OSPF - summarizing routes

This post will go over two ways of summarizing routes in OSPF.

1. Summarizing inter-area OSPF routes on the ABR
2. Summarizing external routes on the ASBR

Here is the topology we will use

R1 <---area0---> R2 <---area3---> R3

R2 is the ABR, R3 is the ASBR
R3 has two loopbacks 3.3.3.3 and 4.4.4.4.
3.3.3.3 is being redistributed so it enters ospf as an external route
4.4.4.4 is being advertised via the network statement so it shows up as an inter- area route on R1.

Here is R1's route table:

R1#show ip route | begin Gateway
Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/65] via 172.12.12.2, 00:15:57, Serial1/0
3.0.0.0/32 is subnetted, 1 subnets
O E1 3.3.3.3 [110/158] via 172.12.12.2, 00:02:42, Serial1/0
4.0.0.0/32 is subnetted, 1 subnets
O IA 4.4.4.4 [110/129] via 172.12.12.2, 00:15:57, Serial1/0
172.12.0.0/24 is subnetted, 2 subnets
C 172.12.12.0 is directly connected, Serial1/0
O IA 172.12.23.0 [110/128] via 172.12.12.2, 00:15:57, Serial1/0

For the first case we perform summarization with the 'area x range' command on R2:

R2(config)#router ospf 1
R2(config-router)#area 3 range 4.0.0.0 255.0.0.0

now check R1;

R1#show ip route | begin Gateway
Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/65] via 172.12.12.2, 00:19:27, Serial1/0
3.0.0.0/32 is subnetted, 1 subnets
O E1 3.3.3.3 [110/158] via 172.12.12.2, 00:02:29, Serial1/0
O IA 4.0.0.0/8 [110/129] via 172.12.12.2, 00:02:34, Serial1/0
172.12.0.0/24 is subnetted, 2 subnets
C 172.12.12.0 is directly connected, Serial1/0
O IA 172.12.23.0 [110/128] via 172.12.12.2, 00:19:27, Serial1/0

4.4.4.4/32 is now being summarized as 4.0.0.0/8

Now on to case 2, summarizing an external route. This needs to be done on R3, the ASBR that is redistributing loopback 3.3.3.3 into OSPF.

On R3:

R3(config)#router ospf 1
R3(config-router)#summary-address 3.0.0.0 255.0.0.0

Verify on R1:

R1#show ip route | begin Gateway
Gateway of last resort is not set

1.0.0.0/32 is subnetted, 1 subnets
C 1.1.1.1 is directly connected, Loopback0
2.0.0.0/32 is subnetted, 1 subnets
O 2.2.2.2 [110/65] via 172.12.12.2, 00:21:15, Serial1/0
O E1 3.0.0.0/8 [110/158] via 172.12.12.2, 00:00:32, Serial1/0
O IA 4.0.0.0/8 [110/129] via 172.12.12.2, 00:04:22, Serial1/0
172.12.0.0/24 is subnetted, 2 subnets
C 172.12.12.0 is directly connected, Serial1/0
O IA 172.12.23.0 [110/128] via 172.12.12.2, 00:21:15, Serial1/0

3.3.3.3/32 has now been summarized into 3.0.0.0/8

In summary,

Inter-area routes are summarized on the ABR
External routes are summarized on the ASBR
NSSA-External routes can be summarized on the ASBR or ABR