Administrative distance (or AD) is a number that an IP routing device uses as a tie breaker when selecting a best path, to a given destination, when two or more paths exists from two or more different routing protocols or sources. During this comparison, the lowest AD value always prevails.
AD values can range from 0 (most preferred) to 255 (least preferred). Every vendor has its own default AD values assigned to each routing protocol however they can be customized by the network engineer as desired. In essence, AD is an indication of reliability or a measure of route preference of the given routing source.
Below is a comparison of Cisco and Juniper default AD values.
|Routing Source||Cisco AD||Juniper AD|
|Directly connected interfaces||0||0|
|Static route with exit interface||1||5|
|Static route with next-hop IP address||1||5|
|EIGRP summary route||5||n/a|
|IS-IS (L1/L2 Internal)||115/115||15/18|
|IS-IS (L1/L2 External)||115/115||160/165|
Both Cisco and Juniper allow one-line configuration to customize a default AD value for a given routing protocol. In order to override the default AD value, you can simply go to routing process or configuration mode, and then issue “distance <new-AD>” or “set protocols <protocol> group <group-number> preference <new-AD>” on Cisco IOS and Juniper Junos OS devices respectively. Please note that there are other ways to change AD values.
For troubleshooting, it is imperative to know the main reasons behind why AD values are modified by the network engineers in the first place. There are three most common reasons that are worth noting here.