Switch the Cisco 678 from CAP to DMT
If you download the DMT firmware upgrade and follow the instructions EXACTLY you will get a working Cisco 678 with DMT that was previously set at CAP. The setup commands for DMT are different than for CAP.
The Cisco 677 and Cisco 678 allow you to choose transmission protocols to match your network configuration by changing the router's configuration file. Use TFTP to transfer files to and from a CPE.
See: Configuration Procedures for the Cisco 67x - section Configure for DMT
The Cisco 678 does not internally support PPPoE authentication. If your ISP is PPPoE only, you will need to put the modem into bridged mode and use a device (PC or router for example) to do the PPPoE authentication.
See also: Ios