HP 3500YL - Manual (Page 228)

IP Routing Features
Configuring OSPF
■ injected into the backbone area and then propagated to any other OSPF
areas (except stub areas) within the local OSPF Autonomous System
(AS). If the AS includes an NSSA, there are two additional options:
• If the NSSA includes an ASBR, you can suppress advertising some or
all of its summarized external routes into the backbone area.
• Replace all type-3 summary LSAs and the default external route from
the backbone area with the default summary route (0.0.0.0/0).
In some cases, multiple ASBRs in an AS can originate equivalent external
LSAs. The LSAs are equivalent when they have the same cost, the same next
hop, and the same destination. In such cases, the ProCurve switch optimizes
OSPF by eliminating duplicate AS External LSAs. That is, 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.
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.
Algorithm for AS External LSA Reduction
The AS External LSA reduction 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 ProCurve switch with the higher router ID floods
the AS External LSAs and the other ProCurve 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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