Administrative distance

Administrative distance is the first criterion that a router uses to determine which routing protocol to use if two protocols provide route information for the same destination. Administrative distance is a measure of the trustworthiness of the source of the routing information. Administrative distance has only local significance, and is not advertised in routing updates. Note: The smaller the administrative distance value, the more reliable the protocol. For example, if a router receives a route to a certain network from both Open Shortest Path First (OSPF) (default administrative distance - 110) and Interior Gateway Routing Protocol (IGRP) (default administrative distance - 100), the router chooses IGRP because IGRP is more reliable. This means the router adds the IGRP version of the route to the routing table. If you lose the source of the IGRP-derived information (for example, due to a power shutdown), the software uses the OSPF-derived information until the IGRP-derived information reappears. Default Distance Value Table This table lists the administrative distance default values of the protocols that Cisco supports:
Route Source

* If the administrative distance is 255, the router does not believe the source of that route and does not install the route in the routing table.