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| 6.6 |  | Multiarea OSPF Configuration and Verification |
| 6.6.2 |  | Configuring OSPF route summarization |

Recall that summarization is the consolidation of multiple routes into one single, supernet advertisement. See Module 2 for more details on this. Proper summarization requires contiguous, sequential, addressing. 200.10.0.0, 200.10.1.0, 200.10.2.0, and so on are examples of contiguous addressing. OSPF routers can be manually configured to advertise a supernet route, which is different from an LSA summary route.

Route summarization directly affects the amount of bandwidth, CPU, and memory resources that are consumed by the OSPF process. With summarization, if a network link fails or flaps, the topology change will not be propagated into the backbone, and other areas by way of the backbone. As discussed in previous modules, route summarization protects routers from needless routing table recalculations. Because the SPF calculation places a significant demand on a router CPU, proper summarization is an important part of OSPF configuration.

OSPF supports the following two types of summarization:

- Interarea route summarization – Interarea route summarization is done on ABRs and applies to routes from within each area. It does not apply to external routes injected into OSPF by way of redistribution. To take advantage of summarization, network numbers within areas should be contiguous.
- External route summarization – External route summarization is specific to external routes that are injected into OSPF by way of redistribution. Here again, it is important to ensure that external address ranges that are being summarized are contiguous. Summarization of overlapping ranges from two different routers could cause packets to be sent to the wrong destination. Only ASBRs can summarize external routes.

To configure an ABR to summarize routes for a specific area before injecting them into a different area, use the following syntax:

> Router(config-router)#area area-id range address mask

To configure an ASBR to summarize external routes before injecting them into the OSPF domain, use the following syntax:

> Router(config-router)#summary-address address mask

Use the following commands to configure RTA for external router summarization as shown in the Figure.

> RTA(config)#router ospf 1
> RTA(config-router)#summary-address 200.9.0.0 255.255.0.0

Once configured, RTA will send only a single summary route, 200.9.0.0/16, into the OSPF domain.

Because RTB sits on the border between Area 0 and Area 1, it should be configured to perform interarea summarization, shown as follows:

> RTB(config)#router ospf 1
> RTB(config-router)#area 1 range 192.168.16.0 255.255.252.0

Notice that the area 1 range command in this example specifies the area containing the range to be summarized before being injected into Area 0.

Also, depending on the network topology, summarizing Area 0 networks may not be wanted. If there is 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 the addresses are summarized, a suboptimal path selection may occur.

| Lab Activity e-Lab Activity: Configuring OSPF Route Summarization In this lab, configure router RTA for external route summarization and router RTB to perform interarea summarization.