diff --git a/neutron/agent/l3/l3_agent_extensions_manager.py b/neutron/agent/l3/l3_agent_extensions_manager.py
index 57cd3932fca..b3362613e45 100644
--- a/neutron/agent/l3/l3_agent_extensions_manager.py
+++ b/neutron/agent/l3/l3_agent_extensions_manager.py
@@ -13,8 +13,11 @@
 #    License for the specific language governing permissions and limitations
 #    under the License.
 
+from neutron_lib.agent import l3_extension
+from neutron_lib import exceptions
 from oslo_log import log
 
+from neutron._i18n import _
 from neutron.agent import agent_extensions_manager as agent_ext_manager
 from neutron.conf.agent import agent_extensions_manager as agent_ext_mgr_config
 
@@ -24,6 +27,13 @@ LOG = log.getLogger(__name__)
 L3_AGENT_EXT_MANAGER_NAMESPACE = 'neutron.agent.l3.extensions'
 
 
+# TODO(ralonsoh): rehome to neutron_lib.
+class L3ExtensionException(exceptions.NeutronException):
+    message = _('The following L3 agent extensions do not inherit from '
+                '``neutron_lib.agent.l3_extension.L3AgentExtension``: '
+                '%(extensions)s.')
+
+
 def register_opts(conf):
     agent_ext_mgr_config.register_agent_ext_manager_opts(conf)
 
@@ -34,57 +44,37 @@ class L3AgentExtensionsManager(agent_ext_manager.AgentExtensionsManager):
     def __init__(self, conf):
         super(L3AgentExtensionsManager,
               self).__init__(conf, L3_AGENT_EXT_MANAGER_NAMESPACE)
+        extensions = []
+        for extension in self:
+            if not isinstance(extension.obj, (l3_extension.L3AgentExtension,)):
+                extensions.append(extension.attr)
+        if extensions:
+            raise L3ExtensionException(extensions=extensions)
 
     def add_router(self, context, data):
         """Notify all agent extensions to add router."""
         for extension in self:
-            if hasattr(extension.obj, 'add_router'):
-                extension.obj.add_router(context, data)
-            else:
-                LOG.error(
-                    "Agent Extension '%(name)s' does not "
-                    "implement method add_router",
-                    {'name': extension.name}
-                )
+            extension.obj.add_router(context, data)
         LOG.debug("L3 agent extension(s) finished router %s "
                   "add action.", data['id'])
 
     def update_router(self, context, data):
         """Notify all agent extensions to update router."""
         for extension in self:
-            if hasattr(extension.obj, 'update_router'):
-                extension.obj.update_router(context, data)
-            else:
-                LOG.error(
-                    "Agent Extension '%(name)s' does not "
-                    "implement method update_router",
-                    {'name': extension.name}
-                )
+            extension.obj.update_router(context, data)
         LOG.debug("L3 agent extension(s) finished router %s "
                   "update action.", data['id'])
 
     def delete_router(self, context, data):
         """Notify all agent extensions to delete router."""
         for extension in self:
-            if hasattr(extension.obj, 'delete_router'):
-                extension.obj.delete_router(context, data)
-            else:
-                LOG.error(
-                    "Agent Extension '%(name)s' does not "
-                    "implement method delete_router",
-                    {'name': extension.name}
-                )
+            extension.obj.delete_router(context, data)
         LOG.debug("L3 agent extension(s) finished router %s "
                   "delete action.", data['id'])
 
     def ha_state_change(self, context, data):
         """Notify all agent extensions for HA router state change."""
         for extension in self:
-            if hasattr(extension.obj, 'ha_state_change'):
-                extension.obj.ha_state_change(context, data)
-            else:
-                LOG.warning(
-                    "Agent Extension '%(name)s' does not "
-                    "implement method ha_state_change",
-                    {'name': extension.name}
-                )
+            extension.obj.ha_state_change(context, data)
+        LOG.debug('L3 agent extension(s) finished HA state change to "%s" '
+                  'for router %s', data['state'], data['router_id'])
diff --git a/releasenotes/notes/l3-extensions-must-inherit-from-L3AgentExtension-12c8f1fe2af26379.yaml b/releasenotes/notes/l3-extensions-must-inherit-from-L3AgentExtension-12c8f1fe2af26379.yaml
new file mode 100644
index 00000000000..ab8385967fe
--- /dev/null
+++ b/releasenotes/notes/l3-extensions-must-inherit-from-L3AgentExtension-12c8f1fe2af26379.yaml
@@ -0,0 +1,7 @@
+---
+other:
+  - |
+    Any L3 agent extension must inherit from
+    ``neutron_lib.agent.l3_extension.L3AgentExtension``. The
+    ``L3AgentExtensionsManager`` makes this check during the initial loading.
+    A ``L3ExtensionException`` will be raised if the condition is not met.