Subject: Recalculating all AS-External routes on reception of ASBR summary LSA

From: "Sudharsana V - CTD, Chennai." <[email-masked]>
Date: Thu, 12 Sep 2002 18:11:39 +0530

Hi,
Section 13.2 of RFC 2328 says -
"Summary-LSAs
            The best route to the destination described by the summary-
            LSA must be recalculated (see Section 16.5).  If this
            destination is an AS boundary router, it may also be
            necessary to re-examine all the AS-external-LSAs."

When route to an ASBR changes (cost changes or becomes unreachable), is it
not sufficient to recalculate external routes to only the destinations
advertised by the concerned ASBR?
Why should we re-examine all the AS-External-LSAs?

Thanks,
Sudharsana.V

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From: Acee Lindem <[email-masked]>
Date: Thu, 12 Sep 2002 09:53:00 -0400

> advertised by the concerned ASBR?

Yes.

> Why should we re-examine all the AS-External-LSAs?

You can make this optimization if you wish. In the case of the intra-area
SPF, RFC 2328 explicitly states that optimizations are possible. The same
statement would apply to incremental calculation corner cases. Keep in
mind that if the path to one ASBR changes, there is chance that the cost
to other ASBRs will change as well.

--
Acee