Cant Ping with packets with MTU larger than 1518 Bytes

Discussion in 'Cisco' started by wilowolf, May 24, 2009.

  1. wilowolf

    wilowolf Guest


    I have two routers connected connected back to back with an Ethernet
    cable. I set MTU size on bother GigaEthernet intefaces to 1524 yet,
    when the routers cant ping each with packets of size more than 1518
    when set the DF on. Any hint ?
    wilowolf, May 24, 2009
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  2. wilowolf

    bod43 Guest

    Maybe you have already taken this into account, however:

    There is no standard for a ping program.

    The size specified as a parameter to the ping command
    may not be the "ethernet" packet length.

    It may be -
    Frame length with CRC - default for ethernet is 1518
    Frame length without the CRC - 1514
    IP packet total length - 1500
    IP packet total length less the IP header
    ICMP payload length - ?
    Some other number the designer fancied at the time

    It may also vary (and does if I recall correctly)
    with the exact ping program being used. Cisco
    router one being different from Windows one I

    I always just check with wireshark on a PC.

    I seem to recall that on Windows you specify -l 1472
    to get a maximal length ethernet packet (however
    that may well be incorrect). This I guess will be the ICMP
    payload length.
    bod43, May 24, 2009
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