RFC 1364 (rfc1364) - Page 2 of 14

BGP OSPF Interaction

RFC 1364                  BGP OSPF Interaction            September 1992


1.  Introduction

   This document defines the various criteria to be used when designing
   Autonomous System Border Routers (ASBR) that will run BGP [RFC 1267]
   with other ASBRs external to the AS, and OSPF [RFC 1247] 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
   [RFC 1267], 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."

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
   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.



Varadhan