Comment 55 for bug 1671951

In my test I modified a multipass launched VM's netplan config to:

multipass@safe-hornet:~$ cat /etc/netplan/50-cloud-init.yaml
# This file is generated from information provided by
# the datasource. Changes to it will not persist across an instance.
# To disable cloud-init's network configuration capabilities, write a file
# /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg with the following:
# network: {config: disabled}
network:
    ethernets:
        ens3:
            dhcp4: true
            mtu: 1496
            match:
                macaddress: 52:54:00:4c:5b:ac
            set-name: ens3
            dhcp4-overrides:
                    use-mtu: false
            dhcp6-overrides:
                    use-mtu: false
            ipv6-mtu: 1284
    version: 2

Also disabled cloud-init overwriting it:

multipass@safe-hornet:~$ cat /etc/cloud/cloud.cfg.d/99-disable-network-config.cfg
network: {config: disabled}

IPv6 MTU is properly set on Eoan after reboot:

multipass@safe-hornet:~$ sysctl net.ipv6.conf.ens3.mtu
net.ipv6.conf.ens3.mtu = 1284

... but not on Disco or Bionic. To be continued...