CCNA 3.3.a: Admin distance

Overview:

Admin distance is what is used to select the best path when a router has two different paths to the same destination via two different routing protocols.

Study Notes:

• Most routing protocols are not compatible with other protocols.
• In a network with multiple routing protocols, being able to select the best path between multiple protocols is critical.
• Admin 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 admin distance value.
• IPv6 uses the same distances as IPv4.
• Admin 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.
• Admin distance is a measure of the trustworthiness of the source of the routing information.
• Admin distance has only local significance, and is not advertised in routing updates.
• The smaller the admin 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

Default Admin Distances:

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

Links:

https://www.cisco.com/c/en/us/support/docs/ip/border-gateway-protocol-bgp/15986-admin-distance.html