With regards to PPPoA & PPPoE explained
Here (post#23) the only reason the Xbox would need to use PPPoA or PPPoE if it needed to dialup your provider first for dsl access as
Bifta has said
the Xbox will just see a network connection and use it.
usually reseting (using the reset button) the BEFSR41 v2 after the Dlink DSL-300T has detected the internet connection works.
what firmware (BEFSR41 v2) are you using is it v1.42.7?