I just had a really bad experience with Comcast support today. This morning my internet access had become intermittent (but mostly not working). I could tell right away that something about the communication between my Linksys router and Motorola SB4100 cable modem was not right. The "link" light which is normally solid on the router was blinking on occasionally, but not staying on.
I did traceroutes between a PC and the modem (through the router), and a high rate of packet failure was evident. I swapped the router for another known good one, and the same behavior occurred. I also swapped cables and other things. The only thing left was the cable modem.
To really make matters confusing for Comcast, the cable modem worked fine when connected directly to a PC. So I spent about an hour on the phone with some tech support guy named "Mike" in which he insisted over and over and over that if the cable modem works with a PC then it is fully functional and is not the problem. WRONG!!!
Since cable modems are cheap now, I decided to run down to Office Max and get a cheap replacement to try out. I found a Best Data CMX300 for $60 and picked that up.
Got home, hooked up the new cable modem, and presto, everything works fine.
So for the record, it is quite possible for a cable modem to work fine with a PC but NOT WORK WITH A ROUTER. I have no idea technically why this can happen, but clearly it can.
I could hope the paid chimps that work for Comcast would read this forum, but I assume they don't.
P.S. We did get competent and friendly help from the Comcast people who helped us out with the new modem setup. It was just the moron named "Mike" that is giving Comcast a bad rep.