If you want to summarize networks into bgp instead of redistributing a lot of smaller networks, you can install the summarized route as a static route pointing to null0 and redistribute it into bgp.

When the traffic for the network arrives, more specific networks are matched. If none are present, the traffic is dropped.

Null0 is actually an interface which has a drop adjacency associated to it. If any traffic is sent to null0, it should be dropped.

There can also be other uses for null0 to fulfill requirements of a perticular network being present in the routing table but you donot want the traffic to use that entry for forwarding purposes and the entry is meant only for control-plane.

http://www.cisco.com/en/US/tech/tk364/technologies_tech_note09186a00801c9a6e.shtml

This link gives other use of null0.

it can alos be used if you want to summarize a n/w in bgp using a network command.

http://www.cisco.com/en/US/tech/tk365/technologies_tech_note09186a00800c95bb.shtml#networkcommand

Since exact match for the summarized n/w is not present in the igp table of the router u can use the static route pointing to null interface and then advertise through network command.

When traffic actually comes for that route, the more specific routes present in the router take over and are used for the forwarding decision.

Regards,

Niranjan