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Thread: MTU question

  1. #1
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    Nov 2008
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    MTU question

    Ive just done a fresh install of 8.10 ubuntu.

    I've connected to the internet usin pppoe no problem (connect via ethernet) no problem.

    But I have got this message: Interface eth0 has mtu of 576 -- should be 1500. You may have serious connection problem.

    No idea what this means but searching around. It sounds like I have to change /etc/network/interface to fix this permantly.

    In the /etc/network/interface file i currently have

    auto lo
    iface lo inet loopback

    no idea what that means but if I simpl add the line "mtu 1492" and save the file will the problem be removed?

  2. #2
    Join Date
    Mar 2008
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    California, USA
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    8,111

    Re: MTU question

    Before you set your MTU, how about following the first part of this tutorial to check what your MTU should be. Also, Intrepid's network manager unfortunately no longer uses the /etc/network/interfaces file, so it won't help if you put the mtu line in that file. I'm not sure what the most elegant solution is now, but one way you could set the MTU automatically on start up is to put:
    Code:
    ifconfig eth0 mtu <value>
    In your /etc/rc.local file. But how about first finding out what your MTU should be, based on the tutorial, and let me know what you come up with.

  3. #3
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    Nov 2008
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    Re: MTU question

    Thanks for your help.

    It looks like my total packet size is 546 (+28 ).

    So maybe I dont need to change anything.

    Bit confusd though because I have ADSL PPPOE and searching on the internet suggests that PPPOE is 1492 whilst Im at a dial up level. Does this affect performance?
    Last edited by kingleviathan; November 29th, 2008 at 04:33 AM.

  4. #4
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    Nov 2008
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    Re: MTU question

    Ok...Now it get really weird

    I thought I would run this equivalent test on my windows laptops in cmd.exe

    And I am able to ping successfully up to 1464

    same bigpond adsl pppoe network but with a wireless care

    So why cant I ping successfully any higher on my ubuntu PC?

  5. #5
    Join Date
    Nov 2008
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    37

    Re: MTU question

    Well, you're running two different cards in two different machines with two different platforms to test this issue (which CAN work, but of course it is better if you're atleast using the same platform). Are there any similarities in the network configurations that might be causing this (grasping at straws and I'm Net+ working on CCNA) I've never seen an MTU below 1500 (or above infact) unless there is some specialized configuration, protocol, or interface type. (But I don't usually offer gobs of attention to things at home [that sounds way worse than it is]).

  6. #6
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    Nov 2008
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    12

    Re: MTU question

    I hope im not sounding like an idiot but i just tried mtu pinging again on ubuntu and could get as hight as 1436 whilst earlier on i could on get to 530 or so.

    Can mtu vary through the day?

  7. #7
    Join Date
    Nov 2008
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    12

    Re: MTU question

    Similarities? no idea. I just did a sudo of the NIC card and its set to 1492.

    But otherwise its just a simple setting for connection nothing fancy.

  8. #8
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    Nov 2008
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    37

    Re: MTU question

    MTU shouldn't vary (much) (if) at all. It is Maximum Transmission Unit. The maxiumum size of a packet. Have you checked the network configuration file?

  9. #9
    Join Date
    Mar 2008
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    Re: MTU question

    Quote Originally Posted by kingleviathan View Post
    I hope im not sounding like an idiot but i just tried mtu pinging again on ubuntu and could get as hight as 1436 whilst earlier on i could on get to 530 or so.

    Can mtu vary through the day?
    MTU is set by the each networking device in the chain between you and your destination, and the device with the lowest MTU limits your overall connection. So you have an ADSL modem? Do you connect to the ADSL modem in Ubuntu via a standard ethernet connection using DHCP or do you have the ADSL modem set in "bridged" mode so that you have to connect via PPPoE in Ubuntu? If you didn't deliberately set your ADSL modem to use "bridged" mode, most ADSL modems use the "PPPoE is on modem" option by default so that you connect to the modem via a standard DHCP ethernet connection. If you look at the bottom of the ADSL modem, usually it gives its local IP address, often "192.168.0.1", so you can access its settings from your web browser by simply going to http://192.168.0.1 as the web address. Let me know what you find.

  10. #10
    Join Date
    Nov 2008
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    37

    Re: MTU question

    Excellent description. Definitely better than mine. And a bit less confusing probably.

    Are you saying that the issue is probably router side?

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