The configs below show how to setup PPPoE for use with CenturyLink. The main difference between the configs is that the 871 can connect directly to the CenturyLink infrastructure where as the ASA requires one of their modems in transparent bridging mode. This is because the 871 has the ability to to tag outgoing packets on 201 (apparently required by Century Link).
871-ISR <-> ONT
ASA-5505 <-> C2000A <-> ONT