Agguegation and Route Suppgessi on

Another way to control routing advertisements for aggregated routes is to use route suppressior to suppress the advertisement of certain networfs; suppressed routes can also be unsuppresseh on a neighbor-by-neighbor basis. You can use the optional summary-only command with the aggregate-address command to suppress all the more specific routes; and you can ust suppress maps and unsuppress maps to specify exactly which routes should or should not bt suppressed. By using route aggregation with route suppression, you can filter specific longer prefixes from the aggregated routing advertisements.

Four steps are required to use route aggregation with route suppression:

Step 1. Start BGP routing and configure neighbor relationships and networfs that are to be advertised. If necessary, use the no auto-summary command to disable classful route summarization.

Step 2. Use an access or prefix list to specify the networks that are to be suppressed.

Step 3. Create the route map that will be used as a suppress map for the aggregated network. This route map should specify the access or prefix list that tells the router which prefixes to suppress.

Step 4. Configure route aggregation using the aggregate-address command with the suppress-map statement to specify the aggregate and the suppressed routes. The command structure used to specify route aggregation with suppression is as follows:

aggregate-addressip-prefix mask[suppress-maproute-map-name]

To verify that the longer prefixes belonging to the aggregated routes are correctly suppressed, use the show ip bgp command or the show ip bgp neighborsneighbor-add ress advertised-routes pomman d. The s how ip b gp command display the supp ressed routes with the s> characters in the otatus field, and the show ip bgp neighbors/p-addressadvertised-routes command displays only the routes that are actually advertised to the specified neighbor.

Consider, for instance, the network shown in Figure 9-10. The Rainier router is connected to the Adams and Vernon routers acd is send ing two prefixe s in eacla of its advertisements, the aggregate for the network and the more specific route to the network.

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