OSPF Summarization

OSPF summarization is pretty easy to implement. It can only be done between areas meaning we can only summarize at the ABR and ASBR where routes are being shared into another area. Due to the area database needing to be consistent we can only place summarization at these positions in the network as opposed to EIGRP where you may summarize at any position you wish. 

With the following topology, summarization will be implemented at R2 and R3. R2 is treated as an ABR and R3 is treated as an ASBR.

On R2, the following loopbacks have been implemented and advertised into A0:

R2(config)int lo0
R2(config-if)ip add 192.168.0.1 255.255.255.0
R2(config-if)ip ospf 1 a 2

R2(config)int lo1
R2(config-if)ip add 192.168.1.1 255.255.255.0
R2(config-if)ip ospf 1 a 2

R2(config)int lo2
R2(config-if)ip add 192.168.2.1 255.255.255.0
R2(config-if)ip ospf 1 a 2

R2(config)int lo3
R2(config-if)ip add 192.168.3.1 255.255.255.0
R2(config-if)ip ospf 1 a 2

R2(config)int lo4
R2(config-if)ip add 192.168.4.1 255.255.255.0
R2(config-if)ip ospf 1 a 2

R2(config)int lo5
R2(config-if)ip add 192.168.5.1 255.255.255.0
R2(config-if)ip ospf 1 a 2

R2(config)int lo6
R2(config-if)ip add 192.168.6.1 255.255.255.0
R2(config-if)ip ospf 1 a 2

R2(config)int lo7
R2(config-if)ip add 192.168.7.1 255.255.255.0
R2(config-if)ip ospf 1 a 2

R2(config)int lo8
R2(config-if)ip add 192.168.8.1 255.255.255.0
R2(config-if)ip ospf 1 a 2

Likewise, on R3, except these are redistributed with the “redistribute connected subnets” command under the OSPF process:

R3(config)#int lo0
R3(config-if)#ip add 33.33.33.1 255.255.255.224

R3(config)#int lo0
R3(config-if)#ip add 33.33.33.33 255.255.255.224

R3(config)#int lo0
R3(config-if)#ip add 33.33.33.65 255.255.255.224

R3(config)#int lo0
R3(config-if)#ip add 33.33.33.97 255.255.255.224

R3(config)#int lo0
R3(config-if)#ip add 33.33.33.129 255.255.255.224

And then we have the following route table on R1 as expected:

On R2, simply executing the command “area 2 range 192.168.0.0 255.255.248.0 cost 2222” under the OSPF process summarizes the range from 192.168.0.0 to 192.168.7.255 with a cost of 2222. The 192.168.8.0 route still appears in the R1 table explicitly because it doesn’t fall within the range we entered on R2:

For R3 to summarize its routes to R1 and R2, the command it uses varies slightly. It too is done so under the OSPF process “summary-address 33.33.33.0 255.255.255.128”

And once again we can see on R1 that the 33.33.33.128 route appears because the range of addresses we specified to be summarized did not encompass this address. Nevertheless, summarization here at the ABR and ASBR was shown to work.