Cisco Tips & Tricks

August 27, 2007

Routing Basics

Filed under: IP Routing — ciscotips @ 3:43 am

One of my team member was asking me the question on routing decision. I thought of putting up here ..Any routing decision is based on the following rules.

1) Longest prefix length :- If we have two different routes in a routing table  for a same network, the longest prefix match is selected. lets say we have both 172.16.0.0/16 & 172.16.16.0/24 in the routing table.  In this case 172.16.16.0/24 will be selected.

2) AD ( Administrative distance) :- If longest prefix match cannot be considered for example if we have two routes with same prefix length then Route with lowest AD will be considered. Incase we have a same route learnt from IGRP and OSPF, IGRP route will be considered as it has alower AD(100) then OSPF (110).

3)Meteric:- If routes have same prefix length and same AD then a route with a better meteric is considered. In OSPF, cost~bandwidth is a meteric where as hop count is meteric for RIP.

4) Load sharing:- In case all the above three conditions match then  data  will be load balanced between the routes.

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