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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:
To configure an ABR to summarize routes for a specific area before injecting them into a different area, use the following syntax:
To configure an ASBR to summarize external routes before injecting them into the OSPF domain, use the following syntax:
Use the following commands to configure RTA for external router summarization as shown in the Figure.
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:
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.
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