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What decides MTU?

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  • 31-08-2008 10:08pm
    #1
    Random
    Closed Accounts Posts: 19,080 ✭✭✭✭


    I can set MTU on my network card, my router and my modem. No doubt I can probably do it in my browser too.

    Which one of these has the last decision as to which one is used?

    i.e. if brower is a, modem is b, router is c and network card is d. Which MTU will I be using to connect to the web?

    Thanks


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    #2
    paulm17781
    Closed Accounts Posts: 4,858 ✭✭✭paulm17781


    The router is the decider. You should set any settable options to be the same. 1500 is standard.


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    #3
    Cabaal
    Moderators, Technology & Internet Moderators, Regional South East Moderators Posts: 28,490 Mod ✭✭✭✭Cabaal


    mtu is sometimes decided by your ISP, alot of rouiters will auto-detect this setting off your Broadband connection, other routers will have a default setting which will work 99% of cases

    In some rare cases ISP's need users to set a specific MTU

    Incorrect MTU can cause issues like problems accessing banking sites at the likes


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    #4
    [Deleted User]
    Posts: 0 ✭✭ [Deleted User]


    Im sure eircom uses 1492, dunno about the other ISPs, their re-sellers probably use 1492 aswell doh


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    #5
    MunsterCycling
    Registered Users Posts: 4,864 ✭✭✭MunsterCycling


    That is to allow for the protocol overhead IIRC


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    #6
    paulm17781
    Closed Accounts Posts: 4,858 ✭✭✭paulm17781


    Deleted User wrote: »
    Im sure eircom uses 1492, dunno about the other ISPs, their re-sellers probably use 1492 aswell doh

    I'm just about certain the default for ethernet is 1500, I know some wireless operators use below 1500. It is decided by ISP but the OP, the router is the deciding factor on your end.


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    #7
    Random
    Closed Accounts Posts: 19,080 ✭✭✭✭Random


    IBB DSL is 1460.

    How is it that the router sets the MTU? I'm a little confused. Would the last link between me and the web (i.e. the modem) not determine it if set in that? Otherwise why does my network card not determine it over the router?


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    #8
    paulm17781
    Closed Accounts Posts: 4,858 ✭✭✭paulm17781


    Random wrote: »
    IBB DSL is 1460.

    How is it that the router sets the MTU? I'm a little confused. Would the last link between me and the web (i.e. the modem) not determine it if set in that? Otherwise why does my network card not determine it over the router?

    It would, the last point to network. I just assumed router.


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    #9
    lmimmfn
    Registered Users Posts: 3,553 ✭✭✭lmimmfn


    Random wrote: »
    IBB DSL is 1460.

    How is it that the router sets the MTU? I'm a little confused. Would the last link between me and the web (i.e. the modem) not determine it if set in that? Otherwise why does my network card not determine it over the router?
    DSL needs the rest for overhead, i.e. PC will be 1460 and modem 1500MTU


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    #10
    Random
    Closed Accounts Posts: 19,080 ✭✭✭✭Random


    Why do I need 40 for the overhead between the PC and the modem?


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    #11
    lmimmfn
    Registered Users Posts: 3,553 ✭✭✭lmimmfn


    Random wrote: »
    Why do I need 40 for the overhead between the PC and the modem?
    you dont,its 1500 from the modem onto the ISP, but theres 0 point in setting it to 1500 on the PC because even if it was accepted by the modem it would have to split it into 2 ethernet packets to forward on. It would probably work at 1500 in a subnet


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    #12
    Random
    Closed Accounts Posts: 19,080 ✭✭✭✭Random


    You have me totally confused. I'd like to think I'm pretty good with networking things but apparently I'm not in this case.

    I guess I'll have to do some serious googling later.

    Thanks though.


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