I tested four modems to see what speeds I get with Xfinity Gigabit (model DLmbs/ULmbs): CM1000 950/46CM700 720/6CM600 720/6CM500 560/600 with , Xfinity offers 200, 600, 300, 600, 1000 in my area. So for gigabit you clearly need a CM1000 as it is DOCSIS 3.1 and xfinity has gigabit optimized for DOCSIS 3.1 and the boot process is also optimized for DOCSIS 3.1. If you have a data plan of 600Mbps or less even the CM500 will suffice as you will only see a maximum of 95% of your data plan, this also assumes you are not overly concerned with UL.
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