Network Sales and Services Handbook (Cisco Press Networking Technology)

Route summarization is the consolidation of multiple routes in a single route advertisement. Route summarization is normally performed at the area boundaries by the ABRs (Area Border Routers). It is recommended that you summarize directly into the backbone (Area 0), although summarization may be configured between any two areas. By summarizing routes directly into the backbone, the backbone will then turn around and inject these routes into other areas as part of the normal link-state advertisement.

There are two types of summarization:

• Inter-area route summarization Inter-area route summarization is done on ABRs and it applies to routes from within the AS. It does not apply to external routes injected into OSPF via redistribution.

• External route summarization External route summarization is specific to external routes that are injected into OSPF via redistribution. It is imperative to ensure that external address ranges being summarized are contiguous. The summarization of overlapping ranges from two different routers could cause packets to be sent to the wrong destination. It is also imperative to ensure that all subnets being summarized are in use within the network; otherwise, routing "black holes" could be created, leading to dropped traffic.