diff --git a/xos/synchronizer/models/models.py b/xos/synchronizer/models/models.py
index eeb5e74..9431488 100644
--- a/xos/synchronizer/models/models.py
+++ b/xos/synchronizer/models/models.py
@@ -12,12 +12,16 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+import random
+
 from core.models.xosbase import *
+from xos.exceptions import XOSValidationError
 
 from models_decl import VOLTService_decl
 from models_decl import VOLTServiceInstance_decl
 from models_decl import OLTDevice_decl
 from models_decl import PONPort_decl
+from models_decl import NNIPort_decl
 from models_decl import ONUDevice_decl
 
 class VOLTService(VOLTService_decl):
@@ -45,7 +49,44 @@
 
 class PONPort(PONPort_decl):
     class Meta:
-        proxy = True 
+        proxy = True
+
+    def generate_tag(self):
+        # NOTE this method will loop if available c_tags are ended
+        tag = random.randint(16, 4096)
+        if tag in self.get_used_s_tags():
+            return self.generate_tag()
+        return tag
+
+    def get_used_s_tags(self):
+        same_olt_device = OLTDevice.objects.filter(device_id=self.olt_device)
+        return [s.c_tag for s in same_olt_device]
+
+    def save(self, *args, **kwargs):
+        # validate s_tag
+        if hasattr(self, 's_tag') and self.s_tag is not None:
+            is_update_with_same_tag = False
+
+            if not self.is_new:
+                # if it is an update, but the tag is the same, skip validation
+                existing = PONPort.objects.filter(s_tag=self.s_tag)
+
+                if len(existing) > 0 and existing[0].s_tag == self.s_tag and existing[0].id == self.id:
+                    is_update_with_same_tag = True
+
+            if self.s_tag in self.get_used_s_tags() and not is_update_with_same_tag:
+                raise XOSValidationError(
+                    "The s_tag you specified (%s) has already been used on device %s" % (self.s_tag, self.onu_device))
+
+        if not hasattr(self, "s_tag") or self.s_tag is None:
+            self.s_tag = self.generate_tag()
+
+        super(PONPort, self).save(*args, **kwargs)
+
+
+class NNIPort(NNIPort_decl):
+    class Meta:
+        proxy = True
 
 
 class ONUDevice(ONUDevice_decl):