Cisco EIGRP metric calculation simplified– the eyes have it!

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Cisco EIGRP metric calculation simplified– the eyes have it!

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Added by August 24, 2012

There are numerous sources online which discuss Cisco’s Enhanced Interior Gateway Routing Protocol (EIGRP) metric formula and its computation. My goal today is to present the formula in a way that is more pleasing to the eye so it is easier to follow the values as they get crunched through it. The following diagram is used for this entire blog post: NOTE: If you are studying for CCNA, this information is not on the exam, but it is fun to learn.

Cisco EIGRP metric formula Enhanced Interior Gateway Routing Protocol

Below is the output from R1 show ip eigrp topology (with all k-values set to default values):

Cisco show IP EIGRP topology output with k values

The full EIGRP formula looks like this:

Cisco-EIGRP-metric–fact-and-not-so-fact

which simplifies (using default k-values) to:

256(min bandwidth + cumulative delay)

To determine min bandwidth and cumulative delay, use the following information:

Cisco min bandwidth cumulative delay formula

And delay is computed as the cumulative delay end to end in tens of microseconds.

Cisco delay computed as cumulative delay microseconds formula
Plugging in the values from the above diagram yields:

Cisco delay computed as cumulative delay microseconds formula yield

Which matches the result shown in R1’s output shown above.

Let’s change some of the values and observe the impact:

Cisco router 1 output show ip eigrp topology formula

Router1’s output (for path from R1 to Loopback 0: 192.168.1.1 of R2): show ip eigrp topology

Cisco show IP EIGRP topology loopback R1 formula

Cisco first value show IP EIGRP topology formula

Router1’s output to compare: show ip eigrp topology

 

Without showing the step, I will simply state that the math returns the same 156160 as shown above, since the truncated value after performing the computation is the same.

Router1’s output: show ip eigrp topology

Cisco router1 output show ip eigrp topology formula

So we see how manipulating various k-values affects the output of the EIGRP metric formula. I have obtained the metric formula from various Cisco Learning websites and there are other numerous informative articles on this subject. I have only skimmed the surface, there is much more to be learned on this topic. A quick search engine effort will reveal additional information for you. As I mentioned at the outset, I just wanted to present the mathematical formula in a way that makes more sense visually. When it is presented with nested parentheses (((()))) like this, it becomes increasingly difficult to follow the flow. I used the really cool equation generator that comes with Microsoft Word to write the formulae. I hope you found it enlightening! Until next time…

Mark Jacob
Cisco Instructor – Interface Technical Training
Phoenix, AZ

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