I recently setup BT infinity with a Cisco router, I was using the Openreach modem to terminate the vDSL connection and then the LAN port from this modem connected to the port on my router (Interface GigabitEthernetX)
The configuration required on the router side was pretty straight forward and can be seen below. No password is actually required and you can send any string, I used '1234' and it worked.
interface GigabitEthernetX description **WAN INTERFACE TO OPENREACH MODEM** no ip address duplex auto speed auto pppoe enable group global pppoe-client dial-pool-number 1 no cdp enable no routing dynamic ! interface Dialer1 ip address negotiated no ip unreachables ip mtu 1492 ip nat outside ip virtual-reassembly in encapsulation ppp ip tcp adjust-mss 1452 dialer pool 1 dialer-group 1 ppp authentication chap pap callin ppp chap hostname [email protected] ppp chap password 0 1234< ppp pap sent-username [email protected] password 0 1234 no cdp enable