"The ifupdown plugin also uses [...] NetworkManager.conf (0.8.1 and later) for some configuration. All ifupdown-specific options go in a "[ifupdown]" section. If the "managed" key is set to "false", then any device listed in /etc/network/interfaces will be completely ignored by NetworkManager. Remember that NetworkManager controls the default route, so because the device is hidden, NetworkManager will assign the default route to some other device."
Here's a reference with some relevance for the present report.
https:/ /live.gnome. org/NetworkMana ger/SystemSetti ngs#ifupdown
"The ifupdown plugin also uses [...] NetworkManager.conf (0.8.1 and later) for some configuration. All ifupdown-specific options go in a "[ifupdown]" section. If the "managed" key is set to "false", then any device listed in /etc/network/ interfaces will be completely ignored by NetworkManager. Remember that NetworkManager controls the default route, so because the device is hidden, NetworkManager will assign the default route to some other device."