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MTU setting for BT.

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  • 24-04-2009 2:58pm
    #1
    digitaldarragh
    Registered Users Posts: 52 ✭✭


    Greetings,

    Am I right to assume that the MTU is the same for BT across the entire country?

    In Drogheda, I've been advised by BT that it's 1432. IN Carlow, I've been advised that it's 1430.

    The Carlow connection is causing problems. Sites that require a lot of bandwidth for example youtube.com do not load. They time out after about a minute of loading when using the wireless connection to the router.

    When connecting via standard ethernet cable sites run fine.

    Any suggestions?

    I'm assuming it's the MTU however I could be wrong. If so, would you have any other suggestions?
    Tagged:


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    #2
    Marlow
    Registered Users Posts: 14,555 ✭✭✭✭Marlow


    In general, the MTU on the lan is 1500, for DSL it should usually be 1492, because PPPoE takes out 8 bytes.

    Your router should handle the MTU change for you.

    If you have problems with MTU, or believe, you have, you can try to decrease the MTU. Another way of testing it is using ping.

    On Linux that is done this way for MTU 1500:
    ping www.hea.net -s 1472 -M do

    if the MTU is broken, you get a message like "icmp_seq=1 Frag needed and DF set (mtu = 1500)"

    The reason, why it is 1472, is that the ICMP header is 28 bytes, which are added.

    There are similar switches for Windows ping, but I don't have a box here to find them right now. -s is the packet size, -M is the Path MTU Discovery strategy

    Hope it helps. If you've got issues, just make the size value smaller until you stop getting errors. Use that value + 28 then for your MTU.

    /M


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