BGP - aggregate-address default route
When configuring BGP, the aggregate-address feature can be used to create an aggregate entry within the BGP table. However, it should not be used to create an aggregate address corresponding to the 0.0.0.0 0.0.0.0 summary.
In other words, the following command should not be used in the BGP router configuration mode:
Router(config-router)aggregate-address 0.0.0.0 0.0.0.0
If this command is issued, the following error statement will be displayed
% Aggregating to create default makes no sense, use a network statement instead.
By attempting to issue this command, you’re telling BGP to consolidate ALL ROUTES into a single aggregate route. This is essentially a default route FOR EVERYTHING, and in the context of aggregation that doesn’t make sense. Taking aggregation to such extremes defeats the purpose of the feature.
If you want to create a default route, it is preferable to use a network statement in the BGP router configuration mode. This is much more efficient (aggregation requires more system resources to perform) and simpler as well.
Links:
https://forum.networklessons.com/t/bgp-aggregate-as-set/3248/29?u=lagapides
https://networklessons.com/cisco/ccie-routing-switching-written/bgp-aggregate-as-set/