for debugging why my router wasn't connecting to pppoe i've tried to connect via the pc, it worked, then i started wireshark just to see what was going on, and i've seen that i can read all the PADI packets from ISP's other clients, now i'm thinking, is that a security problem? Can another user respond with a PADO packet and complete a pppoe session acting as gateway and maybe also sniffing pppoe credentials?
I remember an Article on the German news site heise.de which described a similar vulnerability at a German DSL provider:
The problem in that case was, that not only all the PPPoE packets were visible, but all packets. So if connected directly to a Windows computer, Windows would autoconfigure a private ip-address in the 126.96.36.199/8 range. Furthermore since this is another interface, Firewall rules for the internet-connection were not applied, exposing the Windows Network file and printer sharing.
So this is the next thing to check.