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Configuring EIGRP
EIGRP Configuration Task List
IPC-264
Cisco IOS IP Configuration Guide
Figure 46 Floating Summary Route is Applied to Router-B
The configuration of the default summary route on Router-B sends a 0.0.0.0/0 summary route to
Router-C and blocks all other routes, including the 10.1.1.0/24 route, from being advertised to Router-C.
However, this also generates a local discard route on Router-B, a route for 0.0.0.0/0 to the null 0
interface with an administrative distance of 5. When this route is created, it overrides the EIGRP learned
default route. Router-B will no longer be able to reach destinations that it would normally reach through
the 0.0.0.0.0/0 route.
This problem is resolved by applying a floating summary route to the interface on Router-B that connects
to Router-C. The floating summary route is applied by applying an administrative distance to the default
summary route on the interface of Router-B with the following statement:
Router(config-if)# ip summary-address eigrp 100 0.0.0.0 0.0.0.0 250
The administrative distance of 250, applied in the above statement, is now assigned to the discard route
generated on Router-B. The 0.0.0.0/0, from Router-A, is learned through EIGRP and installed in the
local routing table. Routing to Router-C is restored.
If Router-A loses the connection to Router-B, Router-B will continue to advertise a default route to
Router-C, which allows traffic to continue to reach destinations attached to Router-B. However, traffic
destined to networks to Router-A or behind Router-A will be dropped when it reaches Router-B.
Figure 47 shows a network with two connections from the core, Router-A and Router-D. Both routers
have floating summary routes configured on the interfaces connected to Router-C. If the connection
between Router-E and Router-C fails, the network will continue to operate normally. All traffic will flow
from Router-C through Router-B to the hosts attached to Router-A and Router-D.
103615
interface Serial 0/1
Router-B#show ip route
ip summary-address eigrp 100 0.0.0.0.0.0.0.0 25
. . . .
. . . .
0.0.0.0.0.0.0.0 via <Router-A> (489765/170)
Router-C
Router-A Router-B
0.0.0.0/0
10.1.1.0/24
Router-C#show ip route
0.0.0.0.0.0.0.0 via <Router-B> (489765/90)