When you summarize routes in RIP, IGRP, EIGRP, or OSPF, you're replacing a series of routes with a summary route and mask. With RIP, IGRP, and EIGRP, this actually lessens the size of the routing update packet itself – multiple routes are replaced with the summary route. For instance, the routes 18.104.22.168/8, 22.214.171.124/8, 10.0.0.0/8, and 126.96.36.199/8 can be summarized as 188.8.131.52 252.0.0.0. Only the summary address will be found in the update packet, making it concise yet complete.
Summarizing routes can also make the routing table smaller, yet still allow for complete IP connectivity when done correctly. Using the above example, the four more-specific routes will be replaced by a single summary route. Since the entire routing table is parsed before the routing process is complete, keeping the routing table as small as possible does help speed the routing process as a whole.
Here are some additional tips on route summarization.
With RIP version 2 and EIGRP, manual route summarization is configured on the interface that will be advertising the summary. This is done with the route summarization command "ip summary-address."
RIP version 2 and EIGRP also both perform autosummarization on routes that are advertised across classful network boundaries. This is disabled with the protocol-level command "no auto-summary".
OSPF offers two different route summarization commands. To summarize routes from one OSPF area to another, use the "area range" command to summarize routes learned via redistribution, use the "summary-address" command on the ASBR.