diff --git a/doc/source/usage.rst b/doc/source/usage.rst
index 78f4fd6..b6aef6d 100644
--- a/doc/source/usage.rst
+++ b/doc/source/usage.rst
@@ -168,13 +168,19 @@ Where /tmp/ctlplane-dc contains::
         "cidr": "192.0.2.0/24",
         "allocation_end": "192.0.2.20",
         "allocation_start": "192.0.2.2",
-        "name": "ctlplane",
+        "name": "public",
+        "physical_network": "ctlplane",
         "segmentation_id": 25
       }
     }
 
-This creates a Neutron datacentre 'net' using VLAN tag 25, with the same
-details as the flat network created above.
+This creates a Neutron 'net' called ``public`` using VLAN tag 25, that uses
+the existing 'net' called ``ctlplane`` as a physical transport.
+
+ .. note::
+
+    The key ``physical_network`` is required when creating a network that
+    specifies a ``segmentation_id``, and it must reference an existing net.
 
 setup-neutron can also create two networks suitable for workload clouds::
 
diff --git a/os_cloud_config/neutron.py b/os_cloud_config/neutron.py
index 1b4e9de..28e2a72 100644
--- a/os_cloud_config/neutron.py
+++ b/os_cloud_config/neutron.py
@@ -88,9 +88,10 @@ def _create_net(neutron, network_desc, network_type, admin_tenant):
         network['router:external'] = True
     if type_desc.get('segmentation_id'):
         vlan_tag = type_desc['segmentation_id']
+        physical_network = type_desc['physical_network']
         network.update({'provider:network_type': 'vlan',
                         'provider:segmentation_id': vlan_tag,
-                        'provider:physical_network': 'datacentre'})
+                        'provider:physical_network': physical_network})
     return neutron.create_network({'network': network})
 
 
diff --git a/os_cloud_config/tests/test_neutron.py b/os_cloud_config/tests/test_neutron.py
index 6917c79..c409ab0 100644
--- a/os_cloud_config/tests/test_neutron.py
+++ b/os_cloud_config/tests/test_neutron.py
@@ -41,12 +41,14 @@ class NeutronTest(base.TestCase):
 
     def test_create_net_physical_vlan_tag(self):
         client = mock.MagicMock()
-        network = {'physical': {'name': 'ctlplane', 'segmentation_id': '123'}}
+        network = {'physical': {'name': 'public',
+                                'segmentation_id': '123',
+                                'physical_network': 'ctlplane'}}
         neutron._create_net(client, network, 'physical', 'admin_tenant')
         physical_call = {'network': {'tenant_id': 'admin_tenant',
                                      'provider:network_type': 'vlan',
-                                     'name': 'ctlplane',
-                                     'provider:physical_network': 'datacentre',
+                                     'name': 'public',
+                                     'provider:physical_network': 'ctlplane',
                                      'provider:segmentation_id': '123',
                                      'admin_state_up': True}}
         client.create_network.assert_called_once_with(physical_call)