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    MTU's, PPPoA & PPPoE

    This is a discussion on MTU's, PPPoA & PPPoE within the Technical discussion forums, part of the Broadband Technical Help category; Hi, I'm hoping someone might be able to shed some light on this. If you have time I would appreciate ...

    1. #1
      tentimes's Avatar
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      MTU's, PPPoA & PPPoE

      Hi,

      I'm hoping someone might be able to shed some light on this. If you have time I would appreciate any thoughts you might have - I'm trying to make sure my MTU's on both sides of the bridge are correct. My setup is:

      - Draytek Vigor 120: (NOT a router!!) It's an ADSL modem with true NAT translation over PPPoA to PPPoE bridge

      - Router with PPPoE setup on the other side of the above

      Would I be right in thinking that the correct setup is the PPPoA MTU needs to be 1500, whereas the MTU on the PPPoE router side needs to be 1492? This is what my research has turned up anyway, but I wanted to check with someone who knows a bit more about MTU's. Keep in mind that the MTU Sky side will always be 1500 as required.

      Thanks for any thoughts


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    3. #2
      fuzzy logic's Avatar
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      Re: MTU's, PPPoA & PPPoE

      The MTU doesn't need to be 1500, you can use any sensible value up to a maximum of 1500 on Sky. MTU = Maximum Transmission Unit. Although the only reason I can think of to reduce it on a PPPoA connection on sky's network, is to get a very small increase in throughput by eliminating padding bytes, in which case you'd use 1478 for PPPoA.

      The PPPoE protocol has more header bytes than PPPoA, hence the need to reduce the MTU to 1492.
      Last edited by fuzzy logic; 04-01-11 at 03:13 AM.

    4. #3
      tentimes's Avatar
      tentimes is offline Sky User Member
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      Re: MTU's, PPPoA & PPPoE

      Ok I have the answer. The MTU on the Modem side to Sky's DSLAM is 1500, but the PPP header for the bridge from PPPoA to PPPoE is 8 bytes.

      This means you need to set the MTU of your internal network to 1492 or below, otherwise the modem will have to send extra packets to take the 8 bytes.

     

     

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