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PPPoE connections (WinPoet, RASPPPOE, Enternet, etc.) can only use up to 1492. However, WinPoet and Enternet MTU must be set by hand in the registry. Others may use DRTCP to set MTU.
Verizon FiOS' optimizer tool sets MTU to 1492. If I set my MTU to 1500 (as recommended by the teak test), my speed drops considerably.
How does this relate to packet fragmentation?
1492 seems to be the setting limitation for XP PPPoE att dsl and other tricks like cache cleanup etc (Linksys wrt54g router was cutting out when the sat. TV receivers did their update downloads and and couldn't handle mtu settings higher than 1300 range without cutting out , also found out my c/o ATT server had me competing for bandwidth w/ a nearby hospitals' T1
BT Wholesale do *not* recommend that MTU should be less than 1488. In fact they recommend 1500. Your ISP might have this limitation, but the underlying network has no problems with 1500 MTU on PPPoA. Many years ago when DSL in the UK was rolled out there were some speed issues relating to ATM packet sizes - this hasn't been true for a long time, and as more of the network is switch to 21cn it's travelling over IP anyway (in an L2TP packet, using jumbo frames).
BT recommend an MTU of 1492 for the VDSL2 (Infinity service).