8.7.4 Suppressing advertisement of summarization source routes

After you summarize a route, it is possible to advertise the summarized route but exclude its source routes from being advertised. For example, you can advertise RIP routes that have not been summarized while excluding those that have.

You can suppress the advertisement of source routes on an individual basis or for all summarized routes. To suppress advertisement for an individual summarized route, specify the summary-only parameter when you execute the ip summary-address configuration command. The following figure shows an example in which advertisement is suppressed for a summarized route.

Figure 8-16: Example of suppressing advertisement of summarization source routes

Switch A receives advertisements and learns routes for packets destined for the address range 172.16.1.0/24 to 172.16.20.0/24 from Router 1 and 172.17.1.0/24 from Router 2, and the address range 172.16.21.0/24 to 172.16.40.0/24 from Router 3. Switch A configures the advertised route filter to advertise the summarized route 172.16.0.0/16 and learned route 172.17.1.0/24 to Router 4. At this time, if you specified the summary-only parameter when you configured the switch to generate the summarized route 172.16.0.0/16 from learned routes, you do not need to configure the advertised route filter to prevent the switch from advertising the summary source routes. The following figure shows an example configuration for route summarization, and the entries in the routing table before and after summarization.

Figure 8-17: Example configuration of route summarization and routes before and after route summarization