During BGP configuration on a router that has peered with other BGP speakers, the BGP
command aggregate-address 172.32.0.0 255.255.252.0 is issued. However, the peers do
not receive this aggregate network in BGP advertisements. Also, the router does not have
this aggregate network in its BGP table. Which option indicates a possible reason this
command did not cause the router to advertise the aggregate network to its peers?
A.
Interface NULL 0 is likely shutdown.
B.
The next hop IP address must be a loopback address.
C.
The BGP command no auto-summary is missing.
D.
The BGP command no synchronization is missing.
E.
Subnets of 172.32.0.0/22 do not exist in the BGP table.
F.
The IGP running on this router does not have network 172.32.0.0/22 installed.
Explanation:
BGP allows the aggregation of specific routes into one route using the “aggregate-address
address mask” command. Aggregation applies to routes that exist in the BGP routing table.
This is in contrast to the network command, which applies to the routes that exists in IP
routing table. Aggregation can be performed if at least one or more of the specific routes of
the aggregate address exists in the BGP routing table.
http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800945ff.shtml