Fix the netplan defaults.

defaults/main.yml

@@ -30,7 +30,7 @@ ubuntu_second_int_name: "enp4s0"
 ubuntu_netplan_interfaces:
   - name: "{{ ubuntu_second_int_name }}"
     dhcp4: true
-    mtu: 1500
+    dhcp_overrides: true
 disable_ipv6: false
 ipv6_sysctl_value: 1
 ipv6_sysctl_file: /etc/sysctl.d/10-ipv6-disable.conf

templates/70-ansible.yaml.j2

@@ -4,11 +4,11 @@ network:
 {% for int in ubuntu_netplan_interfaces %}
         {{ int.name }}:
             dhcp4: {{ int.dhcp4 }}
-            mtu: {{ int.mtu }}
 {% if int.dhcp_overrides | default(true) %}
             dhcp4-overrides:
               use-dns: false
               use-routes: false
+              use-mtu: true
 {% endif %}
 {% endfor %}