diff --git a/neutron_lib/api/definitions/l3_ext_gw_mode.py b/neutron_lib/api/definitions/l3_ext_gw_mode.py
index 7d3ca8fa6..7dbc11be1 100644
--- a/neutron_lib/api/definitions/l3_ext_gw_mode.py
+++ b/neutron_lib/api/definitions/l3_ext_gw_mode.py
@@ -13,6 +13,9 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import copy
+import typing
+
 from neutron_lib.api import converters
 from neutron_lib.api.definitions import l3
 
@@ -27,29 +30,20 @@ DESCRIPTION = ('Extension of the router abstraction for specifying whether '
 UPDATED_TIMESTAMP = '2013-03-28T10:00:00-00:00'
 RESOURCE_NAME = l3.ROUTER
 COLLECTION_NAME = l3.ROUTERS
-RESOURCE_ATTRIBUTE_MAP = {
-    COLLECTION_NAME: {
-        l3.EXTERNAL_GW_INFO: {
-            'allow_post': True,
-            'allow_put': True,
-            'is_visible': True,
-            'default': None,
-            'enforce_policy': True,
-            'validate': {
-                'type:dict_or_nodata': {
-                    'network_id': {'type:uuid': None, 'required': True},
-                    'enable_snat': {'type:boolean': None, 'required': False,
-                                    'convert_to':
-                                        converters.convert_to_boolean},
-                    'external_fixed_ips': {
-                        'type:fixed_ips': None,
-                        'required': False
-                    }
-                }
-            }
-        }
-    }
+
+routers: typing.Dict[str, typing.Any] = copy.deepcopy(
+    l3.RESOURCE_ATTRIBUTE_MAP[COLLECTION_NAME]
+)
+routers[l3.EXTERNAL_GW_INFO]['validate']['type:dict_or_nodata'][
+    'enable_snat'] = {
+    'type:boolean': None,
+    'required': False,
+    'convert_to': converters.convert_to_boolean
 }
+RESOURCE_ATTRIBUTE_MAP = {
+    COLLECTION_NAME: routers
+}
+
 SUB_RESOURCE_ATTRIBUTE_MAP = {}
 ACTION_MAP = {}
 REQUIRED_EXTENSIONS = [l3.ALIAS]
diff --git a/neutron_lib/api/definitions/qos_gateway_ip.py b/neutron_lib/api/definitions/qos_gateway_ip.py
index 8dfade819..70943ccc1 100644
--- a/neutron_lib/api/definitions/qos_gateway_ip.py
+++ b/neutron_lib/api/definitions/qos_gateway_ip.py
@@ -11,9 +11,13 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+import copy
+import typing
+
 from neutron_lib.api.definitions import l3
 from neutron_lib.api.definitions import l3_ext_gw_mode
 from neutron_lib.api.definitions import qos
+from neutron_lib.services.qos import constants as qos_consts
 
 
 ALIAS = 'qos-gateway-ip'
@@ -25,18 +29,19 @@ DESCRIPTION = 'The Router gateway IP Quality of Service extension'
 UPDATED_TIMESTAMP = '2018-02-24T00:00:00-00:00'
 RESOURCE_NAME = l3.ROUTER
 COLLECTION_NAME = l3.ROUTERS
-RESOURCE_ATTRIBUTE_MAP = {
-    COLLECTION_NAME: {
-        l3.EXTERNAL_GW_INFO: {
-            'allow_post': True,
-            'allow_put': True,
-            'is_visible': True,
-            'default': None,
-            'enforce_policy': True,
-            'validate': {'type:external_gw_info': None},
-        }
-    }
+
+routers: typing.Dict[str, typing.Any] = copy.deepcopy(
+    l3_ext_gw_mode.RESOURCE_ATTRIBUTE_MAP[COLLECTION_NAME]
+)
+routers[l3.EXTERNAL_GW_INFO]['validate']['type:dict_or_nodata'][
+    qos_consts.QOS_POLICY_ID] = {
+    'type:uuid_or_none': None,
+    'required': False
 }
+RESOURCE_ATTRIBUTE_MAP = {
+    COLLECTION_NAME: routers
+}
+
 SUB_RESOURCE_ATTRIBUTE_MAP = {}
 ACTION_MAP = {}
 REQUIRED_EXTENSIONS = [l3.ALIAS, qos.ALIAS, l3_ext_gw_mode.ALIAS]
diff --git a/releasenotes/notes/router_gw_info_validation-210d27abd4c3fa71.yaml b/releasenotes/notes/router_gw_info_validation-210d27abd4c3fa71.yaml
new file mode 100644
index 000000000..37904919b
--- /dev/null
+++ b/releasenotes/notes/router_gw_info_validation-210d27abd4c3fa71.yaml
@@ -0,0 +1,6 @@
+---
+fixes:
+  - |
+    Fixed the validation of router ``external_gateway_info`` field. Each
+    extension adding a new validator amends the previous definition, but
+    doesn't overwrite it.