Foundry Networks Switch and Router Installation And Configuration Manual page 579

OSPF Autonomous System (AS)
Router A
Router B
Router C
Figure 17.4 AS External LSA reduction
Notice that both Router D and Router E have a route to the other routing domain through Router F. In software
releases earlier than 07.1.00, if Routers D and E have equal-cost routes to Router F, then both Router D and
Router E flood AS External LSAs to Routers A, B, and C advertising the route to 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 Foundry 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 17.4, 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.
December 2000
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
Configuring OSPF
Another routing domain
(such as BGP4 or RIP)
Router F
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