Hello
I have a dell laptop that is connected using an Ethernet cable (50m of cat7) to a openWRT router.
The laptop is running Debian stable and on first connection the speed of the ethernet is 1000, after a certain amount of time/traffic (usually in the 15min to 1h range) the speed drops to 100.
Forcing the connection to 1000 using ethtool and disabling autonegotiation brings back speed to 1000 for a certain amount of time, then it automatically drops to 100 and autoneg goes back on.
I tried to force autoneg off on network manager, tried edit the /etc/network/interfaces to make sure that autoneg is off but it doesn't stay off.
The interface uses the R8169 kernel module, I have tried to interpose a switch but it still drops down to 100...
there are no network errors nor missing packets.
I am out of ideas..
Anything else I could try?
Happens on both Debian and Liquorix kernels. It also happens on R8168 dkms driver.

50m of cat7?

I'm glad to see cat7, but there is maximum distances that you can do with this stuff. I come at this from the hardware end. Before they started using the Henry as a measure of inductance, it used to be measured in feet. 50 metres = 164 feet. There will be also resistance and capacitance added, althougjh I presume they will be minimized in cat7, they can't be eliminated.

There's standard microchips and IP cores for 8169 [Actually 8111/8169/8411] Controllers - I have a couple myself. They just work. My guess is you're borderline on the length, or have low quality cable. I would eliminate the pc as a cause.

If you want to shut off autonegotiation, go at /etc/modprobe.d and a module option, if it's there. Byte checking is a 1 bit checksum, isn't it? If you're transmitting too fast, there is the possibility that two errors will cancel in the same byte. So your checksum will be correct, but your data could be wrong.