What is Administrative Distance

What is Administrative Distance

The router believes a route based on Administrative Distance. Every route that is connected to the router (either directly connected or static route or a dynamic route) is assigned with an arbitrary number.

The number is called as an administrative distance of that network. If there are multiple paths to reach the destination network, router will not add all the possible paths to the routing table. There is a possibility of running multiple protocols like OSPF, RIP can be configured on a router.

 Each protocol advertises administrative distance to other routers. The router uses the values of administrative distance advertised by the processes or protocols to know the best path that is available to the destination network.  This value is advertised by the protocols to the local routers and routers will not advertise the same over the network.

Lower the administrative distance higher the preference given to select the route. 

An administrator can reconfigure the administrative distances and hence change the best path that is available. If there are multiple paths with same administrative distances, the router uses algorithm specified by the vendor to select the best path to reach the destination network.

The administrative distance of various routing protocols by default is as discussed. A directly connected interface has an Administrative Distance of 0. The static route has Administrative Distance as 1.The Administrative Distance of Enhanced Interior Gateway Protocol summary route is 5.

The Administrative Distance of External Border Gateway Protocol (BGP) is 20. The internal EIGRP has an Administrative Distance as 90 whereas IGRP’s Administrative Distance is 100. Open Shortest Path first (OSPF) by default has its Administrative Distance as 110. The Administrative Distance of Intermediate System to Intermediate System (IS-IS) is 115.

The legacy protocol Router Information Protocol (RIP) has an Administrative Distance of 120. The AD of EGP is 140. On Demand Routing, External EIGRP, Internal BGP has their Administrative Distance as 160, 170, and 200 respectively. If the connection has no source that is unknown, it has Administrative Distance as 255

The static rote has Administrative Distance as 1 whereas all the dynamic routing protocols have their Administrative Distance greater than 1.

Hence the static route is given more preference than the dynamic route because the Administrative Distance of the static route is less than that of dynamic routes. The router does not believe the route with Administrative Distance 255 and will not enter the route in the routing table.

To manually change the Administrative Distance of RIP, use the following commands

CISCO-ROUTER> en

CISCO-ROUTER# configure terminal

CISCO-ROUTER(config)# router rip

CISCO-ROUTER(config-router)# distance 7

The above commands change the administrative distance of RIP from 120 to 7.

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