DHCP sets 9000 MTU while wired interface supports 1500 - large packets kill the interface

[liotier]

When told by DHCP, the Raspberry Pi sets a 9000 bit MTU on its wired Ethernet interface that only supports up to 1500. Large packets as good as a cut cable - not recoverable without resetting the network from the local console.

Administrative fix is to comment out "option interface_mtu" in /etc/dhdpcd.conf so that the DHCP client ignores DHCP's MTU setting. But that should not be needed: the Raspberry Pi should just ignore MTU settings larger that supported by the interface.

Reproduced on Pi3B and Pi1B, both with install from 2020-08-20-raspios-buster-armhf image, kernel "5.4.72+ 1356 Thu Oct 22 13:56:00 BST 2020".

This may be related to #2660 but that issue has been closed for two years and that problem should be fixed.

[SkyBeam]

I just faced this problem on OctoPi. Still I do not fully understand it. Most precisely it's described here. The interface MTU visible on Pi OS is clearly 1500 but somehow the DHCP option to set MTU 9000 is interpteted by Pi OS and causing this issue.