Comment 28 for bug 1671951

I launched a bionic image on serverstack, updated the netplan.io to proposed, modified the network config to set ipv6-mtu to 6000 and rebooted.

root@b-test-ipv6-mtu:~# apt-cache policy netplan.io
netplan.io:
  Installed: 0.98-0ubuntu1~18.04.1
  Candidate: 0.98-0ubuntu1~18.04.1
  Version table:
 *** 0.98-0ubuntu1~18.04.1 500
        500 http://nova.clouds.archive.ubuntu.com/ubuntu bionic-proposed/main amd64 Packages
        100 /var/lib/dpkg/status
     0.97-0ubuntu1~18.04.1 500
        500 http://nova.clouds.archive.ubuntu.com/ubuntu bionic-updates/main amd64 Packages
     0.40.1~18.04.4 500
        500 http://security.ubuntu.com/ubuntu bionic-security/main amd64 Packages
     0.36.1 500
        500 http://nova.clouds.archive.ubuntu.com/ubuntu bionic/main amd64 Packages

root@b-test-ipv6-mtu:~# cat /etc/netplan/50-cloud-init.yaml
network:
    version: 2
    ethernets:
        ens3:
            dhcp4: true
            match:
                macaddress: fa:16:3e:4d:3c:6a
            set-name: ens3
            ipv6-mtu: 6000

root@b-test-ipv6-mtu:/run/systemd/network# cat 10-netplan-ens3.link
[Match]
MACAddress=fa:16:3e:4d:3c:6a

[Link]
Name=ens3
WakeOnLan=off
root@b-test-ipv6-mtu:/run/systemd/network# cat 10-netplan-ens3.network
[Match]
MACAddress=fa:16:3e:4d:3c:6a
Name=ens3

[Network]
DHCP=ipv4
LinkLocalAddressing=ipv6
IPv6MTUBytes=6000

[DHCP]
RouteMetric=100
UseMTU=true

~# cat /sys/class/net/ens3/mtu
8958
~# sysctl net.ipv6.conf.ens3.mtu
net.ipv6.conf.ens3.mtu = 8958