Example 3-49 OSPF Routes in R1’s Routing Table R1# show ip route ospf Specific-link LSA flooding outside the area does not occur.įigure 3-25 OSPF Route Summarization TopologyĮxample 3-49 displays OSPF routes in R1’s routing table. Also, if a network link fails, the topology change is not propagated into the backbone (and other areas by way of the backbone). Summarization prevents every router from having to rerun the SPF algorithm, increases the stability of the network, and reduces unnecessary LSA flooding. With route summarization, only the summarized routes are propagated into the backbone (area 0), as illustrated in Figure 3-24. Without route summarization, every specific-link LSA is propagated into the OSPF backbone and beyond, causing unnecessary network traffic and router overhead. Route summarization directly affects the amount of bandwidth, CPU power, and memory resources that the OSPF routing process consumes. Route summarization requires a good addressing plan-an assignment of subnets and addresses that is based on the OSPF area structure and lends itself to aggregation at the OSPF area borders. This is one of the drawbacks of summarization. If the OSPF design includes many ABRs or ASBRs, suboptimal routing is possible. Instead of advertising many specific prefixes, advertise only one summary prefix. ABRs summarize type 3 LSAs, and ASBRs summarize type 5 LSAs. With route summarization, the ABRs or ASBRs consolidate multiple routes into a single advertisement. Normally, type 1 and type 2 LSAs are generated inside each area and translated into type 3 LSAs in other areas. To reduce the size of the area database, you can configure summarization on an area boundary or autonomous system boundary.
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