Foundry Configuration Guide for the FESX, FSX, and FWSX
20 - 6 © Foundry Networks, Inc. December 2005
Figure 20.4 AS External LSA reduction
Notice that both Router D and Router E have a route to the other routing domain through Router F. In earlier
software releases, 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 20.4, 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.
Algorithm for AS External LSA Reduction
Figure 20.4 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
Another routing domain
(such as BGP4 or RIP)
OSPF Autonomous System (AS)
Router A
Router F
Router C
Routers D, E, and F
are OSPF ASBRs
and EBGP routers.
Router B
Router ID: 2.2.2.2
Router D
Router ID: 1.1.1.1
Router E
To Next Page
Loading...
Need help?
General
