diff --git a/validator/interface.py b/validator/interface.py
index 8bc0f1d..702d60a 100644
--- a/validator/interface.py
+++ b/validator/interface.py
@@ -21,6 +21,33 @@ class NullHandler(logging.Handler):
     def emit(self, record):
         pass
 
+def get_qinx_parent_by_name(yaml, ifname):
+    """ Returns the sub-interface which matches a QinAD or QinQ outer tag, or None
+        if that sub-interface doesn't exist. """
+
+    if not is_qinx(yaml, ifname):
+        return None
+    qinx_iface = get_by_name(yaml, ifname)
+    if not qinx_iface:
+        return None
+
+    qinx_encap = get_encapsulation(yaml, ifname)
+    if not qinx_encap:
+        return None
+
+    for ifname, iface in yaml['interfaces'].items():
+        for subid, sub_iface in iface['sub-interfaces'].items():
+            sub_ifname = "%s.%d" % (ifname, subid)
+            sub_encap = get_encapsulation(yaml, sub_ifname)
+            if not sub_encap:
+                continue
+            if qinx_encap['dot1q'] > 0 and sub_encap['dot1q'] == qinx_encap['dot1q']:
+                return sub_ifname
+            if qinx_encap['dot1ad'] > 0 and sub_encap['dot1ad'] == qinx_encap['dot1ad']:
+                return sub_ifname
+    return None
+
+
 def get_parent_by_name(yaml, ifname):
     """ Returns the sub-interface's parent, or None if the sub-int doesn't exist. """
     if not '.' in ifname:
diff --git a/validator/test_interface.py b/validator/test_interface.py
index 64848b2..46c5787 100644
--- a/validator/test_interface.py
+++ b/validator/test_interface.py
@@ -114,3 +114,11 @@ class TestInterfaceMethods(unittest.TestCase):
     def test_l3(self):
         self.assertTrue(interface.is_l3(self.cfg, "GigabitEthernet1/0/0"))
         self.assertFalse(interface.is_l3(self.cfg, "GigabitEthernet3/0/0"))
+
+    def test_qinx_parent(self):
+        self.assertEqual(interface.get_qinx_parent_by_name(self.cfg, "GigabitEthernet1/0/1.202"), "GigabitEthernet1/0/1.200")
+        self.assertEqual(interface.get_qinx_parent_by_name(self.cfg, "GigabitEthernet1/0/1.203"), "GigabitEthernet1/0/1.201")
+        self.assertIsNone(interface.get_qinx_parent_by_name(self.cfg, "GigabitEthernet1/0/1"))
+        self.assertIsNone(interface.get_qinx_parent_by_name(self.cfg, "GigabitEthernet1/0/1.100"))
+        self.assertIsNone(interface.get_qinx_parent_by_name(self.cfg, "GigabitEthernet1/0/1.200"))
+        self.assertIsNone(interface.get_qinx_parent_by_name(self.cfg, "GigabitEthernet1/0/1.201"))