here's the solution to ur problem:
first you need to setup pppoe using the command pppoe-setup
when the setup is over its down for you and now the real thing .whenever you start ur system ,after that type the folowing command:
ifconfig eth0 down
then type
ppoe-start and when u are connected you can ping any site i hope it will do for you just like it worked for me after 2 days full of madness but i still cant understand how this works but sure it does and do reply if it works for you.