the "illegal" connection part must be some type of AUP on the ISP side that says "No, you can NOT network". Because by default windows XP has a 1500 MTU setting (matching that of an ethernet network). PPPoE going across a standard AAL5 ATM network has an optimal MTU of 1456(?) I believe? If ISPs are checking the MTU to enforce thier AUP then they are quite stupid, (considering most OSs have a default MTU of 1500, including LINUX I do believe?) a better way to enforce the no networking policy would be to check the RADIUS (check your RFCs) to see if the customer has authenicated more than once.

Just a thought!
Cheeseball