Random short connection drops behind cable modem

Installing and Using OpenWrt
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I have a Netgear R7800 with the latest version of OpenWRT behind an Arris TG862S which is set to bridge mode. Most of the time everything is working fine, but every few hours the connection drops for a short time. Internet is back up right after about one minute. While the wan connection is, the internal network is fine. If i use the Arris Modem for routing i do not experience such drops. I used to have an Edgerouter X instead of the Netgear and also had these problems, but i put the blame on the Edgerouter. Now it seems it is either the cablemodem or OpenWRT which is causing the problems.
Any idea what could cause this behaviour or how to pin down the problem?

Thanks!

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Anything usefull in the logs?

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My guess would be the short WiFi connectivity breakage fixed yesterday with a backport from upstream Linux mac802011.

I have noticed similar momentary breakages with R7800 in the last few months. Apparently a bug has been introduced to mac80211 late last year, and now it has been debugged and fixed (actually by the ath10k-ct author..)

https://git.openwrt.org/?p=openwrt/openwrt.git;a=commitdiff;h=dfdb28c24aa60cf057ec45cbc11ab48ba2655f53

(That was written assuming that you noticed breakage on wifi devices... I have not seen breakage on wired devices, lately)

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The problems with my setup are with the wired lan connection. I use a raspi connected to one of the ethernet ports of the Netgear to monitor the connection.

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There is nothing going on in the system or the kernel log when the connection fails. I just have disabled SQM to see if this makes any difference.

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May not necessarily apply for your situation, but some Docsis3 Arris cable modems handed out by ISPs are known for memory leaks at least for a year now.
I have even seen github projects recently that aimed to be used to reboots of such buggy Arris routers once every night as workaround, since the ISP so far has not yet patch the memory leak problem.

Could be that your connection drop comes due to a out of mem hangup related reset, which then besides causing a reconnect also causes some RAM to be released, leading to new hours of flawless operation, until RAM fills up again due to memory leaks, restarting the cycle.

And could be that every time you have recently tested without OpenWRT router, with clients directly attached to the cable router that without noticing you have also resetted the Arris router just before starting your test, causing that you effectively cleared up RAM for a few hours, leading to flawless operation for several hours.

Maybe try for a few days, to turn off and on or hard-resetting your Arris at a given time.
Then keep an eye on it, whether the connection problem is temporarily gone for a while after such a „planned“ reset.

You might also want to keep an eye on free mem of your OpenWRT. (But actually the „Arris“ keyword of your story kind of triggered the mem leak deja vu for me.)

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It could be related to this issue: Random packet loss with OpenWrt

I tried to mtr the modem IP (192.168.100.1) with a significant packet loss:

Start: 2021-04-13T09:39:38+0200
HOST: raspberrypi                 Loss%   Snt   Last   Avg  Best  Wrst StDev
  1.|-- OpenWrt.lan               45.0%   100    0.7   0.7   0.5   1.0   0.1
  2.|-- 192.168.100.1              0.0%   100    1.8   2.0   1.5   2.6   0.3
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I now have a new CableModem (Arris TG2492S-85) and the problems seem to be gone (at least there has not been a single connection drop for 24h).

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