diff --git a/tempest/api/network/base.py b/tempest/api/network/base.py
index 0ccddcf..cc768fd 100644
--- a/tempest/api/network/base.py
+++ b/tempest/api/network/base.py
@@ -149,15 +149,16 @@
         return network
 
     @classmethod
-    def create_subnet(cls, network, gateway=None):
+    def create_subnet(cls, network, gateway=None, cidr=None, mask_bits=None):
         """Wrapper utility that returns a test subnet."""
         # The cidr and mask_bits depend on the ip version.
         if cls._ip_version == 4:
-            cidr = netaddr.IPNetwork(CONF.network.tenant_network_cidr)
-            mask_bits = CONF.network.tenant_network_mask_bits
+            cidr = cidr or netaddr.IPNetwork(CONF.network.tenant_network_cidr)
+            mask_bits = mask_bits or CONF.network.tenant_network_mask_bits
         elif cls._ip_version == 6:
-            cidr = netaddr.IPNetwork(CONF.network.tenant_network_v6_cidr)
-            mask_bits = CONF.network.tenant_network_v6_mask_bits
+            cidr = (
+                cidr or netaddr.IPNetwork(CONF.network.tenant_network_v6_cidr))
+            mask_bits = mask_bits or CONF.network.tenant_network_v6_mask_bits
         # Find a cidr that is not in use yet and create a subnet with it
         for subnet_cidr in cidr.subnet(mask_bits):
             if not gateway:
diff --git a/tempest/api/network/base_routers.py b/tempest/api/network/base_routers.py
index b278002..1303bcf 100644
--- a/tempest/api/network/base_routers.py
+++ b/tempest/api/network/base_routers.py
@@ -37,6 +37,15 @@
             routers_list.append(router['id'])
         self.assertNotIn(router_id, routers_list)
 
+    def _add_router_interface_with_subnet_id(self, router_id, subnet_id):
+        resp, interface = self.client.add_router_interface_with_subnet_id(
+            router_id, subnet_id)
+        self.assertEqual('200', resp['status'])
+        self.addCleanup(self._remove_router_interface_with_subnet_id,
+                        router_id, subnet_id)
+        self.assertEqual(subnet_id, interface['subnet_id'])
+        return interface
+
     def _remove_router_interface_with_subnet_id(self, router_id, subnet_id):
         resp, body = self.client.remove_router_interface_with_subnet_id(
             router_id, subnet_id)
diff --git a/tempest/api/network/test_routers.py b/tempest/api/network/test_routers.py
index 7605b8a..d38633f 100644
--- a/tempest/api/network/test_routers.py
+++ b/tempest/api/network/test_routers.py
@@ -300,9 +300,13 @@
 
     @test.attr(type='smoke')
     def test_add_multiple_router_interfaces(self):
-        network = self.create_network()
-        subnet01 = self.create_subnet(network)
-        subnet02 = self.create_subnet(network)
+        network01 = self.create_network(
+            network_name=data_utils.rand_name('router-network01-'))
+        network02 = self.create_network(
+            network_name=data_utils.rand_name('router-network02-'))
+        subnet01 = self.create_subnet(network01)
+        sub02_cidr = netaddr.IPNetwork(CONF.network.tenant_network_cidr).next()
+        subnet02 = self.create_subnet(network02, cidr=sub02_cidr)
         router = self._create_router(data_utils.rand_name('router-'))
         interface01 = self._add_router_interface_with_subnet_id(router['id'],
                                                                 subnet01['id'])
@@ -313,15 +317,6 @@
         self._verify_router_interface(router['id'], subnet02['id'],
                                       interface02['port_id'])
 
-    def _add_router_interface_with_subnet_id(self, router_id, subnet_id):
-        resp, interface = self.client.add_router_interface_with_subnet_id(
-            router_id, subnet_id)
-        self.assertEqual('200', resp['status'])
-        self.addCleanup(self._remove_router_interface_with_subnet_id,
-                        router_id, subnet_id)
-        self.assertEqual(subnet_id, interface['subnet_id'])
-        return interface
-
     def _verify_router_interface(self, router_id, subnet_id, port_id):
         resp, show_port_body = self.client.show_port(port_id)
         self.assertEqual('200', resp['status'])
diff --git a/tempest/api/network/test_routers_negative.py b/tempest/api/network/test_routers_negative.py
index 91ab9d6..feee51b 100644
--- a/tempest/api/network/test_routers_negative.py
+++ b/tempest/api/network/test_routers_negative.py
@@ -13,11 +13,16 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import netaddr
+
 from tempest.api.network import base_routers as base
 from tempest.common.utils import data_utils
+from tempest import config
 from tempest import exceptions
 from tempest import test
 
+CONF = config.CONF
+
 
 class RoutersNegativeTest(base.BaseRouterTest):
     _interface = 'json'
@@ -43,12 +48,30 @@
 
     @test.attr(type=['negative', 'smoke'])
     def test_router_add_gateway_net_not_external_returns_400(self):
-        self.create_subnet(self.network)
+        alt_network = self.create_network(
+            network_name=data_utils.rand_name('router-negative-'))
+        sub_cidr = netaddr.IPNetwork(CONF.network.tenant_network_cidr).next()
+        self.create_subnet(alt_network, cidr=sub_cidr)
         self.assertRaises(exceptions.BadRequest,
                           self.client.update_router,
                           self.router['id'],
                           external_gateway_info={
-                              'network_id': self.network['id']})
+                              'network_id': alt_network['id']})
+
+    @test.attr(type=['negative', 'smoke'])
+    def test_add_router_interfaces_on_overlapping_subnets_returns_400(self):
+        network01 = self.create_network(
+            network_name=data_utils.rand_name('router-network01-'))
+        network02 = self.create_network(
+            network_name=data_utils.rand_name('router-network02-'))
+        subnet01 = self.create_subnet(network01)
+        subnet02 = self.create_subnet(network02)
+        self._add_router_interface_with_subnet_id(self.router['id'],
+                                                  subnet01['id'])
+        self.assertRaises(exceptions.BadRequest,
+                          self._add_router_interface_with_subnet_id,
+                          self.router['id'],
+                          subnet02['id'])
 
     @test.attr(type=['negative', 'smoke'])
     def test_router_remove_interface_in_use_returns_409(self):