diff --git a/neutron/agent/l3_agent.py b/neutron/agent/l3_agent.py
index 0d350a84f1..eb06b380bb 100644
--- a/neutron/agent/l3_agent.py
+++ b/neutron/agent/l3_agent.py
@@ -229,6 +229,8 @@ class L3NATAgent(firewall_l3_agent.FWaaSL3AgentRpcCallback, manager.Manager):
         self.rpc_loop.start(interval=RPC_LOOP_INTERVAL)
         super(L3NATAgent, self).__init__(conf=self.conf)
 
+        self.target_ex_net_id = None
+
     def _check_config_params(self):
         """Check items in configuration files.
 
@@ -311,7 +313,7 @@ class L3NATAgent(firewall_l3_agent.FWaaSL3AgentRpcCallback, manager.Manager):
             ip_wrapper = ip_wrapper_root.ensure_namespace(ri.ns_name())
             ip_wrapper.netns.execute(['sysctl', '-w', 'net.ipv4.ip_forward=1'])
 
-    def _fetch_external_net_id(self):
+    def _fetch_external_net_id(self, force=False):
         """Find UUID of single external network for this agent."""
         if self.conf.gateway_external_network_id:
             return self.conf.gateway_external_network_id
@@ -322,8 +324,13 @@ class L3NATAgent(firewall_l3_agent.FWaaSL3AgentRpcCallback, manager.Manager):
         if not self.conf.external_network_bridge:
             return
 
+        if not force and self.target_ex_net_id:
+            return self.target_ex_net_id
+
         try:
-            return self.plugin_rpc.get_external_network_id(self.context)
+            self.target_ex_net_id = self.plugin_rpc.get_external_network_id(
+                self.context)
+            return self.target_ex_net_id
         except rpc_common.RemoteError as e:
             with excutils.save_and_reraise_exception():
                 if e.exc_type == 'TooManyExternalNetworks':
@@ -759,7 +766,10 @@ class L3NATAgent(firewall_l3_agent.FWaaSL3AgentRpcCallback, manager.Manager):
                 continue
             if (target_ex_net_id and ex_net_id and
                 ex_net_id != target_ex_net_id):
-                continue
+                # Double check that our single external_net_id has not changed
+                # by forcing a check by RPC.
+                if (ex_net_id != self._fetch_external_net_id(force=True)):
+                    continue
             cur_router_ids.add(r['id'])
             if r['id'] not in self.router_info:
                 self._router_added(r['id'], r)
diff --git a/neutron/tests/unit/test_l3_agent.py b/neutron/tests/unit/test_l3_agent.py
index fe11deeb22..6df43edaef 100644
--- a/neutron/tests/unit/test_l3_agent.py
+++ b/neutron/tests/unit/test_l3_agent.py
@@ -926,6 +926,39 @@ class TestBasicRouterOperations(base.BaseTestCase):
 
         agent._process_routers(routers)
         self.assertIn(routers[0]['id'], agent.router_info)
+        self.plugin_api.get_external_network_id.assert_called_with(
+            agent.context)
+
+    def test_process_routers_with_cached_ext_net(self):
+        agent = l3_agent.L3NATAgent(HOSTNAME, self.conf)
+        self.plugin_api.get_external_network_id.return_value = 'aaa'
+        agent.target_ex_net_id = 'aaa'
+
+        routers = [
+            {'id': _uuid(),
+             'routes': [],
+             'admin_state_up': True,
+             'external_gateway_info': {'network_id': 'aaa'}}]
+
+        agent._process_routers(routers)
+        self.assertIn(routers[0]['id'], agent.router_info)
+        self.assertFalse(self.plugin_api.get_external_network_id.called)
+
+    def test_process_routers_with_stale_cached_ext_net(self):
+        agent = l3_agent.L3NATAgent(HOSTNAME, self.conf)
+        self.plugin_api.get_external_network_id.return_value = 'aaa'
+        agent.target_ex_net_id = 'bbb'
+
+        routers = [
+            {'id': _uuid(),
+             'routes': [],
+             'admin_state_up': True,
+             'external_gateway_info': {'network_id': 'aaa'}}]
+
+        agent._process_routers(routers)
+        self.assertIn(routers[0]['id'], agent.router_info)
+        self.plugin_api.get_external_network_id.assert_called_with(
+            agent.context)
 
     def test_process_routers_with_no_ext_net_in_conf_and_two_net_plugin(self):
         agent = l3_agent.L3NATAgent(HOSTNAME, self.conf)