diff --git a/neutron_lib/api/definitions/trunk.py b/neutron_lib/api/definitions/trunk.py
index 1fdb12743..4ef61db03 100644
--- a/neutron_lib/api/definitions/trunk.py
+++ b/neutron_lib/api/definitions/trunk.py
@@ -95,6 +95,8 @@ REQUIRED_EXTENSIONS = [
 ]
 
 # The list of optional extensions.
-OPTIONAL_EXTENSIONS = None
+OPTIONAL_EXTENSIONS = [
+    "provider",  # needed to learn about network segmentation details.
+]
 
 # TODO(armax): add support for modeling custom queries
diff --git a/neutron_lib/api/validators.py b/neutron_lib/api/validators.py
index a37990e53..9f2cecc7b 100644
--- a/neutron_lib/api/validators.py
+++ b/neutron_lib/api/validators.py
@@ -924,12 +924,15 @@ def validate_subports(data, valid_values=None):
             return msg
         subport_ids.add(subport["port_id"])
 
-        # Validate that both segmentation id and segmentation type are
+        # Validate that both segmentation ID and segmentation type are
         # specified, and that the client does not duplicate segmentation
-        # ids
-        segmentation_id = subport.get("segmentation_id")
+        # IDs (unless it is explicitly asked to inherit segmentation
+        # details from the underlying subport's network).
         segmentation_type = subport.get("segmentation_type")
-        if (not segmentation_id or not segmentation_type) and len(subport) > 1:
+        if segmentation_type == 'inherit':
+            return
+        segmentation_id = subport.get("segmentation_id")
+        if (not segmentation_type or not segmentation_id) and len(subport) > 1:
             msg = _("Invalid subport details '%s': missing segmentation "
                     "information. Must specify both segmentation_id and "
                     "segmentation_type") % subport
diff --git a/neutron_lib/tests/unit/api/test_validators.py b/neutron_lib/tests/unit/api/test_validators.py
index 57eca716f..55b9926ea 100644
--- a/neutron_lib/tests/unit/api/test_validators.py
+++ b/neutron_lib/tests/unit/api/test_validators.py
@@ -1013,6 +1013,13 @@ class TestAttributeValidation(base.BaseTestCase):
         ]
         self.assertIsNotNone(validators.validate_subports(body))
 
+    def test_validate_subports_inherit_segmentation_details(self):
+        body = [
+            {'port_id': '00000000-ffff-ffff-ffff-000000000000',
+             'segmentation_type': 'inherit'}
+        ]
+        self.assertIsNone(validators.validate_subports(body))
+
     def test_validate_subports_valid_unique_segmentation_id(self):
         body = [
             {'port_id': '00000000-ffff-ffff-ffff-000000000000',