Not going to lie, this post is for me. There is one small thing that I always forget when it comes to this formula that I will get to in a moment.
The metric of your EIGRP routes can determine which route will be placed in the routing table if there are multiple available. EIGRP does a pretty good job out of the box with selecting the best route, but every network has little things about it that don’t seem to “fit the mold”. Knowing how the calculation for the metric is done is a good way to understanding how the route election is done so you can influence it according to your own individual plan. Here is what the actual calculation looks like:
Metric = 256*((10^7 / minimum bandwidth) + (Sum of all delays in the route))
As for that thing I mentioned I always miss… It concerns the delay value here. The value here is in TENS of microseconds. That is key and will give you headaches if you forget such an easy step like I always do :)
Now for an example. Here is a “show ip eigrp topology 0.0.0.0/0” on my 4506 here at home:
Now you can just pick the values out of that output that you need. The bandwidth for this route is 100000 Kbit and the delay is listed as 110 microseconds.
Our formula then would look like this:
Metric = 256*((10000000/100000)+11)
Metric = 28416
Go ahead and look at the previous output. “Composite metric is (28416/28160), route is External” We can see that the metric is also listed as 28416. Our number matches. That is how the calculation is done. Now in the future should you want to influence one route over another, you can add things like delay to a route to change the result of that calculation.