The summarization in BGP can be done with the aggregate command. However, this aggregate command when enabled the router will automatically send this aggregated updated to all of its neighbor by default. Sometimes, we do not want to advertise the aggregated route to all of the neighbor. We use the unsuppressed-map command to help us accomplish this.

The example below show the command and how to configure aggregate update and unsuppressed-map.

Example

We have R1 trying to advertise the summary route of 172.16.0.0/16 network to R2 and advertise the full updates with the subnets to R3. With the aggregate command alone R3 will only receive an aggregated update. Therefore, we will add the unsuppressed-map command as an option in the neighbor statement to R3. In this example, R3 wants to receive all subnets from R1, so the route-map UNSUPPRESS will not contain any match statement. We can modify the unsuppressed subnet in the router-map command to match the specify subnets we want to see in R3.

router bgp 100
 no synchronization
 bgp log-neighbor-changes
 network 172.16.1.0 mask 255.255.255.0
 network 172.16.2.0 mask 255.255.255.0
 network 172.16.3.0 mask 255.255.255.0
 network 172.16.4.0 mask 255.255.255.0
 aggregate-address 172.16.0.0 255.255.0.0 summary-only
 neighbor 192.168.1.2 remote-as 100
 neighbor 192.168.1.2 unsuppress-map UNSUPPRESS
 neighbor 192.168.2.2 remote-as 100
 no auto-summary

Verify the configuration 

R2>sh ip bgp

BGP table version is 10, local router ID is 192.168.2.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*>i172.16.0.0       192.168.2.1              0    100      0 i

R2>

R3#sh ip bgp

BGP table version is 14, local router ID is 192.168.1.2

Status codes: s suppressed, d damped, h history, * valid, > best, i - internal,

              r RIB-failure, S Stale

Origin codes: i - IGP, e - EGP, ? - incomplete

   Network          Next Hop            Metric LocPrf Weight Path

*>i172.16.0.0       192.168.1.1              0    100      0 i

*>i172.16.1.0/24    192.168.1.1              0    100      0 i

*>i172.16.2.0/24    192.168.1.1              0    100      0 i

*>i172.16.3.0/24    192.168.1.1              0    100      0 i

*>i172.16.4.0/24    192.168.1.1              0    100      0 i

R3#

Now, we can see that R3 has bgp updates for all subnets from R1 as opposed to R2 has only summary routers advertised from R1.