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OSPF path selection posted 01/02/2007
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Hi, all,

If a router receives an external route with type 1 metric coming from
intra-area (higher cost) and inter area lower cost from the same originator,
how OSPF selects the routes? I am having hard time to grasp how RFC says in
this scenario.

********* RFC2328 ***************

16.4.1.  External path preferences

            When multiple intra-AS paths are available to
            ASBRs/forwarding addresses, the following rules indicate
            which paths are preferred. These rules apply when the same
            ASBR is reachable through multiple areas, or when trying to
            decide which of several AS-external-LSAs should be
            preferred. In the former case the paths all terminate at the
            same ASBR, while in the latter the paths terminate at
            separate ASBRs/forwarding addresses. In either case, each
            path is represented by a separate routing table entry as
            defined in Section 11.

            This section only applies when RFC1583Compatibility is set
            to "disabled".

            The path preference rules, stated from highest to lowest
            preference, are as follows. Note that as a result of these
            rules, there may still be multiple paths of the highest
            preference. In this case, the path to use must be determined
            based on cost, as described in Section 16.4.

            o   Intra-area paths using non-backbone areas are always the
                most preferred.

            o   The other paths, intra-area backbone paths and inter-
                area paths, are of equal preference.

I don't quite understand what second bullet means, does it sugguest that
OSPF will loadbalance between intra-area backbone paths and inter-area
Or it is totally implementation dependent?

Thanks a lot,