BGP uses the AS_PATH attribute for loop detection. If a router sees its own AS number in a BGP advertisement, the advertisement is dropped. IBGP routers have the same AS number so the AS number cannot be used for loop detection. IBGP neighbors will not advertise prefixes learned from one IBGP neighbor to another IBGP neighbor; therefore, a full mesh is required.
When a summary address is created with an IGP (EIGRP, OSPF, and IS-IS), the specific routes of the summary are not advertised. BGP advertises the summary, and all the specific routes of the summary unless they are specifically suppressed.
If a router has more than one route to the same IP prefix, the best path is the one with the highest LOCAL_PREF (assuming the WEIGHT attribute for the routes is equal).
The LOCAL_PREF attribute is advertised throughout the autonomous system.
MED is used to prefer a path into an autonomous system. A lower MED value is preferred.