Netgear CG3000D cable modem failure

UPDATE 2! This blog entry is incorrect and has been obsoleted this one: Netgear CG3000D cable modem keeps resetting  Please refer to the updated blog entry for the new, correct information about this problem and how to resolve it.

UPDATE! The brand NEW-IN-BOX Netgear CG3000D cable modem/router that the technician installed yesterday is now doing the exact same thing as the supposedly failed unit!  I guess the problem is something more complicated on Cox's end, and wasn't a cable modem failure after all.  The following is an article I wrote yesterday while I still believed that my original cable modem had failed and the resolution to the problem was to install a new cable modem.  I was wrong about that.  Now I get to call Cox again for more technical support!  (The ticket number is 1542531 for escalation to a higher tier technical support.)

Well I had a lengthy internet service outage that turned out to be a cable modem failure.  This is the first outage of my Cox internet service that could arguably be my fault. I don't know what changed or what failed but the modem was working fine for about 6 months and then last Saturday night I started having a weird problem with my connection.

I have a Netgear CG3000D cable modem+router integrated device connected to Cox cable internet service.  The signals are generally good and are so strong that as a result of one of my previous outages the Cox service technician installed an attenuator to decrease the downlink signal power level which would also cause the modem to boost its uplink signal power.

During this lengthy outage, the modem/router would reset and my internet connection would drop and re-establish itself on a cycle repeating every couple of minutes.  I was running a continuous ping and somewhere between 7-15 pings would get through before it would drop again.  The modem repeated this cycle continuously for hours.  The total outage time was about 2 days - 48 hours!  Here is the sequence of events I observed the Netgear CG3000D cable modem/router continuously repeating:

  • These 6 LEDs were on: power, uplink, downlink, "internet," one of the switch ports, and the wifi LED.
  • The uplink and downlink LEDs would turn blue.
  • All the LEDs would turn off.
  • All the LEDs would flash for a while, then all turn off for a moment. 
  • The power LED would flash for a moment then turn on solid.
  • A moment later (or at the same time?) I think the wifi LED would turn on solid.
  • The downlink LED would flash for a few moments then turn on solid.
  • The uplink LED would flash for a few moments then turn on solid.
  • The internet LED would flash for a few moments then turn on solid.
  • With all 6 LEDs on solid for maybe a minute, the connection would eventually come up and a few pings would get through.
  • Then the uplink and downlink LEDs would turn blue, and this cycle would repeat.

The switch port LED is not really of interest so I didn't mention it much above.  I had an ethernet cable connected to another ethernet switch, but there was no traffic so the light was on solid most of the time.  I think the switch port LED would blink when the other LEDs were flashing but then was on solid after the switch ASIC was initialized.

The internet LED apparently indicates if the router was able to get a DHCP IP address from the network and if network access is allowed.  Generally speaking, if that is on, then you can reasonably expect to have internet access through the router and cable modem, provided your system's networking is set up properly.


I'm not sure what changed in the network or modem on Saturday to make it stop working but the fact that it would come online and stay online for about a minute, long enough for IP networking to stabilize and get through, makes me unsure that the problem is a failed modem. I'm not a DOCSIS expert and I invite corrections to this, but it seems like if it can modulate and demodulate the signals well enough to come online, then the problem of continuous resetting might indicate a problem somewhere else in the network, or a provisioning, or configuration file issue, or something else that Cox changed.  One of the technical support people I spoke with said that there were some really bad signal issues on my line (as reported by their monitoring systems and by my modem) but even after those were resolved it kept resetting according to the pattern I listed above.

Cox routinely releases software updates for the modems that they allow on their network, and I've noticed version number changes in the past.  Maybe a software version update went awry.  I've also had trouble with my modem getting de-provisioned in their back-end system on more than one occasion.  Once the network access was listed as Denied on the modem status web page, which caused an outage of over 12 hours until the problem went away on its own (without me doing anything about it).  Obviously I'm quite certain someone at Cox fixed something, but you'll never find out what was fixed or changed to make it start working again - as a Cox customer your only option is to be thankful that at least it started working again and hope for the best that it stays working for a while.  This probably is the same customer experience as everyone else who has a cable modem.

During this outage I called in several times and I did get one particularly helpful customer service person.  He looked into it and said they were seeing a lot of uncorrectable errors from my modem.  Presumably this means that the packets (frames?) being transmitted by my modem were malformed or corrupt or something and could not be processed by the headend.

I'm still not convinced that I did have a "modem failure" and that the problem was not something that Cox changed or that went wrong with some update they pushed out to my modem, but anyway the service technician installed a new modem which she had in her truck and the connection has been stable ever since then.  I restored my configuration settings from the backup file I made the last time the router lost all my settings, and it's been working OK ever since.

Knock on wood.