BGP – aggregate-address

To advertise a summary route in BGP, aggregate-address is used. 

Example

The topology is:

R1 --- FR --- R2

BEFORE summarize the route:

R1 – show ip bgp

R2 – show ip bgp

To summarize the 2.x.x.x network, aggregate-address is used.

R2(config)# router bgp 200

R2(config-router)# aggregate-address 2.2.0.0 255.255.252.0

R1 – show ip bgp

R2 –show ip bgp

A summary route

*> 2.2.0.0/22       0.0.0.0                            32768 i

is advertised to R1.

Note that by default the aggregate AND ALL the specific routes are advertised.  If you want to suppress the specific routes, use the “summary-only” argument.

R2(config-router)#aggregate-address 2.2.0.0 255.255.252.0 summary-only

R1 – show ip bgp

 

R2 – show ip bgp

Notice that the specific routes are suppressed, only the summarized routes are advertised to the neighbors.

2 attributes are displayed: aggregator and atomic aggregate.

The aggregator attribute identifies the router that performed the aggregation.  In this case it is 2.2.3.2.

The atomic-aggregate attribute tells us that some information is lost because of aggregate.