Someone told me I can configure PPPoA on ethernet router like 831 and 871, but I don't know if it really true.
My goal is to use an ethernet router in conjunction with a ADSL modem indipendently from the tecnology used (ADSL, ADSL over ISDN) and to manage directly the public IP address from the ethernet router.
PPPoA *only* makes sense when you have an ATM interface on the router. A Cisco DSL router has an ATM interfaces (ie. an 837 or 877 you have int ATM 0) as PPPoA describes the format of the ATM AAL5 cells going out of the ATM interface over whatever layer-1 you have.
They may have been confused and meant you could do PPPoE on an 831 or 871, which is a somewhat simular (although yet very different in some ways) of doing virtually the same thing, but over Ethernet instead of AAL5.
You're really going to have to bite the bullet and deal with whatever gets thrown your way, and hang your 831 behind whatever access bridge/router the provider gives you, or stock up on the various 8xx boxes to deal with each situation as needed.