Dell PowerConnect B-RX Configuration Manual page 747

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 BigIron RX 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.
Algorithm for AS external LSA reduction
Figure 104
The behavior changes under the following conditions:
•
BigIron RX Series Configuration Guide
53-1001986-01
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
shows an example in which the normal AS External LSA reduction feature is in effect.
There is one ASBR advertising (originating) a route to the external destination, but one of the
following happens:
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
25
Another routing domain
(such as BGP4 or RIP)
Router F
675