Administrative distance is the router feature that selects the best path when there are more than one different routes to the same destination with different routing protocols and static routes.
If the routing table has more than one route source for the same destination network. For example, if both Enhance Interior Routing Gateway Protocol (EIGRP); and Routing Information Protocol (RIP) are configured on a router for the same destination network.
So, both routing protocol may decide a different path to the destination based on that routing protocol’s metrics because RIP selects a path based on hop count, and EIGRP selects a path based on its composite metric. So, the administrator distance is the value which tells the router which path to use first.
Administrative Distance (AD) counts the reliability and trustworthiness of the route. It is a numeric value which can range from 0 to 255. A smaller AD value is more reliable and trustworthiness, therefore the best AD value is 0 and the worst is 255. The static route AD is 1, whereas the AD of EIGRP is 110; So the static route is more reliable and trustworthiness.
When there are both static and EIGRP routes to the same destination; the router chooses the static route because it is the route with the lowest AD value.
If there EIGRP and OSPF are configured to the same destination, the router will choose EIGRP; because the AD of EIGRP is 90 and the AD of OSPF is 110; So the router will choose the route with the lowest value which is EIGRP.
Default Administrative Distance Values
The figure below illustrates the default values of the router administrative distance. We can change and modify the administrative distance of a routing protocol through the distance command in the routing sub configuration mode. However, a modification in the AD value can show the way to routing loops and black holes. So, use caution if you change the administrative distance.