Comcast isn't consistent with this and it would also depend upon the cable modem they have issued you. They gave me a "SurfBoard" modem one time after an upgrade and it would not, under any circumstances allow me to place anything other then a computer on the other end of the cable, thus my linksys was a doorstop. I bawled them out, got it changed to a different make of modem and away I went.

With regard to the MAC spoofing, I have people within a mile or so of my house that require the linksys to spoof the MAC address of a PC previously connected while I actually don't need to at home. Comcast often seems to upgrade the firmware of the modems serruptitiously at night resulting in MAC spoofing is required I have reached the stage where I set up MAC spoofing anyway for friends since it saves me getting a call later saying that their cable doesn;t work....

I'm not sure I'm entirely understanding the router config part but I can assure you that different modems handle the cable connection and the connection back to the customers device quite differently. For the most past the connection is pretty transparent but some of them make you jump through a load of silly hoops.

Thinking back though, I see less of the hoops today than I did a year or so ago. I know I bawled them out more than once about preventing people from using firewalls/routers and I think their motivation back then was to sell additional IP's. I think that strategy may have changed since they have realized that making it too difficult for the average user to adequately protect themselves is more contentious than not getting the $5/month for an additional IP address.