Dell PowerConnect B-FCXs Configuration Manual page 969

FIGURE 141
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 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
PowerConnect B-Series FCX Configuration Guide
53-1002266-01
AS External LSA reduction
OSPF Autonomous System (AS)
Router A
Router B
Router C
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
29
Overview of OSPF
Another routing domain
(such as BGP4 or RIP)
Router F
927