ip mtu / interface mtu

Discussion in 'Cisco' started by V. Evans, Aug 19, 2005.

  1. V. Evans

    V. Evans Guest

    I am having an issue on a provider network in which they do not pass
    any ip packets over 1400 bytes. This has caused big problems with
    network drives that are mapped over the WAN and with email
    (connectivity to the Exchange server) because packets over 1400 bytes
    are being dropped sporadically (about 50-60% of the time).

    After opening a ticket with the provider, we decided that we would
    alleviate our problems by setting our serial interface IP mtu to 1400
    for all of our remote sites. It seemed to help initially, however we
    are having similar problems again.

    I am a level-4 engineer and I understand this stuff pretty well,
    however I am having a problem understanding the MTU in relation to the
    interface. In the Cisco IOS, there is an interface MTU setting and an
    IP MTU setting. I understand that the second one is a protocol MTU and
    relates to layer-3, while the interface MTU command seems to be more
    related t a layer-2 framing perspective. Additionally, even though I
    have set the IP MTU to 1400 bytes, I can source an ICMP off of the
    ethernet interface of my router and specify a size of 1500 bytes and it
    will go, even though I have set my serial side lower.

    I am not having much luck finding decent documentation on Cisco's
    website; any further understanding from anyone out there would be

    V. Evans
    V. Evans, Aug 19, 2005
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  2. Stig Meireles Johansen
    Senior Consultant

    Provides the following advice:

    MTU is the Maximum Transmission Unit of general frames entering/exiting
    the interface.

    IP MTU is the MTU for IP packets entering/exiting the interface.

    The difference is there because of the different features you can add
    (ie. run IPX etc.)

    If you have MTU-problems, try setting "ip tcp adjust-mss 536" on your
    inside interface... you could always adjust to a bigger value.. use a
    ping with different sizes to see which size gets through..

    If you use the microsoft-ping, remember that the "-l size" parameter
    (-s for unix-version) is the payload of the ICMP-packets, so you should
    subtract 28 bytes (20 bytes IP header + 8 bytes ICMP header) from the
    total.. If you ping with a size of 1472, this would equal a IP-packet
    with MTU of 1500.

    The MSS is the Maximum Segment Size, ie the data payload of
    TCP-packets.. this should be (normally in a ethernet-environment) 1500
    - 20 bytes IP header - 20 bytes TCP header = 1460 bytes. The minimum IP
    MTU which must be supported on the 'net is 576 bytes, which gives an
    MSS of 536.

    If you are running PPPoE, remember to take this into account.

    Troubleshooting MTU Size in PPPoE Dialin Connectivity:



    Hope this helps,

    Brad Reese
    BradReese.Com Cisco Repair Service Experts
    1293 Hendersonville Road, Suite 17
    Asheville, North Carolina USA 28803
    USA/Canada Toll Free: 877-549-2680
    International: 828-277-7272
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    www.BradReese.Com, Aug 19, 2005
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