Simple MTU question.

[husycisco]

Hello,

    Would like to have your opinion about my following concern.

    I have an ISP backbone in which the path MTU is 1500 between any 2 nodes.

    Now right at the border router at datacenter, the interface towards the backbone has MTU 1500, and the interface towards the Internet is also 1500, with the option of being able to increase it , 1700 or more.

    The question is, since pppoe clients have <1500 MTU , and path is 1500 as well, what would be the point of increasing the MTU at border router to 1700?

    Even lets say that I can also accomodate 1700 path MTU. What would be the point of using that high MTU since the client can never send that big packet? I see that almost all ISPs use such high MTUs.

    I am already aware of the overhead caused by MPLS/VPLS has to be accommdated, but +200 or even more in some ISPs shouldnt be only for overhead accommodation.

    The only scenario that I can make sense, is lets say router has 2 packets destined to same address one packet size 1500 and the other is 200 (or less), and it reassembles those two packets into 1 single packet of 1700 bytes, saving the extra header overhead. If this is the case, then I will have additional concerns.

     Any insight is appreciated.

Merry Christmas!

[Giuseppe Larosa · Accepted solution]

Hello Husycisco,

>> The only scenario that I can make sense, is lets say router has 2 packets destined to same address one packet size 1500 and the other is 200 (or less), and it reassembles those two packets into 1 single packet of 1700 bytes, saving the extra header overhead. If this is the case, then I will have additional concerns.

This is never done by network devices. So you don't need to worry about this scenario

The use of high MTU is to be able to carry whatever service without the need for fragmentation.

Actually, the trend is to use MTU as high as 9000 bytes or more in order to accomodate all possible cases

There are some services like Fiber Channel over ethernet or the use of jumbo frames on server farms that use high MTU so the benefit is to be able to carry these services over a backbone.

Be also aware that an MPLS backbone does not support fragmentation, so the need for the use of high MTU

Hope to help

Giuseppe

[husycisco]

Thanks Giuseppe!