BGP OSPF Interaction
RFC 1403

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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

Varadhan                                                        [Page 1]
RFC 1403                  BGP OSPF Interaction              January 1993

1.  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.

Varadhan                                                        [Page 2]
RFC 1403                  BGP OSPF Interaction              January 1993

2.  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