Subject: ABR Summaries causing bad OSPF route paths

From: Erblichs <[email-masked]>
Date: Wed, 14 May 2003 12:14:44 -0700

Group,

        I ran into what I call sys admin error in specifiying
        ABR summaries. This is due to the fact that each summary
        only carries a single weight. These summaries are
        advertised outside of the area. However, a better/shorter
        path may be to one of the summarized subnets via a
        different ABR but that 2nd ABR has summarized the subnet
        with a higher weight.

        Should / are RFCs existing that handle the incorrect
        summarizing and/or the improper weighting of the summaries?

        Should OSPF have a intelligence that tries to identify
        the proper level of summaries and/ or the weighting of such?

        Yes, I do realize that summaries have benefits dealing with
        decreased route tables, fewer OSPF control packets, etc.

        Mitchell Erblich
        Sr Software Engineer

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From: Acee Lindem <[email-masked]>
Date: Wed, 14 May 2003 15:26:07 -0400

>         the proper level of summaries and/ or the weighting of such?

The algorithm is well defined. The summary cost is the highest cost
of any of its components. It is not incorrect - anytime you summarize
of aggregate routes, you're going to run into some level of information
loss. This is true in any routing protocol. OSPF has the advantage allowing
the same ranges to overlap so that an adminstrator can make the tradeoff
between reducing the amount of information advertised and optimal
routing.

--
Acee