How a Router Selects the Best Path

It is important to understand how a router selects the best path to a destination.

1. When there are multiple routes to a destination, a route that has the "longest prefix" length is chosen.
   E: 192.168.1.0 /27 via 10.1.1.1, serial1 <<<< this route will win
   O: 192.168.1.0 /24 via 10.1.1.2, serial0
2. When there are multiple routes to a destination and the prefix is equal between the routes, then the lowest administrative distance will be chosen.
3. When there are multiple routes to a destination that have both the same prefix length and the same administrative distance, then the route with the lowest metric will be chosen.
4. If the multiple routes have the same prefix length, same administrative distance, and the same metric, then the route will be equally load balanced.

You must consider that directly connected networks have a metric of 0 and static routes using the next hop address will have a metric of 1.  Static routes with a exit interface will have a metric of 0.

Here are the Administrative Distances that should be memorized:

• Static routes using exit interface: 0
• Directly connected routes: 0
• Static routes with and next hop address: 1
• EIGRP Summary routes: 5
• External BGP: 20
• Internal EIGRP: 90
• IGRP: 100
• OSPF: 110
• External EIGRP: 170
• Internal BGP: 200
• Unknown networks 255