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rfc:rfc1745

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

Varadhan, Hares & Rekhter [Page 1] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

 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:

Varadhan, Hares & Rekhter [Page 2] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

      (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.

Varadhan, Hares & Rekhter [Page 3] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

 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.

Varadhan, Hares & Rekhter [Page 4] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

         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.

Varadhan, Hares & Rekhter [Page 5] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

 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

Varadhan, Hares & Rekhter [Page 6] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

         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.

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
               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.

Varadhan, Hares & Rekhter [Page 13] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

 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 |
                  +-----+                 +-----+
  1. 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.

Varadhan, Hares & Rekhter [Page 14] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

  1. 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:
  1. The term "set of reachable destinations" is called a NLRI

in [RFC1654].

  1. The term "route" in the BGP context refers to a set of

reachable destinations, and the associated attributes for

             the set.
  1. 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.

Varadhan, Hares & Rekhter [Page 15] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

   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.

Varadhan, Hares & Rekhter [Page 16] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

 [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.

Varadhan, Hares & Rekhter [Page 17] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

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.

Varadhan, Hares & Rekhter [Page 18] RFC 1745 BGP4/IDRP for IP - OSPF Interaction December 1994

 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

Varadhan, Hares & Rekhter [Page 19]

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