Comment 3 for bug 1936842

Looking at /usr/sbin/fanctl, it looks like the Fan MTU is always either 1480 or (usually) 1450.

case "$C_flag_type" in
ipip*)
        C_tun_control="type ipip"
        C_bridge_mtu=1480
        ;;
vxlan)
        # XXX: check that the overlay width is not more than 24 bits.

        # Work out the vxlan ID, which is our overlay >> 8 bits to fix in the
        # vxlan nid.
        local vxlan_id="$(( $overlay_ipnum >> 8 ))"

        C_tun_control="type vxlan id $vxlan_id dev $C_underlay_dev dstport 0"
        C_bridge_mtu=1450
        ;;
esac

The Juju container manager configures the LXD NIC with the MTU of the parent device, so we will have to look at options here.