Administrative distance

Administrative distance (AD) is an arbitrary numerical value assigned to dynamic routes, static routes and directly-connected routes. The value is used by vendor-specific routers to rank routes from most preferred to least preferred. When multiple paths to the same destination are available, the router uses the route with the lowest administrative distance and inserts the preferred route into its routing table. Router vendors typically design their routers to assign a default administrative distance to each kind of route that is used, however, this value can usually be set manually by a network administrator.