to decide which DOCSIS 3.0 Cable Modem to use , 1st go to DOCSIS Devices MyDeviceInfo on Comcast to see what they support and possibly recommend by certification . for my money , only 2 option :  Motorola SB6120 SURFboard DOCSIS 3.0 eXtreme Broadband Cable Modem  choose the later , newer model . more widely use sb6120 be test only to level 2 . Comcast claim to have test sb 6121 to more stringent level 3 certification . Hope to future proof with use this newer model . have not notice , as other , heat to be a problem . Install & SetUp be easy . help reading comment from user -lrb- Comcast employee -rrb- with sb6120 . do not try Comcast self activation web page since this be a modem change and not a new install . plug sb6121 into WAN Cable with nothing else attach . then call Comcast Tech support . change from Linksys BEFCMU10 v4 DOCSIS 2.0 to Motorola SB6121 DOCSIS 3.0 . Comcast see Device , send refresh signal . Voila ! then connected pc direct to Cable Modem . finish with Comcast Tech support . take maybe 20 minute on phone to verify all be work . final hookup with router , in the mix , and clone pc MAC Address into Router . do processing ! accord to sb6121 log , sw download successful - Via Config file , SW Download INIT - Via Config file . MAC Address Identified . light , Camera Action . Houston , we have connection ! be it faster than what we have ? no. . why Upgrade ? concerned about end of Life Devices for what we have .  cisco-linksy befcmu10 Ethernet Cable Modem  prior version do . piece of mind to stay one step ahead of obsolescent technology . why buy vs. rent ? not fond of rent . will pay for its self in less than a year . try RoadRunner Cable Modem then later try Comcast Cable Modem neither of which would connect reliably . both have weak signal require a service call mean down time . purchase Cable modem would always work . have own 2 linksy befcmu10 v4 DOCSIS 2.0 Cable modem . purchase for $ 20.00 each -lrb- elsewhere -rrb- when they be sell for $ 80.00 each , so have a backup . First Linksys befcmu10 v4 buy the farm during a lighten strike . yes , even on a UPS . the lighten strike take out the Cable Modem and a D-Link 8 Port Gigabit switch . two router , in line for security , in between be untouched . now that I think of it , 1 router lose 1 Port leave 3 more port to use . unfortunate but still a usable device . second linksy befcmu10 v4 have work until now . have sit on shelf for backup \/ book end . happy with decision to go sb6121 Gigabit Cable Modem to  D-Link DIR-655 Extreme N Gigabit Wireless Router  and if we ever upgrade we Comcast to 100baset or Comcast upgrade to ipv6 , wow . I can dream !