Networking World

The Administrative Distance (AD) is used to rate the trustworthiness of routing information received on a router from a neighbor router.

An Administrative Distance is an integer from 0 to 255, where 0 is the most trusted and 255 means no traffic will be passed via this route.

If a router receives two updates listing he sane remote network, the first thing the router checks is the AD. If one of the advertised routes has lower AD than the other, then the route with the lowest AD will be placed in the routing table.

If both advertised routes to the same network have the same AD, then routing protocol metrics (such as hop count or bandwidth of the lines) will be used to find the best path to the remote network. The advertised route with the lowest metric will be placed in the routing table.
But if both advertised routes have the same AD as well as the same metrics, then the routing protocol will load-balance in the remote network