EIGRP Manual Route Summarization

EIGRP supports manual route summarization between EIGRP neighbors to help reduce the size of routing tables and to lessen CPU work on the routers. EIGRP enables route summarization at the interface level using the ip summary address command. Below is an example;

R1(config)#router eigrp 100
R1(config-router)#no auto-summary 
R1(config-router)#network 100.1.1.0 0.0.0.255
R1(config-router)#network 100.2.1.0 0.0.0.255
R1(config-router)#network 100.3.1.0 0.0.0.255
R1(config-router)#network 100.4.1.0 0.0.0.255
R1(config-router)#network 100.5.1.0 0.0.0.255
R1(config-router)#network 100.6.1.0 0.0.0.255
R1(config-router)#network 100.7.1.0 0.0.0.255

R1(config-router)#network 10.1.1.0 0.0.0.3

R2(config)#router eigrp 100

R2(config-router)#network 10.1.1.0 0.0.0.3

R2(config-router)#no auto-summary 

*Mar  1 00:21:22.850: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.1.1 (FastEthernet0/0) is up: new adjacency

R2#show ip route eigrp 

     100.0.0.0/24 is subnetted, 7 subnets

D       100.5.1.0 [90/156160] via 10.1.1.1, 00:00:47, FastEthernet0/0

D       100.4.1.0 [90/156160] via 10.1.1.1, 00:00:48, FastEthernet0/0

D       100.7.1.0 [90/156160] via 10.1.1.1, 00:00:48, FastEthernet0/0

D       100.6.1.0 [90/156160] via 10.1.1.1, 00:00:48, FastEthernet0/0

D       100.1.1.0 [90/156160] via 10.1.1.1, 00:00:48, FastEthernet0/0

D       100.3.1.0 [90/156160] via 10.1.1.1, 00:00:48, FastEthernet0/0

D       100.2.1.0 [90/156160] via 10.1.1.1, 00:00:48, FastEthernet0/0

I can see the routes learned from R1 on R2 with the show ip route command. These routes are hierarchical so I will do some binary math to find the summary route. The matching stops at 100.0.0.0 and counting the consecutive bits give us 13 bits, so /13 as subnet mask.

01100100.00000001.00000001.00000000

01100100.00000010.00000001.00000000

01100100.00000011.00000001.00000000

01100100.00000100.00000001.00000000

01100100.00000101.00000001.00000000

01100100.00000110.00000001.00000000

01100100.00000111.00000001.00000000

100.0.0.0  255.248.0.0 is the summary route

R1(config)#interface fastEthernet 0/0

R1(config-if)#ip summary-address eigrp 100 100.0.0.0 255.248.0.0 

*Mar  1 00:35:29.602: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.1.2 (FastEthernet0/0) is down: summary configured

*Mar  1 00:35:33.777: %DUAL-5-NBRCHANGE: IP-EIGRP(0) 100: Neighbor 10.1.1.2 (FastEthernet0/0) is up: new adjacency

R1#show ip route eigrp 

     100.0.0.0/8 is variably subnetted, 8 subnets, 2 masks

D       100.0.0.0/13 is a summary, 00:01:49, Null0

R2#show ip route eigrp 

     100.0.0.0/13 is subnetted, 1 subnets

D       100.0.0.0 [90/156160] via 10.1.1.1, 00:01:04, FastEthernet0/0

R2#ping 100.1.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 100.1.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 1/4/8 ms

R2#ping 100.2.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 100.2.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 4/5/12 ms

R2#ping 100.7.1.1

Type escape sequence to abort.

Sending 5, 100-byte ICMP Echos to 100.7.1.1, timeout is 2 seconds:

!!!!!

Success rate is 100 percent (5/5), round-trip min/avg/max = 4/6/16 ms

Here I verified connectivity to the loopbacks using the summary route