In OSPF, summarization can only be configured on ABRs or ASBRs. Instead of advertising many specific networks, the ABR routers and ASBR routers advertise a summary route. ABR routers summarize type 3 LSAs and ASBR routers summarize type 5 LSAs.

By default, summary LSAs (type 3 LSAs) and external LSAs (type 5 LSAs) do not contain summarized (aggregated) routes; that is, by default, summary LSAs are not summarized.

As illustrated in Figures 1 and 2, route summarization can be configured as follows:

Note: External route summarization is configured on ASBRs using the summary-address address mask router configuration mode command.