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Thread: Haywire optimum MTU

  1. #1
    Junior Member
    Join Date
    Mar 2007

    Question Haywire optimum MTU

    Hi all,

    I have a question dealing with ever changing optimum MTU from my ISP, which is Bellsouth/AT&T in Southeast Florida.

    My modem is the Westell 6100 sold by Bellsouth the firmware automatically upgrades, but there is no user setting for MTU in the firmware. It is now working in Bridge mode since it's connected to my router.

    When I upgraded to DSL Ultra (1500/256) a few weeks ago, I set my router to PPoE with an MTU of 1492, but I was getting disconnected about every 20 minutes or so.

    I soon discovered that optimum MTU for my connection was really 1484, so I set the router accordingly and the disconnects went away.

    About a week went by and I found that I was getting disconnects all over again. Again I did a test to check what the optimum MTU was, and found that it had changed once more, but this time to a much lower one of 1456!

    That looked strange, so I retested two more times to make sure, and then set the MTU in the router to 1456, which is where it is set now, and the disconnects went away.

    At this rate, who knows how low MTU will be in a few days…

    Has anyone else here encountered these crazy and low MTUs with their DSL provider before?

    BTW, I have also been adjusting my Windows XP computer with TCP Optimizer with every MTU change, but this is getting ridiculous.

    All MTU readings have been done with the Westell modem connected directly to the ethernet card in the computer and termporarily switched to PPoE for testing.

    My method of testing for optimum MTU is using a command window and pinging with: –f –e xxxx.

    xxxx being an MTU starting point and going on from there until the packets no longer need to be fragmented, then I add 28 to that number.

    I want to be prepared with as much information as possible before I give Bellsouth DSL team a call on this problem, so I welcome any advice, it will be appreciated!

  2. #2
    Yes, I have seen this before, never on my end, but I have worked on problems like this. Set your internal network LAN to use 128 ms TTL, and Ethernet to use MTU of 1500 regardless. Your modem should be set to auto-discover the WAN MTU number however so you stay connected.

    Anything that uses Ethernet should always be 1500 for MTU.

    I hope this helps you out, if you need more help or want me to explain more, just ask.

  3. #3
    Junior Member
    Join Date
    Mar 2007

    Thanks for your suggestions, I will follow them and post back later on after I give them a chance to work!

    I'll be back...

  4. #4
    Junior Member
    Join Date
    Mar 2007
    Well, latest suggestions implemented, but disconnects remain the same.

    I will try calling my isp to find out if they could help me fix the problem.

    Thanks for the tips!

  5. #5
    Junior Member
    Join Date
    Mar 2007

    Thumbs up


    A BellSouth/ATT technician showed up at the house after I put in a trouble call for multiple DSL disconnects throughout the day and night.

    The tech took various readings and found that somewhere the line was not functioning correctly.

    After a while, he climbed up the telephone pole and corrected the problem there.

    So far, everything is working well and no disconnects at all, so everything seems to have been repaired!

    Thanks to everyone who tried to help, now the problem is solved.
    Last edited by Pilatus; 06-22-07 at 05:46 PM. Reason: missing words

  6. #6
    Good... Glad everything is working for you now...

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