I don't think you can manually adjust the MTU, but if you have Infinity and use the HH3 connected to the OR modem, it automatically uses PPPoE and sets the MTU to 1492, which is the default for PPPoE.
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Brian
From September 2001 on BTopenworld Home 500/Home 1000/Home 2000. Then ADSLMax on <n>ildram. Moved to ADSL2+ from ADSL24. I'm now with plusnet. I'm not saying who I work for. Any opinions expressed here are my own.