5.3 Using and Configuring OSPF Multi-Area Component
5.3.2 Using route summarization
Summarization is the consolidation of multiple routes into one single advertisement. Proper summarization requires contiguous (sequential) addressing, such as 200.10.10.0, 200.10.11.0, 200.10.12.0, and so on. Route summarization is different than an LSA summary route.
Lab Activity
  In this lab, you will learn how to use summarization with OSPF to reduce the number of external routes in the routing table.

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, the topology change will not be propagated into the backbone (and other areas by way of the backbone). As such, flooding outside the area will not occur, so routers outside the area with the topology change will not have to run the SPF algorithm (comment – this is explained in the previous chapter. Running the SPF algorithm is a CPU-intensive activity.

Following are the two types of summarization:

  • Inter-area route summarization --- Inter-area route summarization is done on ABRs and applies to routes from within each area. It does not apply to external routes injected into OSPF via redistribution. In order to take advantage of summarization, network numbers within areas should be assigned in a contiguous way so as to be able to consolidate these addresses into one range. The main figure illustrates where inter-area summarization occurs.
  • External route summarization --- External route summarization is specific to external routes that are injected into OSPF via redistribution. Here again, it is important to ensure that external address ranges that are being summarized are contiguous. Summarization overlapping ranges from two different routers could cause packets to be sent to the wrong destination. Only ASBRs can summarize external routes. These types of routes cannot be summarized by any other router type than an ASBR.

In order to take advantage of summarization, network numbers in areas should be assigned in a contiguous way to be able to group these addresses into one range.

In the main Figure, for example, the list of six networks in router B's routing table can be summarized into two summary address advertisements.