BGP aggregate-address command

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When the aggregate-address command is used within BGP routing, the aggregated address is advertised, along with the more specific routes. The exception to this rule is through the use of the summary-only command. The “summary-only” keyword suppresses the more specific routes and announces only the summarized route.
Using the as-set argument creates an aggregate address with a mathematical set of autonomous systems (AS). This as-set summarizes the AS_PATH attributes of the all of the individual routes. This can be useful to avoid routing loops while aggregating routes. Again, unless the “summary-only” keyword is used with the as-set command the summary route is advertised along with the more specific routes.