from the BGP FAQ:
http://www.cisco.com/warp/public/459/bgpfaq_5816.shtml#5
Q. How does BGP behave differently with auto-summary enabled or disabled?
A. When auto-summary is enabled, it summarizes the locally originated BGP networks to their classfull boundaries. (Auto-summary is enabled by default in BGP). When auto-summary is disabled, the routes introduced locally into the BGP table are not summarized to their classfull boundaries. When a subnet exists in the routing table and the following three conditions are satisfied, then any subnet of that classfull network in the local routing table will prompt BGP to install the classfull network into the BGP table.
Classfull network statement for a network in the routing table
Classfull mask on that network statement
Auto-summary enabled
For example, if the subnet in the routing table is 75.75.75.0 mask 255.255.255.0, and we configure network 75.0.0.0 under the router bgp command, and auto-summary is enabled, BGP will introduce the classfull network 75.0.0.0 mask 255.0.0.0 in the BGP table.
If these three conditions are not all met, then BGP will not install any entry in the BGP table unless there is an exact match in the local routing table.
Note: If the AS doing BGP does not own the complete classfull network, we suggest disabling auto-summary using the no auto-summary command under router bgp.
