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This Question is Not Answered 1 Correct Answer available (4 pts) 1 Helpful Answer available (2 pts)
8897 Views 3 Replies Latest reply: Oct 25, 2011 3:43 AM by Paul Stewart - CCIE Security RSS

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Does MTU size affect throughput / bandwidth / speed

Oct 24, 2011 10:11 AM

Nikhil 38 posts since
Sep 2, 2008


We had an issue of a customer whose last-mile media supports bandwidth of 20 Mbps which he should get. But upon actual testing, the client can achieve only an effective throughput of 4 Mbps. Some of engineers are wondering that it has something to do with the MTU on the path.


PE is a cisco 7206 router and lastmile media is a fiber restricted to 20 Mbps. We have also removed any rate-limitting configuration and do no see any bottlenecks.


The CE device is a non-cisco modem able to handle 24 Mbps traffic, and even this device has been replaced.


Whether MTU has anything to do in this scenario. Any clues on this ?

  • Paul Stewart  -  CCIE Security 7,570 posts since
    Jul 18, 2008
    Currently Being Moderated
    1. Oct 24, 2011 2:41 PM (in response to Nikhil)
    Re: Does MTU size affect throughput / bandwidth / speed

    MTU can have an effect.  However, it typically does not have a huge impact unless it is unnecessarily low.  For example, if it were dropped down to 1492 for PPPoE, that isn't a huge deal.  However, what is a big deal is if there is a Path MTU Discover issue alongside a nonstandard MTU (typical is 1500).  In that case, you can play with TCP MSS to determine if that is in fact an issue.  I always set the MSS to 40 less than the MTU when I have to force down an MTU.  For example--ip tcp adjust-mss 1452.

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  • Paul Stewart  -  CCIE Security 7,570 posts since
    Jul 18, 2008
    Currently Being Moderated
    3. Oct 25, 2011 3:43 AM (in response to Nikhil)
    Re: Does MTU size affect throughput / bandwidth / speed

    A lower MTU adds overhead in the form of headers and protocol interaction with TCP. I seriously doubt the issue you are describing is MTU related (or MSS for that matter). Usually, it is usually only marginally slower (or completely broken with a PMTU-D issue). 

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