Comment 1 for bug 1791253

This is not related just to vlans, it's general configuration that doesn't get applied.

This is my netplan configuration, inside a LXD container:

network:
  version: 2
  ethernets:
    eth0:
      dhcp4: false
      addresses:
        - "10.4.10.110/16"
      gateway4: 10.4.0.1

And this is the output of "ip a":
root@bio-netplan:/etc/netplan# ip -c a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
31: eth0@if32: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 00:16:3e:9d:59:68 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.4.10.110/16 brd 10.4.255.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::216:3eff:fe9d:5968/64 scope link
       valid_lft forever preferred_lft forever

Now I want to change the ip from 10.4.10.110 to 10.4.10.220:

# sed -i "s/10.4.10.110/10.4.10.220/" 51-cloud-init.yaml

Now I run 'netplan apply'. That yields nothing (no error). But when I check my eth0 configuration I see that there are two IPs configured:

root@bio-netplan:/etc/netplan# ip -c a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
31: eth0@if32: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 00:16:3e:9d:59:68 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.4.10.110/16 brd 10.4.255.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet 10.4.10.220/16 brd 10.4.255.255 scope global secondary eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::216:3eff:fe9d:5968/64 scope link
       valid_lft forever preferred_lft forever
root@bio-netplan:/etc/netplan#

Doing the workaround by original poster 'fixes' the configuration:

root@bio-netplan:/etc/netplan# ip link set dev eth0 down
root@bio-netplan:/etc/netplan# ip link set dev eth0 up
root@bio-netplan:/etc/netplan# ip -c a
1: lo: <LOOPBACK,UP,LOWER_UP> mtu 65536 qdisc noqueue state UNKNOWN group default qlen 1000
    link/loopback 00:00:00:00:00:00 brd 00:00:00:00:00:00
    inet 127.0.0.1/8 scope host lo
       valid_lft forever preferred_lft forever
    inet6 ::1/128 scope host
       valid_lft forever preferred_lft forever
31: eth0@if32: <BROADCAST,MULTICAST,UP,LOWER_UP> mtu 1500 qdisc noqueue state UP group default qlen 1000
    link/ether 00:16:3e:9d:59:68 brd ff:ff:ff:ff:ff:ff link-netnsid 0
    inet 10.4.10.220/16 brd 10.4.255.255 scope global eth0
       valid_lft forever preferred_lft forever
    inet6 fe80::216:3eff:fe9d:5968/64 scope link tentative
       valid_lft forever preferred_lft forever
root@bio-netplan:/etc/netplan#

I did not have to wait few seconds between down/up though.