Comment 68 for bug 1815101

Thank you for the minimal reproducer!

Using that, I can reconstruct the issue in a Kinetic LXD container and I can observe the logs as you do, especially the "eth0: Link DOWN" part.

Oct 20 13:43:47 test systemd-networkd[1276]: eth0: Reconfiguring with /run/systemd/network/10-netplan-eth0.network.
Oct 20 13:43:47 test systemd-networkd[1276]: bond_all: Reconfiguring with /run/systemd/network/10-netplan-bond_all.network.
Oct 20 13:43:47 test systemd-networkd[1276]: bond_all: DHCPv6 lease lost
Oct 20 13:43:47 test systemd-networkd[1276]: eth0: Link DOWN
Oct 20 13:43:47 test systemd-networkd[1276]: eth0: Lost carrier
Oct 20 13:43:47 test systemd-networkd[1276]: eth0: Configuring with /run/systemd/network/10-netplan-eth0.network.
Oct 20 13:43:47 test systemd-networkd[1276]: bond_all: Configuring with /run/systemd/network/10-netplan-bond_all.network.
Oct 20 13:43:47 test systemd-networkd[1276]: bond_all: DHCPv6 lease lost
Oct 20 13:43:47 test systemd-networkd[1276]: eth0: Link UP
Oct 20 13:43:47 test systemd-networkd[1276]: eth0: Gained carrier

I tried using override configs, but that didn't change anything.
# cat /etc/systemd/network/10-netplan-{eth0,bond_all}.network.d/override.conf
[Network]
KeepConfiguration=true

The same issue happens when calling "netplan apply", "networkctl reconfigure eth0/bond_all" (which netplan apply is calling under the hood) or "systemctl restart systemd-networkd.service".

I think there isn't a lot that netplan could do to avoid this, if sd-networkd keeps on insisting to restart the interface. Would you mind opening a bug report with the upstream systemd developers about this? https://github.com/systemd/systemd/issues