RFC 1403 (rfc1403) - Page 3 of 17

BGP OSPF Interaction

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
   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-



Varadhan