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IP Routing Features
Configuring OSPF
This enhancement implements the portion of RFC 2328 that describes AS
External LSA reduction. This enhancement is enabled by default, requires no
configuration, and cannot be disabled.
OSPF eliminates duplicate AS External LSAs. When two or more HP Series
5300XL Switches configured as ASBRs have equal-cost routes to the same
next-hop router in an external routing domain, the ASBR with the highest
router ID floods the AS External LSAs for the external domain into the OSPF
AS, while the other ASBRs flush the equivalent AS External LSAs from their
databases. As a result, the overall volume of route advertisement traffic within
the AS is reduced and the Series 5300 Switches that flush the duplicate AS
External LSAs have more memory for other OSPF data.
Algorithm for AS External LSA Reduction. The AS External LSA reduc-
tion feature behavior changes under the following conditions:
There is one ASBR advertising (originating) a route to the external desti-
nation, but one of the following happens:
• A second ASBR comes on-line
• A second ASBR that is already on-line begins advertising an equivalent
route to the same destination.
In either case above, the Series 5300XL Switch with the higher router ID
floods the AS External LSAs and the other Series 5300XL Switch flushes
its equivalent AS External LSAs.
One of the ASBRs starts advertising a route that is no longer equivalent
to the route the other ASBR is advertising. In this case, the ASBRs each
flood AS External LSAs. Since the LSAs either no longer have the same
cost or no longer have the same next-hop router, the LSAs are no longer
equivalent, and the LSA reduction feature no longer applies.
The ASBR with the higher router ID becomes unavailable or is reconfig-
ured so that it is no longer an ASBR. In this case, the other ASBR floods
the AS External LSAs.
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