Route Summarization using Aggregate Address Command in BGP

BGP manual summarization with the aggregate address command can summarize based on any routes in the BGP table creating a summary of any prefix length. It does not always suppress the component subnets although it can be configured to do so.

Let's see the routing table of R1 which I am going to take as the aggregation point.

BGP configuration of R1;

BGP RIBs of all 3 routers;

Let's add a simple aggregate-address command to BGP configuration of R1;

aggregate-address 11.0.0.0 255.0.0.0

As you can see a new NLRI of 11.0.0.0/8 has been injected to the table along with its component subnets in R1 and it is advertised to both iBGP and eBGP neighbors.

Let's extend the command bit.

aggregate-address 11.0.0.0 255.0.0.0 summary-only

Now only the summary is advertised and in aggregation router, it shows subnets are suppressed.

Now let's configure R4 to advertise 11.40.0.0/24 subnet to BGP.. It will be shown in R1 as the following..

Let's see how it is advertised to R3 (It's iBGP neighbor)

You can see the AS path is only set up to the aggregation point even though one component subnet has a different AS path. In order to change this behavior of hiding the  AS path of component subnets, let's extend the command in R1 a little bit further..

 aggregate-address 11.0.0.0 255.0.0.0 summary-only as-set

Now in R3, AS path is shown with the AS path of the component subnet which has has learned from a different AS path.

Now let's configure R2 to advertise 11.200.0.0/24 subnet to BGP.. It will be shown in R1 as the following..

Let's see the output of R3 RIB now..

As you can see, it is showing ASes in a bracket with all the AS paths of component subnets because there are several different AS paths. It is really an unordered list of all ASes from all the component subnets which helps BGP loop prevention mechanism to function.

As a conclusion; while as-set is configured,
if all the component prefixes have same AS_PATH, it will be advertised
if at-least one component prefix is learned from a different AS, it's AS_PATH will be advertised
if more than one prefixes have different AS_PATHs, all will be advertised in a bracket unordered..