diff --git a/neutron/db/l3_db.py b/neutron/db/l3_db.py
index 3fe6d154dbd..f32aab414f0 100644
--- a/neutron/db/l3_db.py
+++ b/neutron/db/l3_db.py
@@ -105,6 +105,15 @@ class L3_NAT_dbonly_mixin(l3.RouterPluginBase,
             l3plugin.prevent_l3_port_deletion(
                 payload.context, payload.resource_id)
 
+    @staticmethod
+    def _validate_subnet_address_mode(subnet):
+        if (subnet['ip_version'] == 6 and subnet['ipv6_ra_mode'] is None and
+                subnet['ipv6_address_mode'] is not None):
+            msg = (_('IPv6 subnet %s configured to receive RAs from an '
+                   'external router cannot be added to Neutron Router.') %
+                   subnet['id'])
+            raise n_exc.BadRequest(resource='router', msg=msg)
+
     @property
     def _is_dns_integration_supported(self):
         if self._dns_integration is None:
@@ -687,6 +696,13 @@ class L3_NAT_dbonly_mixin(l3.RouterPluginBase,
         if not port['fixed_ips']:
             msg = _('Router port must have at least one fixed IP')
             raise n_exc.BadRequest(resource='router', msg=msg)
+
+        fixed_ips = [ip for ip in port['fixed_ips']]
+        for fixed_ip in fixed_ips:
+            subnet = self._core_plugin.get_subnet(
+                context, fixed_ip['subnet_id'])
+            self._validate_subnet_address_mode(subnet)
+
         return port
 
     def _validate_port_in_range_or_admin(self, context, subnets, port):
@@ -811,12 +827,7 @@ class L3_NAT_dbonly_mixin(l3.RouterPluginBase,
             msg = (_('Cannot add interface to router because subnet %s is not '
                      'owned by project making the request') % subnet_id)
             raise n_exc.BadRequest(resource='router', msg=msg)
-        if (subnet['ip_version'] == 6 and subnet['ipv6_ra_mode'] is None and
-                subnet['ipv6_address_mode'] is not None):
-            msg = (_('IPv6 subnet %s configured to receive RAs from an '
-                   'external router cannot be added to Neutron Router.') %
-                   subnet['id'])
-            raise n_exc.BadRequest(resource='router', msg=msg)
+        self._validate_subnet_address_mode(subnet)
         self._check_for_dup_router_subnets(context, router,
                                            subnet['network_id'], [subnet])
         fixed_ip = {'ip_address': subnet['gateway_ip'],
diff --git a/neutron/tests/unit/extensions/test_l3.py b/neutron/tests/unit/extensions/test_l3.py
index f7eeda28015..10f9190090c 100644
--- a/neutron/tests/unit/extensions/test_l3.py
+++ b/neutron/tests/unit/extensions/test_l3.py
@@ -1490,6 +1490,33 @@ class L3NatTestCaseBase(L3NatTestCaseMixin):
                                                   None,
                                                   p['port']['id'])
 
+    def test_update_router_interface_port_ipv6_subnet_ext_ra(self):
+        use_cases = [{'msg': 'IPv6 Subnet Modes (none, slaac)',
+                      'ra_mode': None,
+                      'address_mode': lib_constants.IPV6_SLAAC},
+                     {'msg': 'IPv6 Subnet Modes (none, dhcpv6-stateful)',
+                      'ra_mode': None,
+                      'address_mode': lib_constants.DHCPV6_STATEFUL},
+                     {'msg': 'IPv6 Subnet Modes (none, dhcpv6-stateless)',
+                      'ra_mode': None,
+                      'address_mode': lib_constants.DHCPV6_STATELESS}]
+        for uc in use_cases:
+            with self.network() as network, self.router() as router:
+                with self.subnet(
+                        network=network, cidr='fd00::/64',
+                        ip_version=lib_constants.IP_VERSION_6,
+                        ipv6_ra_mode=uc['ra_mode'],
+                        ipv6_address_mode=uc['address_mode']) as subnet:
+                    fixed_ips = [{'subnet_id': subnet['subnet']['id']}]
+                    with self.port(subnet=subnet, fixed_ips=fixed_ips) as port:
+                        self._router_interface_action(
+                            'add',
+                            router['router']['id'],
+                            None,
+                            port['port']['id'],
+                            expected_code=exc.HTTPBadRequest.code,
+                            msg=uc['msg'])
+
     def _assert_body_port_id_and_update_port(self, body, mock_update_port,
                                              port_id, device_id):
         self.assertNotIn('port_id', body)