Purpose of Administrative Distance

Administrative distance (AD) defines the preference of a routing source. Each routing source - including specific routing protocols, static routes, and even directly connected networks - is prioritized in order of most- to least-preferable using an administrative distance value. Cisco routers use the AD feature to select the best path when it learns about the same destination network from two or more different routing sources.

Administrative distance is an integer value from 0 to 255. The lower the value the more preferred the route source. An administrative distance of 0 is the most preferred. Only a directly connected network has an administrative distance of 0, which cannot be changed.

It is possible to modify the administrative distance for static routes and dynamic routing protocols. This is discussed in CCNP.

An administrative distance of 255 means the router will not believe the source of that route and it will not be installed in the routing table.

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