Comment # 20 on bug 903553 from Dominique Leuenberger

(In reply to Reinhard Max from comment #19)
> I still don't get why NetworkManager wants to use rp-pppoe at all, given
> that the default pppd from the ppp package does the job just fine. Or am I
> missing something?

The changelog of NetworkManager clearly states:
* PPPoE now requires userland "rp-pppoe" to work around kernel bugs
    that prevent detecting server-side termination of the PPP link