LAB: EIGRP Floating Summarization

R5:
interface FastEthernet0/0
ip summary-address eigrp 100 150.1.4.0 255.255.254.0 5
!
ip route 150.1.4.0 255.255.254.0 155.1.0.4

When summaries are created in EIGRP, OSPF, and BGP the router automatically installs a route to Null0 to match the summary. This is used to prevent the router from forwarding traffic for destinations inside the summary that it does not have a longer match for. However in certain designs this can be an undesirable behavior. To resolve this EIGRP sets its interface level summaries to have an administrative distance of 5 by default. This means that any other route with a distance of 1 – 4 will take precedence over the summary.

In this particular case before summarization is configured on R5, SW2 has the subnet route 150.1.5.0/24, and a default route to reach 150.1.4.4. This is because R4 is generating a default route and suppressing its subnet advertisements.

Rack1SW2#sho ip route 150.1.4.4
Routing entry for 150.1.4.0/24
Known via "eigrp 100", distance 90, metric 2298112, type internal
Redistributing via eigrp 100
Last update from 155.1.58.5 on Vlan58, 00:52:46 ago
Routing Descriptor Blocks:
* 155.1.58.5, from 155.1.58.5, 00:52:46 ago, via Vlan58
Route metric is 2298112, traffic share count is 1
Total delay is 25010 microseconds, minimum bandwidth is 1544 Kbit
Reliability 255/255, minimum MTU 1500 bytes
Loading 1/255, Hops 2

Rack1SW2#sho ip route 150.1.4.4
% Subnet not in table

Rack1R5#sho ip route 150.1.4.4
% Subnet not in table

Rack1R5#sho ip route 150.1.5.5
Routing entry for 150.1.5.0/24
Known via "connected", distance 0, metric 0 (connected, via interface)
Redistributing via eigrp 100
Routing Descriptor Blocks:
* directly connected, via Loopback0
Route metric is 0, traffic share count is 1

Rack1SW2#traceroute 150.1.4.4

Type escape sequence to abort.
Tracing the route to 150.1.4.4

1 155.1.58.5 16 msec 9 msec 8 msec
2 155.1.0.4 42 msec * 17 msec
Rack1SW2#
Rack1SW2#traceroute 150.1.5.5

Type escape sequence to abort.
Tracing the route to 150.1.5.5

1 155.1.58.5 9 msec * 8 msec
Rack1SW2#

after adding the summary route in R5.

Rack1R5#sho ip route 150.1.4.4
Routing entry for 150.1.4.0/23
Known via "eigrp 100", distance 5, metric 128256, type internal
Redistributing via eigrp 100
Routing Descriptor Blocks:
* directly connected, via Null0
Route metric is 128256, traffic share count is 1
Total delay is 5000 microseconds, minimum bandwidth is 10000000 Kbit
Reliability 255/255, minimum MTU 1514 bytes
Loading 1/255, Hops 0

Rack1R5#sho ip route 150.1.5.5
Routing entry for 150.1.5.0/24
Known via "connected", distance 0, metric 0 (connected, via interface)
Redistributing via eigrp 100
Routing Descriptor Blocks:
* directly connected, via Loopback0
Route metric is 0, traffic share count is 1

Rack1R5#show ip route | include 150.1.4.0
D 150.1.4.0/23 is a summary, 00:06:31, Null0


Rack1SW2#traceroute 150.1.4.4

Type escape sequence to abort.
Tracing the route to 150.1.4.4

1 155.1.58.5 25 msec 8 msec 0 msec
2 155.1.0.4 42 msec * 17 msec