be run the sb6121 for a few week with a old e1000 router . I feel a little warm spot on top of the case , the rest of the box be cool , a little chunkier than the sb5100 , and full of perforation . have to call Comcast before the new modem start work : -lrb- Occupy Comcast after repower the sb6121 , it take longer to reconnect than the 5100 , then it work fine . the modem 's web-interface show that instead of one download frequency , now I have four , thanks to the Docsis 3 , although my download speed have not increase . url for the web interface be slightly different than the sb5100 , but if you start at the traditional 192.168.100.1 , you 'll get in . the web-interface webpage be very similar to the 5100 . slightly different light on the front panel . one day , I 'll look they up and see what they mean . -lrb- Update -- on my modem , the downstream light be blue , which mean the connection be use multiple downstream channel bond together , else the light would be green to indicate use a single downstream channel . the upstream light be green since my connection use only one upstream channel , but it would turn blue if the upstream bond multiple channel . whether you connection use single channel or multiple channel be determine automatically when the modem connect to the cable system . -rrb- I replace my 5100 because it be start to have trouble communicate ; long wait to connect , have to use process of elimination to determine the problem be the 5100 . my connection be stable again , but it be the same speed as before . take the router out of the system -- go straight from the sb6121 to a computer , I see a powerboost of 30 mbits\/sec for the first 15 seconds of download , and then 13 mbits\/sec for the duration . I think that be about what I get on the old modem , too . -lrb- with the e1000 router in line , the download speed be only 12 mbits\/sec and there be not any boost -- bottleneck be the router . -rrb-