Question Random intermittent bursts of high ping when router plugged in


Dec 20, 2013
Best answers
I have an ethernet connection from my desktop to my Linksys AC5400 router, which is plugged into the Comcast-provided Cisco DPC3941B modem. Most of the time, my Internet is fine, but approximately once every 2 to 3 minutes I get a 15-to-30-second span of high (>300ms) ping. I've had two Comcast technicians come out and say that there are no problems with the line, but I can ping the modem from my desktop and it's always normal.

Today I tried plugging my desktop into the modem directly, and I still had the ping spikes. Then I unplugged everything else from it, and the ping spikes stopped, as far as I can tell. (They're intermittent, so it's possible that I just didn't wait for long enough, but that's unlikely.) I plugged my router back in, and the ping spikes came back, even though my desktop was still connected directly.

I'm surprised that the connection quality from the modem would be affected by another device plugged into it when it's not under heavy load. If it is, is there anything I can do about it?


Aug 9, 2012
Best answers
The device you are calling a modem is actually a modem/router. Since you have another router I would see if you can convert the cisco box into a modem only. It likely is a bridge option but I don't know I have never used that box. Your ISP may help you to accomplish this.

This should make it simpler to troubleshoot and maybe you get lucky and the problem goes away.

It is kinda strange because to get high ping times the data must be stored someplace. The hard part is finding that device. Normally this is because of overload and this problem is called bufferbloat. When it does it when there is no load it almost has to be some kind of bug. Normally you could try firmware upgrades but with cable modem the ISP is in control even when you own your own router. They like to keep all the devices on the same patch level just to make support easier.

SO after I posted this I pulled that device up since I never looked at it. This is one of the devices that had the PUMA intel bug. The symptoms you get are from that bug are not the same as you describe but are pretty close. Many modems/routers have been patched and the problem is undetectable in real world traffic. The test program looking for puma can still detect it but at a much lower amount.

If this was a arris I could give you the patch level so you could check. Still only the ISP can update the firmware.
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