Comment 21 for bug 1721223

Starting with ephemeral lxd container, on a system with app/promote_secondaries set to 0:

root@systemd-sru-artful:~# grep -e 0 -e 1 /proc/sys/net/ipv4/conf/*/promote_secondaries
/proc/sys/net/ipv4/conf/all/promote_secondaries:0
/proc/sys/net/ipv4/conf/default/promote_secondaries:0
/proc/sys/net/ipv4/conf/eth0/promote_secondaries:0
/proc/sys/net/ipv4/conf/lo/promote_secondaries:0
root@systemd-sru-artful:~# dpkg-query -W systemd
systemd 234-2ubuntu12.1
root@systemd-sru-artful:~# grep DHCP= /run/systemd/network/10-netplan-eth0.network
DHCP=ipv4

And DHCP on eth0 interface.

Upgraded systemd to:
root@systemd-sru-artful:~# dpkg-query -W systemd
systemd 234-2ubuntu12.3

Observed that promote_secondaries, got enabled on networkd DHCP managed link:

root@systemd-sru-artful:~# grep -e 0 -e 1 /proc/sys/net/ipv4/conf/*/promote_secondaries
/proc/sys/net/ipv4/conf/all/promote_secondaries:0
/proc/sys/net/ipv4/conf/default/promote_secondaries:0
/proc/sys/net/ipv4/conf/eth0/promote_secondaries:1
/proc/sys/net/ipv4/conf/lo/promote_secondaries:0