Explain brief Introduction of Administrative Distance of router with practical example

In this article we learn what is administrative distance , How is impact the routing decision and default Administrative value of routing protocols.

What is Administrative Distance

Administrative distance is the feature of all routers to select the best trusted route when two or more different routes are there to reach the same destination. Administrative Distance number always in between 0 to 255.

Default Administrative Distance value of different routing protocols

For IGRP Administrative Distance value is 100

For EIGRP Administrative Distance value is 90

For RIP Administrative Distance value is 120

For OSPF Administrative Distance value is 110

For ISIS Administrative Distance value is 115

For STATIC ROUTE Administrative Distance value is 1

For DIRECTLY CONNECTED ROUTE Administrative Distance value is 0

Sample diagram to explain Administrative Distance

Explain brief Introduction of Administrative Distance of router with practical example

Suppose route1 wants to reach to router4.To reach the router4 there are 3 possible route.

One is router1=>router0=>router4

Second is router1=>router2=>router3=> router4

And third is router1=>router5=>router6=>router7=>router8=>router4.

First route takes 2 hops to reach router4.Second route takes 3 hops to reach route4 and third route takes 5 hops to reach router4.

If we taken rip protocol we know that rip is calculate its best route using hop counts. Least hop count consider as the best route among the three. Here route one is the best route for the rip protocol.

For ospf and eigrp bset route is calculated using band width. if first route has band width 3mbps,second route has bandwidth 4mbps and third route has band width of 8mbps,then eigrp and ospf takes third route to reach the destination.

When network admin configure static route he/she decides the best route among the three. Suppose here network admin select second route to reach the destination.

Now router have consider best trusted route using the administrative distance. In the above scenario router select second route to reach the destination as static route has administrative distance which is 1.

Explain brief Introduction of Administrative Distance of router with practical example