I have a few minutes this morning that i want to use to clarify a special BGP feature which i had misunderstood until a few days ago.
This has to do with the aggregate-address that you use to create a summary address. One of its many options includes the “advertise-map” parameter.
Previously i was under the impression that it functioned in the same manner as the advertise-map thats used when selecting what routes to advertise based on conditions (exist and non-exist maps). Thats incorrect.
What the advertise-map in the aggregate-address is used for is actually to select what attributes the aggregate-address will include. The routes specified in the advertise-map and their associated attributes (community values) will be carried along in the aggregate.
So as an example, lets say you have 4 routes that will be covered by your aggregate-address. 22.214.171.124/24, 126.96.36.199/24, 188.8.131.52/24 and 184.108.40.206/24. If 220.127.116.11/24 had the community value of no-export, the entire aggregate-address would include the no-export community value. You can then create an aggregate-address with an advertise-map that only selects the 18.104.22.168, 22.214.171.124 and 126.96.36.199 routes and use their attributes in the aggregate.
So a configuration example of an advertise map to perform this would be:
access-list 2 permit 188.8.131.52
route-map TST deny 10
match ip add 2
route-map TST permit 20
aggregate-address 184.108.40.206 255.255.0.0 as-set summary-only advertise-map TST
Steps are as follows:
1) Create an access-list to identify the 220.127.116.11/24 network.
2) Create a route-map to explicitly deny this route’s attributes.
3) Create a catch-all in the route-map to permit all other route’s attributes.
4) Create the aggregate-address (18.104.22.168/16), set the as-set parameter to include a list of all the AS’s, except the 22.214.171.124/24 network, only send a summary and not the individidual routes as well, and then use our advertise-map to implement the logic.
I hope this has clarified this detail a bit.
Update: There is a good post about the same thing here: http://amplebrain.blogspot.com/2010/02/advertise-maps-in-bgp.html