Comment 12 for bug 1709135

# dpkg-query -W systemd nplan
nplan 0.23~16.04.1
systemd 229-4ubuntu19

# netplan generate
Error in network definition //etc/netplan/bond-primary.yaml line 13 column 8: unknown key primary

Enabled proposed, retweaked netplan to have correct interface names

# dpkg-query -W systemd nplan
nplan 0.29~16.04.1
systemd 229-4ubuntu20

root@srux:~# netplan generate
root@srux:~# echo $?
0

My current netplan is
$ cat /etc/netplan/bond.yaml
network:
  version: 2
  ethernets:
    ens7:
      match:
        name: ens7
    ens8:
      match:
        name: ens8
  bonds:
    bond1:
      interfaces: [ ens7, ens8 ]
      parameters:
        mode: active-backup
        mii-monitor-interval: 100
        primary: ens8
      dhcp4: true

Active slave is ens8. Changing netplan primary to ens7, and re-running apply changes the active slave.