A Simple Way to Manually Sumamrize BGP Routes

First the router will be configured with a static route, typically with destination Null 0. Then prefix/length can be matched with a network command to inject the summary. Note that this will not filter any component subnets but for simple route sumarization can be achieved.

Let's see the routing table and the BGP config of the aggregating (summarizing) router..

network 11.0.0.0 mask 255.255.0.0 is inserted into the BGP configuration.

RIB of the aggregating router shows the summary route. But you can see the component routes are not suppressed. So You will have to remove advertising commands of component routes in R1. But to do so, you will have to make sure they are reachable from the aggregating router. In the above case, it is not a problem because the routes are originated from this router..