPF at an ASBR:

       OSPF cost and type      vs. BGP INTER-AS METRIC
       OSPF tag                vs. BGP ORIGIN and AS_PATH
       OSPF Forwarding Address vs. BGP NEXT_HOP

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-3 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 networks are reachable through it.  From the standpoint
    of exterior routing, an AS can be viewed as monolithic:
    reachability to networks 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."
 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.

Route Exchange

This section discusses the constraints that must be met to exchange routes between an external BGP session with a peer from another AS and internal OSPF routes. BGP does not carry subnet information in routing updates. Therefore, when referring to a subnetted network in the OSPF routing domain, we consider the equivalent network route in the context of BGP. Multiple subnet routes for a subnetted network in OSPF are collapsed into one network route when exported into BGP. 2.1. Exporting OSPF routes into BGP

  1.   The administrator MUST be able to selectively export OSPF
       routes into BGP via an appropriate filter mechanism.
       This filter mechanism MUST support such control with the
       granularity of a single network.
       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.  A single configuration parameter MUST permit all
            OSPF inter-area and intra-area routes to be exported
            into BGP.
            OSPF external routes of type 1 and type 2 MUST never be
            exported into BGP unless they are explicitly configured.
  2.   When configured to export a network, the ASBR MUST advertise
       a network route for a subnetted network, as long as at least
       one subnet in the subnetted network is reachable via OSPF.
  3.   The network administrator MUST be able to statically
       configure the BGP attribute INTER-AS METRIC to be used for
       any network route.
       o    By default, the INTER_AS METRIC MUST not be set.  This
            is because the INTER_AS METRIC is an optional attribute
            in BGP.
       Explanatory text: The OSPF cost and the BGP INTER-AS METRIC
       are of different widths.  The OSPF cost is a two level
       metric.  The BGP INTER-AS METRIC is only an optional non-

       transitive attribute.  Hence, a more complex BGP INTER-AS
       METRIC-OSPF cost mapping scheme is not necessary.
  4.   When an ASBR is advertising an OSPF route to network Y to
       external BGP neighbours and learns that the route has become
       unreachable, the ASBR MUST immediately propagate this
       information to the external BGP neighbours.
  5.   An implementation of BGP 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 route via OSPF and subsequent
       advertisement of the route via BGP to the external
       neighbours.
       o    The default value for this setting MUST be 0, indicating
            that the route is to be advertised to the neighbour BGP
            peers instantly.
            Note that [RFC1267] mandates a mechanism to dampen the
            inbound advertisements from adjacent neighbours.

2.2. Importing BGP routes into OSPF

  1.   BGP implementations SHOULD allow an AS to control
       announcements of BGP-learned routes into OSPF.
       Implementations SHOULD support such control with the
       granularity of a single network.  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 route or the autonomous
       system that advertised the route to the local system
       (adjacent autonomous system).
       o    The default MUST be to export no routes from BGP into
            OSPF.  Administrators must configure every route they
            wish to import.
            A configuration parameter MAY allow an administrator to
            configure an ASBR to import all the BGP routes into the
            OSPF routing domain.
  2.   The administrator MUST be able to configure the OSPF cost and
       the OSPF metric type of every route imported into OSPF.
       o    The OSPF cost MUST default to 1; the OSPF metric type
            MUST default to type 2.

  3.   Routes learned via BGP from peers within the same AS MUST not
       be imported into OSPF.
  4.   The ASBR MUST never generate a default route into the OSPF
       routing domain unless explicitly configured to do so.
       A possible criterion for generating default into an IGP is to
       allow the administrator to specify a set of (network route,
       AS_PATH, default route cost, default route type) tuples.  If
       the ASBR learns of the network route for an element of the
       set, with the corresponding AS_PATH, then it generates a
       default route into the OSPF routing domain, with cost
       "default route cost" and type, "default route type".  The
       lowest cost default route will then be injected into the OSPF
       routing domain.
       This is the recommended method for originating default routes
       in the OSPF routing domain.

BGP Identifier and OSPF router ID

The BGP identifier MUST be the same as the OSPF router id at all times that the router is up. This characteristic is required for two reasons.

 i    Synchronisation between OSPF and BGP
      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 have routes to an external network X and
      import it into the OSPF routing domain.  RT3 is advertising
      the route to network X to other external BGP speakers.  RT3

      must use the OSPF router ID to determine whether it is using
      RT1 or RT2 to forward packets to network X and hence build the
      correct AS_PATH to advertise to other external speakers.
      More precisely, RT3 must determine which ASBR it is using to
      reach network X by matching the OSPF router ID for its route
      to network X with the BGP Identifier of one of the ASBRs, and
      use the corresponding route for further advertisement to
      external BGP peers.
 ii   It will be convenient for the network administrator looking at
      an ASBR to correlate different BGP and OSPF routes based on
      the identifier.

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] 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 from other BGP speakers within the same autonomous system via BGP. 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 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-

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

 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

       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.

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>

  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.

  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>
     This SHOULD be used for importing routes into OSPF from an IGP.
  4.4.5.  Routes with complete 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|1|0|1|     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
     routing protocol also has additional information about the
     neighbour AS of the route.
     The OSPF tag to BGP attribute mappings for these routes MUST be
     Automatic=1, Completeness=1, PathLength=01, AS=next_hop_AS
                    => ORIGIN=<IGP>, AS_PATH=<local_AS, next_hop_AS>
     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 BGP routes whose
     ORIGIN=<IGP> and AS_PATH=<next_hop_AS>;  if the BGP learned
     route has no other transitive attributes, then its propagation
     via BGP to other ASBRs internal to the AS MAY be suppressed.

  4.4.6.  Routes with complete 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|1|1|0|     ArbitraryTag      |       AutonomousSystem        |
  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
     These are routes imported from routing protocols with complete
     path information and carry the AS path information as part of
     the routing information.
     The OSPF tag MUST be set to
     Automatic=1, Completeness=1, PathLength=10, AS=don't care
     These routes MUST not be exported into BGP because these routes
     are already imported from BGP into the OSPF RD.  Hence, it is
     assumed that the BGP speaker will convey this information to
     other BGP speakers within the same AS via BGP.  An ASBR
     learning of such a route MUST wait for the BGP update from its
     internal neighbours before advertising this route to external
     BGP peers.
     Note that an implementation MAY import BGP routes with a path
     length of 1 and no other transitive attributes directly into
     OSPF and not send these routes via BGP to ASBRs within the same
     AS.  In this situation, it MUST use tag settings corresponding
     to 4.4.2, or 4.4.5.

4.5. 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
  routes into the OSPF routing domain.  ASBRs MUST ignore tags which
  indicate a PathLength=11.

4.6. Summary of the tag sub-field setting

  The following table summarises the various combinations of
  automatic tag settings for the Completeness and PathLength sub-
  field of the OSPF tag and the default behaviour permitted for each
  setting.
              Completeness := 0 | 1
              PathLength := 00 | 01 | 10 | 11
              ORIGIN := <INCOMPLETE> | <IGP> | <EGP>
              AS_PATH := valid AS path settings as defined in BGP

PathLength ==> 00 01 10 11 Completeness

 ||     +--------------------------------------------------------------
 vv     |
 =  NO  |    <EGP>            <EGP>             never export   reserved
     |  <local_AS>  <local_AS,next_hop_AS>
     |
 = YES  |    <IGP>            <IGP>             out of band    reserved
     |  <local_AS>  <local_AS,next_hop_AS>
     |
  The "out of band" in the table above implies that OSPF will not be
  able to carry everything that BGP needs in its routing
  information.  Therefore, some other means must be found to carry
  this information.  In BGP, this is done by running BGP to other
  ASBRs within the same AS.

Setting OSPF Forwarding Address and BGP 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. Both BGP and OSPF have equivalents of forwarding addresses. In BGP, 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 with each other. RT3 and RT4 are talking OSPF with each other.

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

 - Importing network X to OSPF:
      Consider an external network X, learnt via BGP from RT1.
      RT3 MUST always fill the OSPF Forwarding Address with the BGP
      NEXT_HOP attribute for the route to network X.
 - Exporting network Y to BGP:
      Consider a network Y, internal to the OSPF routing domain,
      RT3's route to network Y is via RT4, and network Y is to be
      exported via BGP to RT1.
      If network Y is not a subnetted network, RT3 MUST fill the
      NEXT_HOP attribute for network Y 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].

Security Considerations

Security issues are not discussed in this memo.

Acknowledgements

I would like to thank Yakov Rekhter, Jeff Honig, John Moy, Tony Li, Dennis Ferguson, and Phil Almquist for their help and suggestions in writing this document, without which I could not have written this document. I would also like to thank them for giving me the opportunity to write this document, and putting up with my muddlements through various phases of this document.

I 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, who went through the document thoroughly, and came back with questions and comments, which were very useful. These suggestions have also been carried over into the next version of this document for dealing with BGP 4 and OSPF.

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", STD 34,

          RFC 1058, Rutgers University, June 1988.

[RFC1388] Malkin, G., "RIP Version 2 - Carrying Additional

          Information", RFC 1388, Xylogics, Inc., January 1993.

[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.

[RFC1267] Lougheed, K., and Y. Rekhter, "A Border Gateway

          Protocol 3 (BGP-3)", RFC 1267, cisco Systems,
          T.J. Watson Research Center, IBM Corp., October 1991.

[RFC1268] Rekhter, Y., and P. Gross, Editors, "Application of the

          Border Gateway Protocol in the Internet", RFC 1268,
          T.J. Watson Research Center, IBM Corp., ANS, October 1991.

[RFC1247] Moy, J., "The OSPF Specification - Version 2:", RFC 1247,

          Proteon, January 1991.

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

                Work in Progress.

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

           Work in Progress.

Author's Address:

  Kannan Varadhan
  Internet Engineer, OARnet,
  1224, Kinnear Road,
  Columbus, OH 43212-1136.
  Phone: (614) 292-4137
  Email: [email protected]