default to no network-device-mtu

config.yaml

@@ -50,9 +50,10 @@ options:
     description: Name of the external network for floating IP addresses provided by Neutron.
   network-device-mtu:
     type: int
-    default: 1500
+    default: 0
     description: |
-      The MTU size for the interfaces managed by neutron.
+      The MTU size for the interfaces managed by neutron. If set to 0, no value
+      will be applied.
   neutron-plugin:
     default: ovs
     type: string

hooks/neutron_api_hooks.py

@@ -282,8 +282,12 @@ def neutron_plugin_api_relation_joined(rid=None):
             'neutron-security-groups': config('neutron-security-groups'),
             'l2-population': get_l2population(),
             'overlay-network-type': get_overlay_network_type(),
-            'network-device-mtu': config('network-device-mtu'),
         }
+
+        mtu = config('network-device-mtu')
+        if mtu:
+            relation_data['network-device-mtu'] = mtu
+
     relation_set(relation_id=rid, **relation_data)
 
 

unit_tests/test_neutron_api_hooks.py

@@ -273,6 +273,21 @@ class NeutronAPIHooksTests(CharmTestCase):
         self.assertTrue(self.CONFIGS.write.called_with(NEUTRON_CONF))
 
     def test_neutron_plugin_api_relation_joined_nol2(self):
+        _relation_data = {
+            'neutron-security-groups': False,
+            'l2-population': False,
+            'overlay-network-type': 'vxlan',
+        }
+        self.get_l2population.return_value = False
+        self.get_overlay_network_type.return_value = 'vxlan'
+        self._call_hook('neutron-plugin-api-relation-joined')
+        self.relation_set.assert_called_with(
+            relation_id=None,
+            **_relation_data
+        )
+
+    def test_neutron_plugin_api_relation_joined_w_mtu(self):
+        self.test_config.set('network-device-mtu', 1500)
         _relation_data = {
             'neutron-security-groups': False,
             'l2-population': False,