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

    Roman2K~ 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. wilsonhaven 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.


Share This Page