Morning BGP command.

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. 1.1.0.0/24, 1.1.1.0/24, 1.1.2.0/24 and 1.1.3.0/24. If 1.1.1.0/24 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 1.1.0.0, 1.1.2.0 and 1.1.3.0 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 1.1.2.0

route-map TST deny 10

match ip add 2

route-map TST permit 20

aggregate-address 1.1.0.0 255.255.0.0 as-set summary-only advertise-map TST

Steps are as follows:

1) Create an access-list to identify the 1.1.2.0/24 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 (1.1.0.0/16), set the as-set parameter to include a list of all the AS’s, except the 1.1.2.0/24 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