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.

Now let’s check out the RIB on R4:

R4#sh ip route ospf
     1.0.0.0/32 is subnetted, 1 subnets
O E2    1.1.1.1 [110/20] via 10.1.34.3, 00:01:26, Serial0/0
     2.0.0.0/32 is subnetted, 1 subnets
O E2    2.2.2.2 [110/20] via 10.1.34.3, 00:01:26, Serial0/0
     3.0.0.0/32 is subnetted, 1 subnets
O E2    3.3.3.3 [110/20] via 10.1.34.3, 00:01:26, Serial0/0
     172.30.0.0/24 is subnetted, 4 subnets
O E2    172.30.6.0 [110/20] via 10.1.34.3, 00:01:26, Serial0/0
O E2    172.30.7.0 [110/20] via 10.1.34.3, 00:01:26, Serial0/0
O E2    172.30.5.0 [110/20] via 10.1.34.3, 00:01:26, Serial0/0
O E2    172.30.8.0 [110/20] via 10.1.34.3, 00:01:26, Serial0/0
O IA 192.168.4.0/24 [110/75] via 10.1.34.3, 00:01:26, Serial0/0
     10.0.0.0/24 is subnetted, 2 subnets
O IA    10.1.123.0 [110/74] via 10.1.34.3, 00:01:26, Serial0/0
O IA 192.168.1.0/24 [110/75] via 10.1.34.3, 00:01:26, Serial0/0
O IA 192.168.2.0/24 [110/75] via 10.1.34.3, 00:01:26, Serial0/0
O IA 192.168.3.0/24 [110/75] via 10.1.34.3, 00:01:26, Serial0/0

Here we see the loopbacks from R1 as Inter-Area, and the loopbacks from R2 as External Type 2.

Now we’ll configure the “area range” command to summarize R1′s loopbacks on R3 (ABR):

R3(config)#router ospf 100
R3(config-router)#area 0 range 192.168.0.0 255.255.248.0

Seems almost too easy. We use “area 0 range [IP] [Summary Mask]“.

Let’s verify on R4:

R4#sh ip route ospf
     1.0.0.0/32 is subnetted, 1 subnets
O E2    1.1.1.1 [110/20] via 10.1.34.3, 00:04:51, Serial0/0
     2.0.0.0/32 is subnetted, 1 subnets
O E2    2.2.2.2 [110/20] via 10.1.34.3, 00:04:51, Serial0/0
     3.0.0.0/32 is subnetted, 1 subnets
O E2    3.3.3.3 [110/20] via 10.1.34.3, 00:04:51, Serial0/0
     172.30.0.0/24 is subnetted, 4 subnets
O E2    172.30.6.0 [110/20] via 10.1.34.3, 00:04:51, Serial0/0
O E2    172.30.7.0 [110/20] via 10.1.34.3, 00:04:51, Serial0/0
O E2    172.30.5.0 [110/20] via 10.1.34.3, 00:04:51, Serial0/0
O E2    172.30.8.0 [110/20] via 10.1.34.3, 00:04:51, Serial0/0
     10.0.0.0/24 is subnetted, 2 subnets
O IA    10.1.123.0 [110/74] via 10.1.34.3, 00:04:51, Serial0/0
O IA 192.168.0.0/21 [110/75] via 10.1.34.3, 00:00:13, Serial0/0

It worked! We shrunk all those loopbacks from R1 into a single summary route.

Now we’ll summarize on R2 (ASBR) using the “summary-address” command.

R2(config)#router ospf 100
R2(config-router)#summary-address 172.30.0.0 255.255.240.0

Again, pretty easy stuff, we used the “summary-address [IP] [Summary Mask]” command on R2 (ASBR) to summarize its loopbacks.

Let’s look at R4′s RIB now:

R4#sh ip route ospf
     1.0.0.0/32 is subnetted, 1 subnets
O E2    1.1.1.1 [110/20] via 10.1.34.3, 00:37:55, Serial0/0
     2.0.0.0/32 is subnetted, 1 subnets
O E2    2.2.2.2 [110/20] via 10.1.34.3, 00:02:15, Serial0/0
     3.0.0.0/32 is subnetted, 1 subnets
O E2    3.3.3.3 [110/20] via 10.1.34.3, 00:04:01, Serial0/0
     172.30.0.0/20 is subnetted, 1 subnets
O E2    172.30.0.0 [110/20] via 10.1.34.3, 00:02:15, Serial0/0
     10.0.0.0/24 is subnetted, 2 subnets
O IA    10.1.123.0 [110/74] via 10.1.34.3, 00:37:55, Serial0/0
O IA 192.168.0.0/21 [110/75] via 10.1.34.3, 00:33:17, Serial0/0

It worked this time too. We see two summaries now, one Inter-Area summary for R1′s loopbacks, which we summarized on R3 (ABR) and also a External Type 2 summary for R2′s loopbacks which we configured on R2 (ASBR) itself.

Something to note before I end this one, when we create summary routes the router will install a “discard route” to null locally. This helps prevent routing loops. It will not interfere with the networks we summarize for as they are longer matches. Here are the two examples:

R2#sh ip route | i Null
O       172.30.0.0/20 is a summary, 00:09:02, Null0
 
R3#sh ip route | i Null
O    192.168.0.0/21 is a summary, 00:39:27, Null0

That’s OSPF Summarization in a nutshell. There are some other tricks you can use when summarizing, I may go into them in another post. Or you guys could talk about them in the comments. :)

Colby

Colby Glass has been in IT since 2002. He is currently a network engineer with a Cisco Gold partner and holds the CCNP R/S, CCNP DC, CCDP, CCIP, JNCIA-ER and ITILv3: Foundations certifications.

More Posts