Aug 13

To find the proper MTU Size, you’ll have to do a special ping of the destination you’re trying to go to. A destination could be another computer, or a URL.

Step 1 Click on Start and then click Run.

Step 2 Windows NT, 2000, and XP users type in cmd and press Enter (or click OK).

Step 3 Once the window opens, you’ll need to do a special ping. Use the following syntax:

ping [url] [-f] [-l] [MTU value]

Example: ping yahoo.com -f -l 1472

dlink-mtu

You should start at 1472 and work your way down by 10 each time. Once you get a reply, go up by 2 until you get a fragmented packet. Take that value and add 28 to the value to account for the various TCP/IP headers. For example, lets say that 1452 was the proper value, the actual MTU size would be 1480, which is the optimum for the network we’re working with (1452+28=1480).

Once you find your MTU, you can now configure your router with the proper MTU size.

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1 Comment

  • At 2019.03.23 07:32, Danilo Piazzalunga said:

    On Linux, the equivalent ping options are:

    -c 4 : to stop pinging after 4 packets
    -n : to disable reverse name resolution
    -M do : to prevent fragmentaion
    -s MTU : to set the packet size

    For example:

    $ ping -n -c 4 -M do -s 1462 yahoo.com
    PING yahoo.com (72.30.35.9) 1462(1490) bytes of data.
    ping: local error: Message too long, mtu=1488
    ping: local error: Message too long, mtu=1488
    ping: local error: Message too long, mtu=1488
    ping: local error: Message too long, mtu=1488

    — yahoo.com ping statistics —
    4 packets transmitted, 0 received, +4 errors, 100% packet loss, time 76ms

    $ ping -n -c 4 -M do -s 1452 yahoo.com
    PING yahoo.com (72.30.35.9) 1452(1480) bytes of data.
    1460 bytes from 72.30.35.9: icmp_seq=1 ttl=50 time=178 ms
    1460 bytes from 72.30.35.9: icmp_seq=2 ttl=50 time=124 ms
    1460 bytes from 72.30.35.9: icmp_seq=3 ttl=50 time=222 ms
    1460 bytes from 72.30.35.9: icmp_seq=4 ttl=50 time=146 ms

    — yahoo.com ping statistics —
    4 packets transmitted, 4 received, 0% packet loss, time 7ms
    rtt min/avg/max/mdev = 123.953/167.699/222.462/37.020 ms


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