Routing Protocols review - metric

This morning I reviewed routing protocols again, which should be known but I want to make sure I'm absolutely clear on this since it is the foundation of routing.

After being clear on Administrative Distance yesterday, this morning I wanted to make sure I understood what was being described by "Metric," which is something you see in a route table when you execute a "show ip route" command. So for instance, you might get output that looks like:

R 172.17.0.0/16 [120/1] via 192.168.1.10, Serial0/0/0

This is telling us that the network 172.17.0.0/16 was learned using RIP, that its Administrative Distance is 120, the metric is 1, and that it came from 192.168.1.10 (the IP address of our neighbouring router) through the Serial 0/0/0 interface (on our router). Cool enough.

The Administrative Distance is the number that Cisco IOS uses to determine the most reliable route to the network with the lower AD being the more reliable. The *metric* is what the routing protocol uses to determine the best path to that network. So, if we learned about the 172.17.0.0/16 network through another interface, indicating we had multiple paths to that network, and that path was using RIP also, RIP would choose the path with the lowest metric and place that in the routing table. So, if RIP had another entry that looked like:

R 172.17.0.0/16 [120/3] via 192.168.1.129, Serial0/0/1

it would place the route with the lowest metric into the table and use it for any traffic destined for that network. It is then important to know how each routing protocol determines its metric - RIP uses "hop count," or a count of the number of networking devices between the source and the destination networks. Our first entry tells us that there is one router between source and destination, while the second tells us there are 3. Going through one stop theoretically is faster than going through 3, right?

So, that is what the metric is. Administrative Distance is used by IOS to determine the most reliable path based on routing protocol, and metric is used within a routing protocol to determine the most efficient path.

I probably could have just summarized that and saved 15 minutes of typing...