FIGURE 21
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.
Reduction of equivalent AS external LSAs
FastIron Ethernet Switch Layer 3 Routing
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