Change MTU settings? iMac or at 2Wire 3800?

Discussion in 'macOS' started by mindquest, Oct 31, 2010.

  1. mindquest macrumors regular

    Oct 25, 2009
    Working on troubleshooting my uVerse to iMac internet connection. I want to hard code my MTU setting to 1472 but not sure if I should do it on the iMac? 2wire 3800 VDSL modem? Or both?

    I am assuming that if I change it on the iMac to (MTU 1472) that even if my 2Wire is set to MTU 1500 the iMac should take priority.

    Any thoughts are appreciated!

    My set-up:

    Internet(AT&T Uverse)=>2Wire (3800HGV VDSL)=Ethernet Cable=>iMac 27" i7
  2. mindquest thread starter macrumors regular

    Oct 25, 2009
    Forgot to mention my Uverse set-up is running internet & TV.
  3. ghboard2010 macrumors member

    Aug 7, 2010
    Dont look now, I'm right behind you
    MTU Settings. . .

    Out of curiosity, what is the significance of an MTU setting of 1472 versus 1500? I too am an AT&T U-verse customer so this piqued my interest (it caught my attention).
  4. wrldwzrd89 macrumors G5


    Jun 6, 2003
    Solon, OH
    MTU = Maximum Transmission Unit (size) for those who don't know. Its value determines how big packets can be across a network.

    A quick search of Apple Support found this:
  5. maflynn Moderator


    Staff Member

    May 3, 2009
    Why lower it?

    I'm not seeing the justification to reducing the packet size, and to me, it would decrease performance, i.e., more packets = more traffic.
  6. diyguy macrumors newbie

    Jul 22, 2010
    first off, sorry to hear you have uverse ;) im just kidding i used to install uverse and its a good product if it is installed right and that was few and far between at least with the 'burger flippers' i worked with....

    and like maflynn said, it would decrease performance, decreasing the size=more packets=more lost packets=more retransmitted=more traffic=slower network?? i could be wrong...??
  7. mindquest thread starter macrumors regular

    Oct 25, 2009
    Looking to optimize my Uverse connection and this topic pop up several times:

    From this article it seems like the right sized packet will reduce fragmentation. And Uverse seems to have a sweet spot for a less than 1500 value.

    Any thoughts are appreciated!
  8. mindquest thread starter macrumors regular

    Oct 25, 2009
    I think my biggest question is what MTU setting with take priority? Uverse modem or iMac?
  9. logana macrumors 65816


    Feb 4, 2006
    If you change it at the modem/router then all the computers will take advantage of the MTU setting....

    If you change it at the iMac then only that machine will get the advantage.

    I have to use an MTU of 1432 but I changed the router and the Mac to this value.

    Just change the the 2 wire box...
  10. mindquest thread starter macrumors regular

    Oct 25, 2009
    Thanks logana,
  11. Galtiero, Jan 29, 2011
    Last edited: Jan 29, 2011

    Galtiero macrumors newbie

    Jan 29, 2011
    Why Set MTU to other than default 1500??

    The primary reason is that in most if not all cases if you send packets at 1500 they will need to be fragmented resulting in degradation of performance. I would not reccomend changing the MTU settings without first checking the optimal settings for your environment, do not just take the word of your ISP, they are making a generalized statement that covers the masses, it is not a total optomization.

    Checking is easy to do. First make sure that your MTU setting is in fact 1500 or the test is compromised and will result in a number that is too low.

    The command is to see the largest packet is "ping -f -l XXXX" -f -l forces the ping packet size to be what you specify, any size to lage returns a reply that the packet needed to be fragmented.

    -l is a lowwercase L
    XXXX is mtu speeds start at 1500 and decrease in multiples of 10 (1500, 1490, 1480 etc...) until you get a ping result, then increase in multiples of 1 until you again get the reply that the packets needed to be fragmented. The highest numbet that did not have to be fragmented is your highest optimal number. I would actually suggest decreasing it by 2. So if 1464 is the highest you could send without fragmentation set the MTU to 1462.

    and XXX is a website of your choosing.


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