Brocade BigIron RX Series Configuration Guide - Manual (Page 835)

BigIron RX Series Configuration Guide 757
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Overview of OSPF (Open Shortest Path First)
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FIGURE 114 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 BigIron RX switches 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 BigIron RX switches that flush the duplicate AS External LSAs have more memory for other
OSPF data. In
Figure 114 , 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 114 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:
Router B
Router A
Router C
Router D
Router ID: 2.2.2.2
OSPF Autonomous System (AS)
Router E
Router ID: 1.1.1.1
Routers D, E, and F
are OSPF ASBRs
and EBGP routers.
Another routing domain
(such as BGP4 or RIP)
Router F