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Re: Check network MTU size

by gsiems (Chaplain)
on May 23, 2013 at 22:07 UTC ( #1035031=note: print w/ replies, xml ) Need Help??


in reply to Check network MTU size

++ We recently had this happen to us and it took a while to figure out what was going on. I shall have to try this.


Comment on Re: Check network MTU size
Re^2: Check network MTU size
by 5mi11er (Deacon) on Jun 04, 2013 at 13:46 UTC
    I've had to rethink the "check for MTU larger than it should be" section. Checking for a larger MTU size is not as simple as this, because it will only work if the network segment (vlan) connected to the computer running the check is the one with the MTU set too large.

    As an example imagine the semi-simple case of a network like this

    -------A--------B--------C-------
      (1)     (2)       (3)     (4)
    
    Each of the four numbered, dashed areas is a network segment (vlan), the letters being routers.

    If our computer is located on segment 1, and sends out a ping larger than the actual MTU size allowed for segment 1, router A will drop that packet and not forward it on. So, if segment 2 or 3 or 4 is set to have an MTU size larger than segment 1, we would not be able to tell that from segment 1.

    To actually find out if the MTU size of any of the segments were larger than 1500, we would have to perform the large MTU ping test from each of the routers A, B and C.

    The testing for an MTU size being smaller in one network than the rest, is still valid, but it is unable to tell you which segment was at fault.

    -Scott

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