Cisco Network Fundamentals // Administrative Distance
Routers need a way of determining which path to use to a destination network if two or more routing protocols are in use and both advertise a route. Administrative distance is Cisco’s answer. Cisco has assigned an administrative distance (AD) to each routing protocol that outlines which protocol a router will prefer. The AD values can be between 0 and 255 with the lowest values being used for routing.
Default AD values
For example, if router R1 receives a route to network 10.10.200.0 from both EIGRP and OSPF, the router will compare the administrative distance of the EIGRP-learned route (90), to that of OSPF (110). The router will then add EIGRP’s route to the routing table because its AD is lower (90 < 110).