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How to figure out max MTU size

Discussion in 'Networking Issues' started by ifican, Nov 14, 2006.

  1. ifican

    ifican Network Guru Member

    Ok ladies and gents,

    There has been enough mtu talk going around and without really getting into the technical side of MTU here is a quick and easy way to check what the maximum MTU size is for the network you are sending from / too. Keep in mind that it may or may not be the maximum size of your ISP but by trying several different domains you can figure out if its you or them and set your MTU accordingly if you feel that its the cause of your problem.

    What you need to do is this, pull up a command prompt and ping whatever ip or website you like that will respond. The trick is you need to set two flags, the -f (tells the packet to not allow itself to be fragmented) and the -l (changes the size of the packet being sent). In my opinion its easier to set the -f first as you will be changing the -l size so get to the right one. Your command and output string will look something like this:

    C:\Documents and Settings\>ping google.com -f -l 1272 (size of the packet)

    Pinging google.com [64.233.187.99] with 1272 bytes of data:

    Reply from 64.233.187.99: bytes=56 (sent 1272) time=73ms TTL=244
    Reply from 64.233.187.99: bytes=56 (sent 1272) time=72ms TTL=243
    Reply from 64.233.187.99: bytes=56 (sent 1272) time=72ms TTL=244
    Reply from 64.233.187.99: bytes=56 (sent 1272) time=72ms TTL=243

    Ping statistics for 64.233.187.99:
    Packets: Sent = 4, Received = 4, Lost = 0 (0% loss),
    Approximate round trip times in milli-seconds:

    As you will see the maximum size for the link i am currently on is 1272 as 1273 and greater does not work (see below).

    C:\Documents and Settings>ping google.com -f -l 1275

    Pinging google.com [64.233.167.99] with 1275 bytes of data:

    Packet needs to be fragmented but DF set.
    Packet needs to be fragmented but DF set.
    Packet needs to be fragmented but DF set.
    Packet needs to be fragmented but DF set.

    Ping statistics for 64.233.167.99:
    Packets: Sent = 4, Received = 0, Lost = 4 (100% loss),
    Minimum = 72ms, Maximum = 73ms, Average = 72ms

    The simplest way to quickly get to your maximum is start at 1500 and move down in 100 size segments until you get a reply, then move up, for instance: 1500,1400,1300,1200 (got reply) 1250, 1275 (no reply) 1274,1273,1272 (found max).

    For the most part you only need to do this once as usually it will be your ISP link that is the limiting factor, however i have seen in very few cases where it was not the source isp but somewhere in the middle that needed smaller packets.
     

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