Preventing Routing Domain Loops with Administrative Distance
Each router associates an administrative distance (AD) with every route it considers to be added to the routing table. When a router must consider multiple routes from different sources for the exact same prefix/length, the first item considered by the router is not the metric, but rather the AD. The lower the AD, the better the route.
Note that the AD is a local setting on a router and cannot be advertised to neighboring routers.
Each routing source has a default AD according to Cisco IOS. In some cases, a given routing source has different defaults for different types of routes inside that routing source. For example, EIGRP has different AD values for EIGRP internal routes (AD 90) ...