Routing decision criteria

Routers must determine the best route to send traffic on toward itsdestination. This is accomplished as follows (note that the order of operations is critical and fixed):

1. Valid next-hop IP address—when updates are received, the router first verifies that the next-hop IP address to reach the potential destination is valid.

2. Metric—the router then examines the metrics for the various routes that might exist from a particular protocol. For example, if OSPF has several routes to the destination, the router tries to install the route with the best metric (in this case, cost) into the routing table.

3. Administrative distance—if multiple routing protocols are running on the device, and multiple protocols are all presenting routes to the destination with valid next hops, the router examines administrative distance. The route sourced from the lowest administrative distance protocol or mechanism is installed in the routing table.

4. Prefix—the router examines the route’s prefix length. If no exact match exists in the routing table, the route is installed. Note that this might cause the routing table to be filled with the following entries: EIGRP 172.16.2.0/24 and RIP 172.16.2.0/19.

The subject of prefix length and the routing table, remember that when a router is looking for a match in the IP routing table for the destination address, it always looks for the longest possible prefix match. For example, if the routing table contains entries of 10.0.0.0/8, 10.2.0.0/16, and 10.2.1.0/24, and your traffic is destined for 10.2.1.0/24, the longest match prefix is selected.