CCNP ROUTE - Administrative Distance (AD)

In some cases, usually when we are talking for WAN topology, there is a huge chance to have more than one path from the source to destination. Router has to decide which path is the best, that will be shown in its routing table. To make this decision a router goes through a four-step process. First, the route with the longest prefix length takes the place of the best route. Second, if both routes have the same prefix length, then the router with the lowest administrative distance is used. Third, if the previous two steps are exactly the same, then the route with the lowest metric becomes the best route. Fourth, if again, all the previous steps are exactly the same, then all of these routes will be used in load balancing based on which protocol is used. 

Administrative Distance Metric
Directly Connected--- 0
Static Route--- 1
EIGRP Summary--- 5
Internal EIGRP--- 90
OSPF--- 110
RIP--- 120
External EIGRP--- 170
Internal BGP--- 200