diff --git a/neutron_lib/api/extensions.py b/neutron_lib/api/extensions.py
index d7cd26fb4..24bcd7cb9 100644
--- a/neutron_lib/api/extensions.py
+++ b/neutron_lib/api/extensions.py
@@ -220,32 +220,50 @@ class APIExtensionDescriptor(ExtensionDescriptor):
         cls._assert_api_definition('UPDATED_TIMESTAMP')
         return cls.api_definition.UPDATED_TIMESTAMP
 
-    def get_extended_resources(self, version):
-        """Retrieve the resource attribute map for the API definition."""
+    @classmethod
+    def get_extended_resources(cls, version):
+        """Retrieve the extended resource map for the API definition.
+
+        :param version: The API version to retrieve the resource attribute
+            map for.
+        :returns: The extended resource map for the underlying API definition
+            if the version is 2.0. The extended resource map returned includes
+            both the API definition's RESOURCE_ATTRIBUTE_MAP and
+            SUB_RESOURCE_ATTRIBUTE_MAP where applicable. If the version is
+            not 2.0, an empty dict is returned.
+        """
         if version == "2.0":
-            self._assert_api_definition('RESOURCE_ATTRIBUTE_MAP')
-            return self.api_definition.RESOURCE_ATTRIBUTE_MAP
+            cls._assert_api_definition('RESOURCE_ATTRIBUTE_MAP')
+            cls._assert_api_definition('SUB_RESOURCE_ATTRIBUTE_MAP')
+            # support api defs that use None for sub attr map
+            sub_attrs = cls.api_definition.SUB_RESOURCE_ATTRIBUTE_MAP or {}
+            return dict(
+                list(cls.api_definition.RESOURCE_ATTRIBUTE_MAP.items()) +
+                list(sub_attrs.items()))
         else:
             return {}
 
-    def get_required_extensions(self):
+    @classmethod
+    def get_required_extensions(cls):
         """Returns the API definition's required extensions."""
-        self._assert_api_definition('REQUIRED_EXTENSIONS')
-        return self.api_definition.REQUIRED_EXTENSIONS
+        cls._assert_api_definition('REQUIRED_EXTENSIONS')
+        return cls.api_definition.REQUIRED_EXTENSIONS
 
-    def get_optional_extensions(self):
+    @classmethod
+    def get_optional_extensions(cls):
         """Returns the API definition's optional extensions."""
-        self._assert_api_definition('OPTIONAL_EXTENSIONS')
-        return self.api_definition.OPTIONAL_EXTENSIONS
+        cls._assert_api_definition('OPTIONAL_EXTENSIONS')
+        return cls.api_definition.OPTIONAL_EXTENSIONS
 
-    def update_attributes_map(self, attributes, extension_attrs_map=None):
+    @classmethod
+    def update_attributes_map(cls, attributes, extension_attrs_map=None):
         """Update attributes map for this extension.
 
         Behaves like ExtensionDescriptor.update_attributes_map(), but
-        if extension_attrs_map is not given the extension's API
-        definition RESOURCE_ATTRIBUTE_MAP is used.
+        if extension_attrs_map is not given the dict returned from
+        self.get_extended_resources('2.0') is used.
         """
         if extension_attrs_map is None:
-            extension_attrs_map = self.get_extended_resources('2.0')
-        super(APIExtensionDescriptor, self).update_attributes_map(
+            extension_attrs_map = cls.get_extended_resources('2.0')
+        super(APIExtensionDescriptor, cls()).update_attributes_map(
             attributes, extension_attrs_map=extension_attrs_map)
diff --git a/neutron_lib/tests/unit/api/test_extensions.py b/neutron_lib/tests/unit/api/test_extensions.py
index 5e4a7da39..51fe8ef9c 100644
--- a/neutron_lib/tests/unit/api/test_extensions.py
+++ b/neutron_lib/tests/unit/api/test_extensions.py
@@ -119,6 +119,7 @@ class TestAPIExtensionDescriptor(base.BaseTestCase):
     DESCRIPTION = 'A test API definition'
     UPDATED_TIMESTAMP = '2017-02-01T10:00:00-00:00'
     RESOURCE_ATTRIBUTE_MAP = {'ports': {}}
+    SUB_RESOURCE_ATTRIBUTE_MAP = {'ports': {'debug': {}}}
     REQUIRED_EXTENSIONS = ['l3']
     OPTIONAL_EXTENSIONS = ['fw']
 
@@ -162,8 +163,10 @@ class TestAPIExtensionDescriptor(base.BaseTestCase):
         self.assertRaises(NotImplementedError, _EmptyAPIDefinition.get_updated)
 
     def test_get_extended_resources_v2(self):
-        self.assertEqual(self.RESOURCE_ATTRIBUTE_MAP,
-                         self.extn.get_extended_resources('2.0'))
+        self.assertEqual(
+            dict(list(self.RESOURCE_ATTRIBUTE_MAP.items()) +
+                 list(self.SUB_RESOURCE_ATTRIBUTE_MAP.items())),
+            self.extn.get_extended_resources('2.0'))
 
     def test_get_extended_resources_v2_unset(self):
         self.assertRaises(NotImplementedError,
diff --git a/releasenotes/notes/subresource-update-attrmap-and-classmethods-76accdd5c56a3bd4.yaml b/releasenotes/notes/subresource-update-attrmap-and-classmethods-76accdd5c56a3bd4.yaml
new file mode 100644
index 000000000..a790f8362
--- /dev/null
+++ b/releasenotes/notes/subresource-update-attrmap-and-classmethods-76accdd5c56a3bd4.yaml
@@ -0,0 +1,11 @@
+---
+features:
+  - All methods defined in ``APIExtensionDescriptor`` are now class methods.
+    This allows consumers to call them without a reference to an actual
+    extension object instance.
+fixes:
+  - The ``get_extended_resources`` method of the ``APIExtensionDescriptor``
+    was updated to also include the underlying API definition's
+    ``SUB_RESOURCE_ATTRIBUTE_MAP`` in the returned dict. As a result, the
+    ``update_attributes_map`` method now also includes the sub-resources if no
+    ``extension_attrs_map`` is passed to it.