Dell PowerConnect B-RX Configuration Manual page 751

Figure 104
and E are OSPF ASBRs, and thus communicate route information between the OSPF AS, which
contains Routers A, B, and C, and another routing domain, which contains Router F. The other
routing domain is running another routing protocol, such as BGP4 or RIP. Routers D, E, and F,
therefore, are each running both OSPF and either BGP4 or RIP.
FIGURE 104
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 device switches that flush the duplicate AS External LSAs have more memory for other
OSPF data. In
AS External LSAs for Router F to Routers A, B, and C. Router E flushes the equivalent AS External
LSAs from its database.
BigIron RX Series Configuration Guide
53-1001810-01
shows an example of the AS External LSA reduction feature. In this example, Routers D
AS external LSA reduction
OSPF Autonomous System (AS)
Router A
Router B
Router C
Figure 104, since Router D has a higher router ID than Router E, Router D floods the
Overview of OSPF (Open Shortest Path First)
Routers D, E, and F
are OSPF ASBRs
and EBGP routers.
Router D
Router ID: 2.2.2.2
Router E
Router ID: 1.1.1.1
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Another routing domain
(such as BGP4 or RIP)
Router F
679