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From what I have found on the internet, there is confusion about the role of MTU in EIGRP metric calculation.

I performed an experiment in GNS with few routers. I was not able to find any change in path cost after the change in MTU value. This was obvious since MTU is not included in the formula of calculating the cost in EIGRP.

Recently I found a "draft-savage-eigrp-00" on ietf explaining the EIGRP protocol. Yet this document also presents some sort of confusion.

Under 'EIGRP Metric Calculation' on page 33, it states that

The composite metric is based on bandwidth, delay, load, and reliability. MTU is not an attribute for calculating the composite metric.

Again on page 35 under 'Wide Metric Vectors' it states that

EIGRP uses five 'vector' metrics: minimum throughput, latency, load, reliability, and maximum transmission unit (MTU).

From my own practical observation I was able to deduce that "MTU is not even used as tie breaking when comparing equal cost paths".

This whole adds up to my confusion. I am not able to deduce the meaning of MTU is a vector and an attribute in EIGRP metric calculation.

I accept that if its not an attribute, it has nothing to do with cost estimation. Hence it is not included in the formula for metric.

metric = {K1*BW + [(K2*BW)/(256-load)]+(K3*delay)}*{K5/(reliability+K4)}

If it is considered as one the vectors in find the path, the change in MTU of successor must trigger some sort of process (an update of path if it is too low as compared to feasible successor). But my observation says it does not.

Can someone please help me.

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The MTU field in EIGRP is informational in nature, it is not used in any path selection algorithms.

It was most likely put in to the scope of the protocol as a way to pass values through route redistribution. This is pure speculation as I was not there myself. :) You could always reach out to your local SE and press the issue if you want more information.

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  • I know it is informational in nature. But there is no documentation to verify. But in the RFC for eigrp, MTU is considered to be a factor to evaluate the cost. – Damon Jul 29 '15 at 6:39
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The MTU metric is only used as a tie-breaker. The formula doesn't calculate MTU value. The MTU is used if the router has to choose between routes (theory).

"EIGRP routing updates are triggered only by a change in network topology (interface up/down event, IP addressing change or configured bandwidth/delay change) and not by change in interface load or reliability. The load/reliability numbers are thus a snapshot taken at the moment of the topology change and should be ignored."

I think that MTU make part like load/reliability procedure, and doesn't triggers route update

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  • In order to implement the change in MTU after giving command for "ip tcp adjust-mss *** ", you must reboot the router. This in turn, would cause all the lines to go down. When the router reboots with the changed MTU (Given :- write command is used before reloading), there has to be change in the previously calculated best path and the new best path in case of a tie-breaking situation. But from my practical observations, no change was found. Please wait while I add the actual scenario diagram and config files, with results. – Damon Jun 25 '15 at 13:54
  • I did some tests too using wireshark and GNS3. As I think the metric calculation doesn't change. If the path is on same metric for both routes it will use both with a traffic share 50/50. So maybe this "tie-breaker" thing must be something of the past. take a look here: learningnetwork.cisco.com/thread/66465 – emirjonb Jun 26 '15 at 7:24
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MTU Used as tie breaker.

Full explanation : https://www.edanesh.com/article/MTU-in-eigrp-metric-calculation

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