Need help connecting to xbox live?

Hello, i'm connecting to xbox live using the direct modem method (just connecting with the DSL modem without router or adapters). Well i tested the connection and everything worked fine until it reached the part between internet and xbox live. An error popped up saying i had an MTU error. I checked the MTU on my modem and it said it had 1500 which is enough. Can anyone find out the problem? The dsl modem is a speedstream 4100.