This post is about OSPF Summarization. We’ll be using a familiar topology and going over two ways to summarize with OSPF.

There are two conventional ways to summarize networks in OSPF, we can use the “area range” command and the “summary-address” command. “Area range” is used on the ABR to summarize networks between areas. The “summary-address” command is used on the ASBR to summarize external networks.

Here’s the topology:

I’m not going through the basic OSPF config, so assume everything is configured as the diagram suggests. On R1 I’ve added Lo11-14 and used “ospf 100 area 0″ under the respective interfaces. On R2 I’ve added Lo15-18 and used “redistribute connected subnets”. Let’s look at the RIBs on a couple routers:

First we’ll check out “sh ip route” on R1:

R1#sh ip route
...
     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 E2    2.2.2.2 [110/20] via 10.1.123.2, 01:13:32, FastEthernet0/0
     3.0.0.0/32 is subnetted, 1 subnets
O E2    3.3.3.3 [110/20] via 10.1.123.3, 01:12:38, FastEthernet0/0
     4.0.0.0/32 is subnetted, 1 subnets
O E2    4.4.4.4 [110/20] via 10.1.123.3, 01:03:47, FastEthernet0/0
     172.30.0.0/24 is subnetted, 4 subnets
O E2    172.30.6.0 [110/20] via 10.1.123.2, 00:06:11, FastEthernet0/0
O E2    172.30.7.0 [110/20] via 10.1.123.2, 00:06:11, FastEthernet0/0
O E2    172.30.5.0 [110/20] via 10.1.123.2, 00:06:11, FastEthernet0/0
O E2    172.30.8.0 [110/20] via 10.1.123.2, 00:06:11, FastEthernet0/0
C    192.168.4.0/24 is directly connected, Loopback14
     10.0.0.0/24 is subnetted, 2 subnets
O IA    10.1.34.0 [110/74] via 10.1.123.3, 01:12:39, FastEthernet0/0
C       10.1.123.0 is directly connected, FastEthernet0/0
C    192.168.1.0/24 is directly connected, Loopback11
C    192.168.2.0/24 is directly connected, Loopback12
C    192.168.3.0/24 is directly connected, Loopback13

Lots of routes here. The ones to note are the 172s showing as External Type 2, which are R2′s loopbacks. Also notice our connected loopbacks.