MB8611 Modem Breaks my working Seeed Carrier CM4 Based Router

I have a RPi CM4 based installation. It is running the the Seeed CM4 carrier board and has worked fine for a couple of years on 2 different ISPs (Verizon and Optimum). Verizon provided gigabit service via an ONT.

I find myself now in an Xfinity area with 1 GB service and purchased a Motorola MB8611 DOCSIS 3.1 cable modem. The router simply refused to work reliably with this modem. After a few minutes, it would constantly disconnect the WAN port and finally become inaccessible via the network. At one point I was able to ascertain that there were numerous hardware checksum errors on eth1.

I thought back to other issues that folks were having with some Verizon ONTs and IPv6 with hardware offload breaking connections. I decided to use ethtool to disable rx-checksumming, which is on for eth1 (a USB connected LAN7811 port) by default.

That did the trick and stabilized the router. So if anyone is using the Seeed CM4 based carrier board and is having speed based issues on the WAN port, try the following:

If not installed, install ethtool.

Try:

ethtool --offload eth1 rx-checksum off

If that stabilizes things, then create /etc/hotplug.d/iface/10-ethtool with the following contents:

 Turn off rx-checksum on WAN interface

[ ifup = "$ACTION" ] || exit 0
[ -n "$DEVICE" ] || exit 0

if [ "$DEVICE" == eth1 ] ; then
   ethtool --offload "$DEVICE" rx-checksum off
fi

Reboot and you should be stable.

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