Got first sb6121 unit in prep for move to new house . Comcast tech groan when he hear I have `` Motorola DOCSIS 3 '' because of they extraordinarily long set-up time . Tech wait around for more than 70min after plugging-in modem to coax & power . keep cycling power-on , sync ` ing , then power-off - repeat . Tech claim be download firmware . as a `` Geek Class 5 ~ 6 '' -lrb- of 10 -rrb- who be I to argue as he do this for living . he be on phone with Comcast office with acronym , blah-blah while look at diagnostic tool . Modem would link-up with Internet long enough to see Comcast registration\/activation webpage , only to shut-down again . grrrr ! finally tech say must be bad unit - replace . Got rental `` Ubee 3.0 '' from van & I be on in -lrb- 5 min . Amazon to rescue with great replacement service . last night started-over with 2nd new Mtrola sb6121 - same thing ! go to bed with unit connect to coax & power hope any `` firmware '' download would sort-themselve by morning . unit stable in morning , until I plugged-in cat5e to computer - cycle back ! grrrr ! contact Comcast . give they the basics you would expect : serial # , MAC # , account info . they register modem detail on they side . they instruct I to reconnect Mtrla & my computer -lrb- include router between -rrb- . do so . launch browser . activation page stable & hit authorization . Modem cycle ! -lrb- `` oh carp ''  , I think -rrb- ... but thank God . . when modem come back-up authorization complete & now stable & equivalent speed as rental ubee . be my ~ 4th modem and by far the most aggravating process ! who be to blame ? Motorola , Tech , Comcast system ? no idea ... at least work now . better luck to you