This allows a user to restore the previous behavior where NetworkManager
would not reconfigure the MTU during device activation, if no MTU is
available (commit "22e8af6 device: set a per-device default MTU on
activation").
Well, not exactly. The previous behavior was to use per-connection
configuration, then DHCP provided value, or finally leave the MTU
unspecified.
Now, we prefer a per-connection configuration, followed by a global
connection default. If "ethernet.mtu=0", the MTU is left unspecified.
In absense of a global connection default, the value from DHCP is used
or finally a per-device-type default. That is effectively 1500 for most
types, except for infiniband where the MTU is still left unspecified.
be813707f0