I just had a cable for Xfinity Internet installed. I purchased a new Motorola SB6121 cable modem for my internet connection. The installer could not get the modem to work as it continually would reboot after initializing. To find out if the problem was the cable or the modem, the installer brought in a Motorola SB6120 he had in his truck. He hooked it up and it worked perfectly. We both figured that the SB6121 modem was DOA. The next day I went to BestBuy and purchased a new Motorola SBG6580. I called Comcast to get the SBG6580 activated and the tech on the phone and I worked for for 45 minutes to get it going, but no luck. The tech was telling me that he just couldn't connect to the gateway even though it appeared that ethernet was working. Since we couldn't get it to work, I went back to the SB6120. Does anyone have any idea what might be the problem with the SBG6580, or what I could try next? I'm looking for a gateway (modem+router) or if there is no choice, a separate modem and wireless router.
I recontacted Comcast and the tech I spoke to told me to try one more new SBG6580 and if that did not work go to an Infinity gateway (at $7/month). I went back to Best Buy and got the second SBG6580, brought it back, called Comcast and it was up and running in 10 minutes! That's two brand new Motorola modems (SB6210 and SBG6580) from 2 different vendors both DOA! Incredible! A customer service person at Best Buy told me that 10% of the electronics fails out of the box! Whaaaat?? Anyway, it's been running perfectly for a little over a month so I have my fingers crossed. Good luck!