I have a C4000 router, and managed to get it working last night in transparent bridge mode. Settings to note follow:
(Interfaces/WAN)
WAN device pppoe0
IPv4 Config type PPPoE
MTU 1492
MSS 1460
Vlan priority "best effort"
(Interfaces/devices/point to point)
Type PPPoE
Link Interfaces (same as WAN is assigned)
Username
Password
(Interfaces/devices/VLAN)
Nothing needed.
On the centurylink router, I had to use Port 1, not the one that is labeled LAN/WAN
Forgot to mention:
(rules/settings/advanced)
Reflection for port forwards CHECKED
Automatic outbound NAT for Reflection CHECKED