Lets try to recap BGP aggregation
Its one of the method for advertising a network into BGP domain, others are via network statement, redistribution and inject map
An aggregate prefix is only advertise when at least one subset prefix of aggregate prefix is present in the BGP table (NOT ROUTING TABLE)
key points to remember in BGP aggregation
1. Without summary-only command, both aggregate and specific prefixes are advertise
2. "as-set" key word adds an unodered ASes list and addive community attribute from original prefixes to the new aggregated prefix as attribute.
3. "attribute-map" key word assigns attributes to the newly generated aggregated prefix. for example if aggregated prefix obtained its attribute "no-export" community from its one of the original prefix then using advertise-map this attribute can be reset as "none" community for the aggregated prefix or any attribute can be set using attribute-map.
4. "advertise-map" allows filtering of attributes which are passing to the aggregated prefix from original prefixes based on removal of some specific prefix . for example deny specific prefix in a route map so that its attribute will not contribute to the aggregated prefix. Though aggregate is still composed of all the original prefixes. "REMOVE THIS PREFIX SO THAT ITS ATTRIBUTE IS NOT ADDED IN THE AGGREGATED PREFIX ATTRIBUTE"
