3.3.d Implement, optimize and troubleshoot administrative distance

Most routing protocols have metric structures and algorithms that are not compatible with other protocols. In a network with multiple routing protocols, the exchange of route information and the capability to select the best path across the multiple protocols are critical.

Administrative distance is the feature that routers use in order to select the best path when there are two or more different routes to the same destination from two different routing protocols. Administrative distance defines the reliability of a routing protocol . Each routing protocol is prioritized in order of most to least reliable (believable) with the help of an administrative distance value.

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 . The smaller the administrative distance value, the more reliable the protocol. For example, if a Cisco 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.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 2677-2683).  . Kindle Edition.