HP ProCurve 5300xl Series Management Manual page 523

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 5300xl,
3400cl, and/or 6400cl 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 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
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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 HP switch with the higher router ID floods the
AS External LSAs and the other HP 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.
IP Routing Features
Configuring OSPF
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