diff --git a/config.yaml b/config.yaml
index ac543d3c..580987cd 100644
--- a/config.yaml
+++ b/config.yaml
@@ -156,8 +156,8 @@ options:
       HA Cluster nodes.
   phy-nic-mtu:
     type: int
-    default: 1500
+    default: 0
     description: |
       To improve network performance of VM, sometimes we should keep VM MTU as
       1500 and use charm to modify MTU of tunnel nic more than 1500 (e.g. 1546
-      for GRE).
+      for GRE). A value of zero means no mtu will be set/modified.
diff --git a/hooks/quantum_contexts.py b/hooks/quantum_contexts.py
index eb52dca4..6e8769a1 100644
--- a/hooks/quantum_contexts.py
+++ b/hooks/quantum_contexts.py
@@ -176,12 +176,15 @@ class L3AgentContext(OSContextGenerator):
 class ExternalPortContext(NeutronPortContext):
 
     def __call__(self):
+        ctxt = {}
         port = self.resolve_port('ext-port')
         if port:
-            return {"ext_port": port,
-                    "mtu": config('phy-nic-mtu')}
-        else:
-            return None
+            ctxt = {"ext_port": port}
+            mtu = config('phy-nic-mtu')
+            if mtu:
+                ctxt['ext_port_mtu'] = mtu
+
+        return ctxt
 
 
 class DataPortContext(NeutronPortContext):
diff --git a/templates/ext-port.conf b/templates/ext-port.conf
index 41bafc49..1d850240 100644
--- a/templates/ext-port.conf
+++ b/templates/ext-port.conf
@@ -6,7 +6,7 @@ task
 
 script
     EXT_PORT="{{ ext_port }}"
-    MTU="{{ mtu }}"
+    MTU="{{ ext_port_mtu }}"
     if [ -n "$EXT_PORT" ]; then
         ip link set $EXT_PORT up
         if [ -n "$MTU" ]; then
diff --git a/unit_tests/test_quantum_contexts.py b/unit_tests/test_quantum_contexts.py
index be8c23bb..f3f8b00e 100644
--- a/unit_tests/test_quantum_contexts.py
+++ b/unit_tests/test_quantum_contexts.py
@@ -144,7 +144,7 @@ class TestNeutronPortContext(CharmTestCase):
     def test_no_ext_port(self, mock_config):
         self.config.side_effect = config = self.fake_config({})
         mock_config.side_effect = config
-        self.assertIsNone(quantum_contexts.ExternalPortContext()())
+        self.assertEquals(quantum_contexts.ExternalPortContext()(), {})
 
     @patch('charmhelpers.contrib.openstack.context.config')
     def test_ext_port_eth(self, mock_config):
@@ -152,7 +152,7 @@ class TestNeutronPortContext(CharmTestCase):
         self.config.side_effect = config
         mock_config.side_effect = config
         self.assertEquals(quantum_contexts.ExternalPortContext()(),
-                          {'ext_port': 'eth1010', 'mtu': None})
+                          {'ext_port': 'eth1010'})
 
     @patch('charmhelpers.contrib.openstack.context.get_nic_hwaddr')
     @patch('charmhelpers.contrib.openstack.context.list_nics')
@@ -173,13 +173,13 @@ class TestNeutronPortContext(CharmTestCase):
         mock_get_nic_hwaddr.side_effect = self._fake_get_hwaddr
 
         self.assertEquals(quantum_contexts.ExternalPortContext()(),
-                          {'ext_port': 'eth2', 'mtu': None})
+                          {'ext_port': 'eth2'})
 
         config = self.fake_config({'ext-port': self.absent_macs})
         self.config.side_effect = config
         mock_config.side_effect = config
 
-        self.assertIsNone(quantum_contexts.ExternalPortContext()())
+        self.assertEquals(quantum_contexts.ExternalPortContext()(), {})
         self.assertTrue(mock_config.called)
 
     @patch('charmhelpers.contrib.openstack.context.get_nic_hwaddr')
@@ -205,7 +205,7 @@ class TestNeutronPortContext(CharmTestCase):
         self.config.side_effect = config
         mock_config.side_effect = config
         self.assertEquals(quantum_contexts.ExternalPortContext()(),
-                          {'ext_port': 'eth2', 'mtu': 1234})
+                          {'ext_port': 'eth2', 'ext_port_mtu': 1234})
 
     @patch('charmhelpers.contrib.openstack.context.config')
     def test_data_port_eth(self, mock_config):