Motorola is a legendary brand for communications equipment.  The SB6121 was instantly one of the most desirable dedicated cable modems when it was released.  It was supplied in brown-box editions by almost all of the major cable broadband providers due to it's excellent balance of low-cost, sturdy construction, advanced features, rock-solid reliability and and ease of configuration and maintenance. 

It is still one of the most popular Customer-Owned broadband modems.  Designed to DOCSIS 3.0 specifications, the SB6121 is a true Gigabit modem, with download speeds of up to 1216 Mbit/s (152 MByte/s) and Upload speeds of 126 Mbit/s (27 MBytes/s). DOCSIS 3.0 is still a current specification for all the major cable providers, and contrary to what some poorly trained customer service reps will tell you, the SB6121 is still supported by all major cable providers.  They will always recommend the most advanced modem available (DOCSIS 4.0), but you will not actually need these more advanced modems until you get over the 150 Mbps service tier.

So if you are an avid gamer or video freak, you will find this unit can support your performance requirements unless you have a large number of users sharing the connection.  A typical gaming connection requires only 5~30 MB/s, so you can easily have a game and a couple of video streams running over a 50 MB/s connection.  This modem can support up to 152 MB/s, so your Internet connection will be the only limiting bottleneck up to that level of service.  People with a large family or several gamers or several 4K video streams will need a more advanced connection, and this modem will top out at the 150 MB/s tier.

This device was my family's broadband device until we were offered a fiber connection for less money.  My network had almost 30 devices and 6 users, and we never had bandwidth issues sharing this modem.  We never had one service outage due to the modem.  I bought it because Comcast was going to start charging us a rental fee of $10 a month for a modem, and I got this one from Best Buy and never looked back.  There is a device setup screen with diagnostic information, but you will probably only use it once, if at all.  The cable provider does all setup remotely.  You will basically plug it in and call them to activate.  You just read them the MAC address from the bottom label or the setup screen, and they will do everything else. 

This is not a router or a wireless gateway, so you will need to do some configuration of your router or gateway to connect to the modem, but you probably already know how to do that. 

I did knock the unit off the back of the console once, and when it hit the floor, the case popped open on the bottom.  I heard some plastic rattling around, so I opened it up to inspect (I used to repair electronics for a living, so I know what I'm doing).  The identification label with the MAC address is glued across the seam at the bottom of the case, so I carefully sliced the label with a razor so it didn't become distorted or illegible from attempting to peel it off and reapply it after opening.  This was successful, as the label is still absolutely legible and appears perfect when pressing the case together.  It turned out that the two plastic tabs that lock the edges of the case together at the bottom had broken off and were loose inside.  There was no other damage and the unit continued to function perfectly.  There is a screw holding the case together, and the tabs are mainly for cosmetic reasons. 

The case is very sturdily built of thick, solid plastic and you would probably not even notice anything wrong if I didn't point it out, but I hate even minor disappointments like this if they are not disclosed, and I assume most of you are the same.  Motorola's quality and reliability are legendary for a reason.  There are no operational, or even appearance issues due to this, and the unit appears visually to be brand new except that you can see a small separation if you turn it over and look at the bottom.  If you think this will bother you, please don't order this device. I want you to be happy with your purchase, and let's not waste our time and money shipping this device back and forth.  There are other listings, and I won't be offended, I promise.

If this modem meets your performance requirements, I recommend you get it now.  You will not be disappointed.

As a little bonus gift to help convince you to buy my modem, I have prepared a bonus CD with the User Guide and other documentation I have gathered about this device over the years, and I will put the CD in the box for you.  This will put the User Manual and all the other information in your hands immediately and save you the time and effort of finding it.  I hope this will give my listing a little edge over the other similar listings and convince you to buy from me.  Thanks for reading all this and I hope you are satisfied with your purchase.