MSU problem ?

Hi all, recently bought a new router and am having trouble connecting to xbox live now. I am trying to connect wirelessly and my xbox can connect to the router, the router to the internet but when it comes to xbox live, it says 'your network doesnt have a sufficient MTU setting'. So i went to my router setup and im connected usuing PPPoa and the mtu is 1432 (well over the minimum xbox says it needs to be). Can anyone tell me why this isnt working because its getting pretty frustrating and my mtu should be fine with it ?