> I believe the 1490 MTU is used when (MPPE) encryption has been 
> negotiated.  MPPE encrypts each packet to the same size as the 
> unencrypted packet, but also encrypts the protocol id, increasing the 
> payload by two bytes.

I do not pretend to have read the MPPE RFC yet, so probably just am 
misunderstanding what you said. In PPPoE the 6 byte PPPoE header is always
followed by the PPP protocol ID (two bytes), followed by payload data,
that's why the MTU is 1500-6-2 = 1492.

Besides, MPPP over PPPoE sounds - strange.


Martin