3.7.e Implement and troubleshoot scalability

3.7.e [iii] Aggregation, AS set

Aggregation, AS set

Border Gateway Protocol (BGP) allows the aggregation of specific routes into one route with use of the aggregate-address address mask [as-set] [summary-only] [suppress-map map-name] [advertise-map map-name] [attribute-map map-name] command. When you issue the aggregate-address command without any arguments, there is no inheritance of the individual route attributes (such as AS_PATH or community), which causes a loss of granularity. Use of the as-set argument creates an aggregate address with a mathematical set of autonomous systems (ASs). This as-setargument summarizes the AS_PATH attributes of all the individual routes. These sample configurations enable you to examine this feature and how this argument helps BGP detect and avoid loops.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 4037-4044).  . Kindle Edition.

http://packetlife.net/blog/2008/sep/19/bgp-route-aggregation-part-1/