BGP - auto-summary feature
Auto-summary is a feature of BGP that changes the way the
network command functions under the BGP router configuration.
With auto-summary enabled, you can advertise a classful network and you don’t have to add the mask parameter. BGP will automatically advertise the classful network if you have the classful network or a subnet of this network in your routing table.
auto-summary feature is disabled by default on all Cisco IOS devices