Systemd's networkd will drop IP-Addresses configured by other processes like Keepalived's VRRP, when it is restarted. This may happens on updates.

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To prevent this following settings are required in /run/systemd/network/10-netplan-eth0.network

v242 or earlier (even if you do not use DHCP client in networkd)

[DHCP]
CriticalConnection=yes

v243 or newer

[Network]
KeepConfiguration=yes

If there is netplan in place these files will be generated.

Add critical: yes to /etc/netplan/50-cloud-init.yaml

eth1:
  addresses:
    - 192.0.2.67/24
  gateway4: 192.0.2.1
  nameservers:
    addresses:
      - 198.51.100.42
      - 198.51.100.53
    search:
      - example.lcal
  critical: yes

run netplan generate

This will add the CriticalConnection to your networkd configuration.

• This is the Upstream issue Show archive.org snapshot
• The Ubuntu Launchpad issue Show archive.org snapshot