Brocade Multi-Service IronWare Routing Configuration Guide (Supporting R05.6.00) - Manual (Page 189)

Multi-Service IronWare Routing Configuration Guide 161
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OSPF point-to-point links
FIGURE 11 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. In
Figure 11 , 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.
Algorithm for AS external LSA reduction
Figure 11 shows an example in which the normal AS External LSA reduction feature is in effect. 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:
• A second ASBR comes on-line