Brocade Virtual ADX Switch and Router Guide (Supporting ADX v03.1.00) - Manual (Page 119)

Brocade Virtual ADX Switch and Router Guide 105
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Overview of OSPF
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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.
Since both routers are flooding equivalent routes, Routers A, B, and C receive multiple routes with
the same cost to the same destination (Router F). For Routers A, B, and C, either route to Router F
(through Router D or through Router E) is equally good.
OSPF eliminates the duplicate AS External LSAs. When two or more Brocade Layer 3 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 Layer 3 Switches that flush the duplicate AS External LSAs have more memory for other
OSPF data. In
Figure 22 , since 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.