When can I used L2 forwarding? If I configured L2 forwarding, do I still need to configure the proper commands (ip wccp 61 redirect in, etc) on the router?
The forwarding method determines how traffic is transmitted between the WCCP-enabled router/switch and the WAAS device. You still have to configure the global and interface command in IOS. In fact, the forwarding method used in configured on the WAAS device, not the router/switch.