MTU in EIGRP metric?

[Peter Paluch]

Hello everyone,

Many pages describing EIGRP say that one of used metrics is the interface MTU. However, the formula for calculating the composite metric does not take MTU into account so the MTU can not affect the composite metric of a route.

I would like to ask how exactly is the MTU used in the EIGRP, then. Thank your for any insights!

With best regards,

Peter

[Richard Burts]

Peter

You ask a very good question and I believe that you are on the right track in trying to understand it. Your observation is correct that MTU is a required parameter in specifying EIGRP metric (for redistribution) and is one of the parameters that is advertised. You are also correct in observing that there is nothing in the EIGRP calculation that uses MTU. So the real answer is that the EIGRP specification included MTU but that it does not play any active role in the path selection done by EIGRP.

HTH

Rick

[Peter Paluch]

Rick,

It's kind of funny when the answer can be so straightforward :)

Anyway, thank you very much for your immediate and elaborate answer. I enjoyed reading it very much!

With best regards,

Peter

[burak-isiksoy · Accepted solution]

Hi Peter,

Actually it is used but not directly effective.MTU is advertised with the routing updates and it is stored in topology database, and if your router has equal paths to same destination and has to choose one of them MTU comes in to play

link with the low MTU ignored.

BR

Burak

[Peter Paluch]

Hello Burak,

Thank you very much for answering my query!

I have asked this question about the MTU in the EIGRP quite a long time ago (it was back in 2006). Since then, I've come to know exactly what you have suggested - that if the metrics of two competing routes are the same, the EIGRP will choose the path with the larger MTU. Thank you so much for completing this thread!

Best regards,

Peter