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Routing - Administrative Distance


Most routing protocols have metric structure that are not comparable with each other, in case multiple protocols are being used in a network. For eg. RIP uses hop count and OSPF uses cost for best path selection to a particular route. In order to have a common parameter which can be used for computing the best path, the concept of Administrative Distance (AD) was introduced. AD is simply a number which helps identity a particular route learned from a particular protocol. For eg, if the route is learned from Routing Information Protocol (RIP), then it would have a default AD value of 120 (unless it has been explicitly modified). Similary, a route learned from OSPF would have the AD value of 110.





Rule: A route with lower Administrative Distance (AD) value would always be preferred over the one with higher AD value.





In the above scenario, if the route is being learned from both RIP as well as OSPF, then the route learned via OSPF would be preferred since OSPF has a lower AD value of 110 than RIP (which has an AD value of 120)





The list of protocols with their administrative distances can be found here.


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