@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)
@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)