5.3 Using and Configuring OSPF Multi-Area Component
5.3.3 Configuring route summarization
Lab Activity
  In this lab, you will learn how to configure OSPF to summarize addresses within an area.

Summarization is off by default. To configure route summarization on the ABR, perform the following steps:

  1. Configure OSPF.
  2. Instruct the ABR to summarize routes for a specific area before injecting them into a different area, as follows:
  3. router(config-router)#area area-id range address mask

To configure route summarization on an ASBR to summarize external routes, perform the following steps:

  1. Configure OSPF.
  2. Instruct the ASBR to summarize external routes before injecting them into the OSPF domain, as follows:
  3. router(config-router) #summary-address address mask 

Figure shows that route summarization can occur in both directions.

In Router 2's configuration shown in Figure , the following commands are used:

  • area 0 range 172.16.96.0 255.255.224.0 --- Identifies area 0 as the area containing the range of networks to be summarized into area 1. The ABR R1 is summarizing the range of subnets from 172.16.96.0 to 172.16.127.0 into one range: 172.16.96.0 255.255.224.0. This summarization is achieved by masking the first three left-most bits of subnet 96 using the mask 255.255.224.0.
This summarization was successful because you are summarizing two distinct subnet ranges into the backbone: 32 to 63 and 64 to 95.
  • area 1 range 172.16.32.0 255.255.224.0 --- Identifies area 1 as the area containing the range of networks to be summarized into area 0. The ABR R1 is summarizing the range of subnets from 172.16.32.0 to 172.16.63.0 into one range: 172.16.32.0 255.255.224.0.

The configuration on the right works exactly the same way. Note that, depending on your network topology, you may not want to summarize area 0 networks. If you have more than one ABR between an area and the backbone area, for example, sending a summary LSA with the explicit network information will ensure that the shortest path is selected. If you summarize the addresses, a suboptimal path selection may occur.