diff --git a/xos/synchronizer/model_policies/model_policy_fabriccrossconnectserviceinstance.py b/xos/synchronizer/model_policies/model_policy_fabriccrossconnectserviceinstance.py
index ea591dd..db439a3 100644
--- a/xos/synchronizer/model_policies/model_policy_fabriccrossconnectserviceinstance.py
+++ b/xos/synchronizer/model_policies/model_policy_fabriccrossconnectserviceinstance.py
@@ -14,7 +14,7 @@
 # limitations under the License.
 
 
-from synchronizers.new_base.modelaccessor import FabricCrossconnectServiceInstance, model_accessor
+from synchronizers.new_base.modelaccessor import FabricCrossconnectServiceInstance, ServiceInstance, model_accessor
 from synchronizers.new_base.policy import Policy
 from synchronizers.new_base.exceptions import *
 
@@ -39,6 +39,48 @@
                 service_instance.delete()
             return
 
+        # If there is a westbound link, then make sure the SerivceInstance is consistent with the
+        # westbound fields.
+
+        if service_instance.provided_links.exists():
+            updated_fields = []
+
+            si = ServiceInstance.objects.get(id=service_instance.id)
+            s_tag = si.get_westbound_service_instance_properties("s_tag")
+            switch_datapath_id = si.get_westbound_service_instance_properties("switch_datapath_id")
+            source_port = si.get_westbound_service_instance_properties("switch_port")
+
+            if (s_tag is None):
+                raise Exception("Westbound ServiceInstance s-tag is None on fcsi %s" % service_instance.id)
+
+            if (not switch_datapath_id):
+                raise Exception("Westbound ServiceInstance switch_datapath_id is unset on fcsi %s" % service_instance.id)
+
+            if (source_port is None):
+                raise Exception("Westbound ServiceInstance switch_port is None on fcsi %s" % service_instance.id)
+
+            s_tag = int(s_tag)
+            source_port = int(source_port)
+
+            if (s_tag != service_instance.s_tag):
+                if service_instance.s_tag is not None:
+                    raise Exception("Westbound ServiceInstance changing s-tag is not currently permitted")
+                service_instance.s_tag = s_tag
+                updated_fields.append("s_tag")
+            if (switch_datapath_id != service_instance.switch_datapath_id):
+                if service_instance.switch_datapath_id:
+                    raise Exception("Westbound ServiceInstance changing switch_datapath_id is not currently permitted")
+                service_instance.switch_datapath_id = switch_datapath_id
+                updated_fields.append("switch_datapath_id")
+            if (source_port != service_instance.source_port):
+                if service_instance.source_port is not None:
+                    raise Exception("Westbound ServiceInstance changing source_port is not currently permitted")
+                service_instance.source_port = source_port
+                updated_fields.append("source_port")
+
+            if updated_fields:
+                service_instance.save(update_fields = updated_fields)
+
     def handle_delete(self, service_instance):
         log.info("Handle_delete Fabric-Crossconnect Service Instance", service_instance=service_instance)
 
diff --git a/xos/synchronizer/model_policies/test_model_policy_fabriccrossconnectserviceinstance.py b/xos/synchronizer/model_policies/test_model_policy_fabriccrossconnectserviceinstance.py
new file mode 100644
index 0000000..0099881
--- /dev/null
+++ b/xos/synchronizer/model_policies/test_model_policy_fabriccrossconnectserviceinstance.py
@@ -0,0 +1,255 @@
+# Copyright 2017-present Open Networking Foundation
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+# http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+
+import unittest
+
+import functools
+from mock import patch, call, Mock, PropertyMock, MagicMock
+import requests_mock
+import multistructlog
+from multistructlog import create_logger
+
+import os, sys
+
+# Hack to load synchronizer framework
+test_path=os.path.abspath(os.path.dirname(os.path.realpath(__file__)))
+xos_dir=os.path.join(test_path, "../../..")
+if not os.path.exists(os.path.join(test_path, "new_base")):
+    xos_dir=os.path.join(test_path, "../../../../../../orchestration/xos/xos")
+    services_dir = os.path.join(xos_dir, "../../xos_services")
+sys.path.append(xos_dir)
+sys.path.append(os.path.join(xos_dir, 'synchronizers', 'new_base'))
+# END Hack to load synchronizer framework
+
+# generate model from xproto
+def get_models_fn(service_name, xproto_name):
+    name = os.path.join(service_name, "xos", xproto_name)
+    if os.path.exists(os.path.join(services_dir, name)):
+        return name
+    else:
+        name = os.path.join(service_name, "xos", "synchronizer", "models", xproto_name)
+        if os.path.exists(os.path.join(services_dir, name)):
+            return name
+    raise Exception("Unable to find service=%s xproto=%s" % (service_name, xproto_name))
+# END generate model from xproto
+
+def mock_get_westbound_service_instance_properties(props, prop):
+    return props[prop]
+
+def match_json(desired, req):
+    if desired!=req.json():
+        raise Exception("Got request %s, but body is not matching" % req.url)
+        return False
+    return True
+
+class TestPolicyFabricCrossconnectServiceInstance(unittest.TestCase):
+
+    def setUp(self):
+        global DeferredException
+
+        self.sys_path_save = sys.path
+        sys.path.append(xos_dir)
+        sys.path.append(os.path.join(xos_dir, 'synchronizers', 'new_base'))
+
+        # Setting up the config module
+        from xosconfig import Config
+        config = os.path.join(test_path, "../test_fabric_crossconnect_config.yaml")
+        Config.clear()
+        Config.init(config, "synchronizer-config-schema.yaml")
+        # END Setting up the config module
+
+        from synchronizers.new_base.mock_modelaccessor_build import build_mock_modelaccessor
+        build_mock_modelaccessor(xos_dir, services_dir, [get_models_fn("fabric-crossconnect", "fabric-crossconnect.xproto")])
+        import synchronizers.new_base.modelaccessor
+
+        from mock_modelaccessor import MockObjectList
+        self.MockObjectList = MockObjectList
+
+        from model_policy_fabriccrossconnectserviceinstance import FabricCrossconnectServiceInstancePolicy, \
+            model_accessor
+
+        # import all class names to globals
+        for (k, v) in model_accessor.all_model_classes.items():
+            globals()[k] = v
+
+        self.policy_step = FabricCrossconnectServiceInstancePolicy
+        self.policy_step.log = Mock()
+
+        # mock onos-fabric
+        self.onos_fabric = Service(name = "onos-fabric",
+                              rest_hostname = "onos-fabric",
+                              rest_port = "8181",
+                              rest_username = "onos",
+                              rest_password = "rocks")
+
+        self.service = FabricCrossconnectService(name = "fcservice",
+                                                 provider_services = [self.onos_fabric])
+
+    def mock_westbound(self, fsi, s_tag, switch_datapath_id, switch_port):
+        # Mock out a ServiceInstance so the syncstep can call get_westbound_service_instance_properties on it
+        si = ServiceInstance(id=fsi.id)
+        si.get_westbound_service_instance_properties = functools.partial(
+            mock_get_westbound_service_instance_properties,
+            {"s_tag": s_tag,
+             "switch_datapath_id": switch_datapath_id,
+             "switch_port": switch_port})
+
+        fsi.provided_links=Mock(exists=Mock(return_value=True))
+
+        return si
+
+    def test_handle_update(self):
+        with patch.object(ServiceInstance.objects, "get_items") as serviceinstance_objects, \
+            patch.object(FabricCrossconnectServiceInstance, "save") as fcsi_save:
+
+            fsi = FabricCrossconnectServiceInstance(id=7777, owner=self.service, s_tag=None, source_port=None,
+                                                    switch_datapath_id=None)
+
+            serviceinstance_objects.return_value = [fsi]
+
+            si = self.mock_westbound(fsi, s_tag=111, switch_datapath_id = "of:0000000000000201", switch_port = 3)
+            serviceinstance_objects.return_value = [si]
+
+            self.policy_step().handle_update(fsi)
+
+            self.assertEqual(fsi.s_tag, 111)
+            self.assertEqual(fsi.switch_datapath_id, "of:0000000000000201")
+            self.assertEqual(fsi.source_port, 3)
+
+            fcsi_save.assert_called_with(update_fields=["s_tag", "switch_datapath_id", "source_port"])
+
+    def test_handle_update_west_si_bad_s_tag(self):
+        with patch.object(ServiceInstance.objects, "get_items") as serviceinstance_objects, \
+            patch.object(FabricCrossconnectServiceInstance, "save") as fcsi_save:
+
+            fsi = FabricCrossconnectServiceInstance(id=7777, owner=self.service, s_tag=None, source_port=None,
+                                                    switch_datapath_id=None)
+
+            serviceinstance_objects.return_value = [fsi]
+
+            si = self.mock_westbound(fsi, s_tag=None, switch_datapath_id = "of:0000000000000201", switch_port = 3)
+            serviceinstance_objects.return_value = [si]
+
+            with self.assertRaises(Exception) as e:
+                self.policy_step().handle_update(fsi)
+
+            self.assertEqual(e.exception.message, "Westbound ServiceInstance s-tag is None on fcsi 7777")
+
+    def test_handle_update_west_si_bad_port(self):
+        with patch.object(ServiceInstance.objects, "get_items") as serviceinstance_objects, \
+            patch.object(FabricCrossconnectServiceInstance, "save") as fcsi_save:
+
+            fsi = FabricCrossconnectServiceInstance(id=7777, owner=self.service, s_tag=None, source_port=None,
+                                                    switch_datapath_id=None)
+
+            serviceinstance_objects.return_value = [fsi]
+
+            si = self.mock_westbound(fsi, s_tag=111, switch_datapath_id = "of:0000000000000201", switch_port = None)
+            serviceinstance_objects.return_value = [si]
+
+            with self.assertRaises(Exception) as e:
+                self.policy_step().handle_update(fsi)
+
+            self.assertEqual(e.exception.message, "Westbound ServiceInstance switch_port is None on fcsi 7777")
+
+    def test_handle_update_west_si_bad_switch_datapath_id(self):
+        with patch.object(ServiceInstance.objects, "get_items") as serviceinstance_objects, \
+            patch.object(FabricCrossconnectServiceInstance, "save") as fcsi_save:
+
+            fsi = FabricCrossconnectServiceInstance(id=7777, owner=self.service, s_tag=None, source_port=None,
+                                                    switch_datapath_id=None)
+
+            serviceinstance_objects.return_value = [fsi]
+
+            si = self.mock_westbound(fsi, s_tag=111, switch_datapath_id = None, switch_port = 3)
+            serviceinstance_objects.return_value = [si]
+
+            with self.assertRaises(Exception) as e:
+                self.policy_step().handle_update(fsi)
+
+            self.assertEqual(e.exception.message, "Westbound ServiceInstance switch_datapath_id is unset on fcsi 7777")
+
+    def test_handle_update_no_links(self):
+        with patch.object(ServiceInstance.objects, "get_items") as serviceinstance_objects, \
+            patch.object(FabricCrossconnectServiceInstance, "save") as fcsi_save:
+
+            fsi = FabricCrossconnectServiceInstance(id=7777, owner=self.service, s_tag=None, source_port=None,
+                                                    switch_datapath_id=None)
+
+            fsi.provided_links = Mock(exists=Mock(return_value=False))
+
+            serviceinstance_objects.return_value = [fsi]
+
+            self.policy_step().handle_update(fsi)
+
+            fcsi_save.assert_not_called()
+
+    def test_handle_update_change_s_tag(self):
+        with patch.object(ServiceInstance.objects, "get_items") as serviceinstance_objects, \
+            patch.object(FabricCrossconnectServiceInstance, "save") as fcsi_save:
+
+            fsi = FabricCrossconnectServiceInstance(id=7777, owner=self.service, s_tag=100, source_port=None,
+                                                    switch_datapath_id=None)
+
+            serviceinstance_objects.return_value = [fsi]
+
+            si = self.mock_westbound(fsi, s_tag=111, switch_datapath_id = "of:0000000000000201", switch_port = 3)
+            serviceinstance_objects.return_value = [si]
+
+            with self.assertRaises(Exception) as e:
+                self.policy_step().handle_update(fsi)
+
+            self.assertEqual(e.exception.message, "Westbound ServiceInstance changing s-tag is not currently permitted")
+
+    def test_handle_update_change_switch_datapath_id(self):
+        with patch.object(ServiceInstance.objects, "get_items") as serviceinstance_objects, \
+            patch.object(FabricCrossconnectServiceInstance, "save") as fcsi_save:
+
+            fsi = FabricCrossconnectServiceInstance(id=7777, owner=self.service, s_tag=None, source_port=None,
+                                                    switch_datapath_id="foo")
+
+            serviceinstance_objects.return_value = [fsi]
+
+            si = self.mock_westbound(fsi, s_tag=111, switch_datapath_id = "of:0000000000000201", switch_port = 3)
+            serviceinstance_objects.return_value = [si]
+
+            with self.assertRaises(Exception) as e:
+                self.policy_step().handle_update(fsi)
+
+            self.assertEqual(e.exception.message, "Westbound ServiceInstance changing switch_datapath_id is not currently permitted")
+
+    def test_handle_update_change_source_port(self):
+        with patch.object(ServiceInstance.objects, "get_items") as serviceinstance_objects, \
+            patch.object(FabricCrossconnectServiceInstance, "save") as fcsi_save:
+
+            fsi = FabricCrossconnectServiceInstance(id=7777, owner=self.service, s_tag=None, source_port=5,
+                                                    switch_datapath_id=None)
+
+            serviceinstance_objects.return_value = [fsi]
+
+            si = self.mock_westbound(fsi, s_tag=111, switch_datapath_id = "of:0000000000000201", switch_port = 3)
+            serviceinstance_objects.return_value = [si]
+
+            with self.assertRaises(Exception) as e:
+                self.policy_step().handle_update(fsi)
+
+            self.assertEqual(e.exception.message, "Westbound ServiceInstance changing source_port is not currently permitted")
+
+
+    def tearDown(self):
+        self.o = None
+        sys.path = self.sys_path_save
+
+if __name__ == '__main__':
+    unittest.main()