Brocade FastIron Ethernet Switch Layer 3 Routing Configuration Guide - Manual (Page 250)

FIGURE 22 AS external LSA reduction
Notice that both Router D and Router E have a route to the other routing domain through Router F.
OSPF eliminates the duplicate AS External LSAs. When two or more devices are 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 devices that flush the
duplicate AS External LSAs have more memory for other OSPF data. Because Router D has a higher
router ID than Router E, Router D floods the AS External LSAs for Router F to Routers A, B, and C.
Router E flushes the equivalent AS External LSAs from its database.
Algorithm for AS external LSA reduction
The AS external LSA reduction example shows the normal AS External LSA reduction feature. The
behavior changes under the following conditions:
• There is one ASBR advertising (originating) a route to the external destination, but one of the
following happens:
Algorithm for AS external LSA reduction
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