SB6121 Cable Modem



This is currently a popular cable modem in use today and it has a very picky Ethernet negotiation characteristic.

The fix is simple, just lock down the Ethernet WAN port speed to 1000Mbps Full Duplex and uncheck the “Advertise Speed”. Save and apply and then reboot the cable modem, you are good to go.

You may not see this issue on all of our devices but it definitely applies to the Surf SOHO.


Surf SOHO MK3 fix with Arris /Motorola SB modems?
Surf_SOHO Not connecting to wlan
WAN interface failure with B20? Or?

So glad I found this post in the forum. This trick got me connected immediately, so please update your post to say that it also works on the ARRIS SB6183. Thanks!


I have this modem attached to a Balance 580. I get WAN failiure warnings from that device several times a day. The cable company says no problem on their end. I just made this change, hope this cures the problem!


Thank you so much for this great tip! I was getting desperate with that SB6121 failing to connect each time Comcast’s connection goes down and then back up, or when the modem powered was cycled. It took a manual procedure every time, to power cycle the SB6121 with the link between the Surf MK3 and the modem unplugged and then connecting the link after the modem had fully initialized… If I was not in the office when Comcast had an ‘event’ which they have a lot of, I would have no way to get the office back online until I get there…
With the fix above, the modem can cycle as much as it wants and the Surf MK3 just reconnects as it should…
Just another piece of info from when the I had the problem: when the modem cycles, it first succesfully connects to the Surf assigning a local IP address (it turns out they have a DHCP server for when the cable link is down). Then when the cable link gets established, it tries to renegotiate the connection and that is when the Surf Mk3 is unable to get it… that is, prior to the WAN port being locked at 1Gbps as recommended here.
Anyhow all fixed now, I don’t have to upgrade to another modem :grin: so thank you.