4 Replies Latest reply: Feb 12, 2011 11:18 PM by Andrew G CCIE RSS

    Optimal MTU settings ADSL connections

    Andrew G CCIE

      Hi All,


      Sorry If im posting in the wrong place


      I have been looking long and hard for a answer but have come up empty handed.


      The question is what is the optimal MTU setting for an ADSL connection? And more specifically how is it calculated?


      After alot of forum searching I have found that the most common optimal MTU for PPPoA and PPPoE is 1462 and 1454 respectively and that this is due to filling the ATM cells more efficiently. example.  http://whirlpool.net.au/wiki/ADSL_Theory_PPP


      This doesn't make sense to me, an ATM cell is 53bytes long 5bytes are for the ATM cell header and the remain 48bytes is the payload, neither 1462 or 1454 divide equally into 48.


      Using this thinking wouldn't an MTU of 1488 be more optimal (1488/48=31).


      Is the website I linked in above wrong or am I missing something?





        • 1. Re: Optimal MTU settings ADSL connections

          Hi dear,


          The default MTU is 1500 and if you expeircne any issue with default MTU ... e.g. MSN /Yahoo mail not logging in or any site login issue. You can chnage the MTU to 1496.





          • 2. Re: Optimal MTU settings ADSL connections
            Andrew G CCIE

            I understand that the default is 1500 for PPPoA and 1492 for PPPoE as it requires an additional 8byte header.


            The question is about optimizing the link. Understanding why I am seeing suggestions to use and 1462 for the MTU and how this has been calculated.

            • 3. Re: Optimal MTU settings ADSL connections

              I will try some math...  The maximum Ethernet payload MTU is 1500. Let's fill all cells with data and avoid padding. 48 * 31 = 1488. One thing you are missing is that the last cell will have 8 bytes trailer (RFC 2364), so we need to reduce our data chunk by 8 bytes 1488 - 8 = 1480. Finally, 2 bytes PPP overhead gives us 1478 MTU. That's assuming PPPoA with IP over AAL5/VC-MUX.


              Now, if we look at PPPoE  bridged over ATM, we need to consider Ethernet frame overhead that also should be placed into ATM cells. So, from the above 1480 we take out 8 bytes PPPoE overhead (6 bytes PPPoE and 2 bytes protocol Id), then additional 18 bytes of Ethernet overhead and now we have 1454 (one is out of the way). What did "bright" DSL specialists do next? They said: "Ok, since PPPoE has 8 bytes overhead and that overhead does not apply to PPPoA let's simply add those 8 bytes to our payload" and that's how they came up with 1454 and 1462. That, however, makes no sense because for PPPoA calculations they overlooked the fact that 18  bytes of "used to be" Ethernet overhead no longer there and they also forgot to subtract 2 bytes of PPPoA PPP overhead.  PPPoE calculations are also questionable because of in case of LLC (vs VC-MUX) which is normally used for PPPoE there are further 8 bytes of LLC overhead (RFC 2684).


              But to answer your question about most optimal setting from perspective of minimising any ATM padding ...


              1478 for PPPoA/VC-MUX

              1470 for PPPoA/LLC

              1446 for PPPoE over ATM/LLC


              Hope this makes sense.




              • 4. Re: Optimal MTU settings ADSL connections
                Andrew G CCIE

                Thanks Andrei,


                Thats Excellent I really wanted an indepth answer and that is what you have provided!


                Thank you