OS X Lion: Wi-Fi problems? Set the MTU manually

Discussion in 'Mac OS X Lion (10.7)' started by Roman2K~, Dec 25, 2011.

  1. Roman2K~, Dec 25, 2011
    Last edited: Dec 25, 2011

    macrumors 6502a

    Mar 11, 2011

    2011 11" MBA
    OS X Lion v10.7.2


    The last few days, I've been pulling my hair out because throughput via my Wi-Fi connection had become unstable out of the blue. While the signal was strong and stable, download speeds (on my local network) were slow and fluctuating, like ~160 KB/s at the most for one second, then 0 B/s for 5 seconds and cycling. Other computers (two MacBook Pros) connected to the same Wi-Fi router had stable, normal-throughput connections.

    It reminded me of an issue I had on Windows over 10 years ago, where I had to play with the MTU to restore throughput stability (100 Mbits/s Ethernet).


    Set the MTU manually:

    1. Determine the maximum MTU following this guide.

    2. It's a connection-specific setting. Configure it in System Preferences > Network > [a connection] > Advanced... > Hardware > Configure: {Manually,Automatically}

    3. Disable and re-enable Wi-Fi.


    I usually am not comfortable resetting things to "solve" problems, because usually, it's only a temporary patch and the real problem is left unsolved. So the abnormal behaviour will come back sooner or later.

    This time, I had a hunch this was MTU-related. Until now, the MTU was configured automatically. The current automatically-determined value must be stored somewhere. Not knowing where, I thought I'd got a try to the famous PRAM- / NVRAM-reset.

    After that, download speeds were back to normal.

    Then I went a bit further and determined the maximum MTU manually. I found it was the regular 1500, but setting this manually increased the speed from a stable 850 KB/s (far away from a crappy 802.11g router) to 1.9 ~ 2.3 MB/s!


    Automatic MTU setting, at least paired with my crappy-ol' Wi-Fi router, is not optimal, and may lead to extremely poor speeds at some point. So, in case you're encountering instability, try setting it manually.
  2. macrumors member

    Oct 12, 2009
    Useful exercise. I used the very simple suggestion for checking your effective MTU in the link provided.

    My MTU was automatically set to 1500. I did a traceroute to my provider and used the address for the first external device that gave me a reply.

    My actual capability fragments at 1480. I set it to 1472 and at least speedtest.net thinks I'm getting much closer to what I pay for.


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