I have a 2811 router and no space for equipment. I put a NM-16ESW switch module in it and it worked great. Later I needed POE for a number of devices, so I installed an HWIC-D-9ESW-POE card with a new power supply. Now I can connect on any device on the HWIC but not the NM-16ESW.
It's all the same VLAN, but I can't even ping the VLAN interface from a device connect to the NM-16ESW. Plug it into the HWIC and the device can ping up a storm.