Comment 51 for bug 1671951

@Balint, after further testing, here's where things are:

-----------------
On LXD Containers
-----------------

On Bionic
  1) First boot: neither interface mtu, nor ipv6 mtu is applied.
  2) Restarting systemd-networkd: neither interface nor ipv6-mtu is applied
  3) netplan apply: interface mtu is set correctly, ipv6 mtu is not.

On Disco
  1) First boot: neither interface mtu, nor ipv6 mtu is applied.
  2) Restarting systemd-networkd: neither interface nor ipv6-mtu is applied
  3) netplan apply: both interface and ipv6 MTU are set correctly.

On Eoan
  1) First boot: neither interface mtu, nor ipv6 mtu is applied
  2) Restarting systemd-networkd: neither interface nor ipv6-mtu is applied
  3) netplan apply: both interface and ipv6 MTU are set correctly.

------
On VMs
------

On Bionic
  1) First boot: interface MTU is set, ipv6 MTU is not applied.
  2) Restarting systemd-networkd: both interface and ipv6 MTU are set correctly
  3) netplan apply not needed
  4) After reboot, same as (1)

On Disco
  1) First boot: interface MTU is set, ipv6 MTU is not applied.
  2) Restarting systemd-networkd: both interface and ipv6 MTU are set correctly
  3) netplan apply not needed
  4) After reboot, same as (1)

On Eoan
  1) First boot: interface MTU is set, ipv6 MTU is not applied.
  2) Restarting systemd-networkd: both interface and ipv6 MTU are set correctly
  3) netplan apply not needed
  4) After reboot, same as (1)