Top tier providers "peer" with each other. They share routes via BPG with their routing "peers". When you know an AS only is authoritative for XYZ, you can ACL them to only be authoritative for XYZ.

http://www.dasblinkenlichten.com/bgp-route-filtering-access-... http://blog.ine.com/2008/01/08/using-extended-acls-for-bgp-f...