fix(neutron-understack): wire up better compat for evpn alias

python/neutron-understack/neutron_understack/conf/policies/evpn.py

@@ -1,6 +1,8 @@
 from neutron.conf.policies import base
 from oslo_policy import policy
 
+from neutron_understack import evpn_compat
+
 COLLECTION_PATH = "/routers"
 RESOURCE_PATH = "/routers/{id}"
 
@@ -31,4 +33,9 @@
 
 
 def list_rules():
+    # When core owns EVPN it registers these same policies; registering them
+    # again makes neutron.policy.register_rules() raise DuplicatePolicyError
+    # and abort policy initialization for the whole neutron-server.
+    if evpn_compat.core_provides_evpn():
+        return []
     return rules

python/neutron-understack/neutron_understack/evpn_compat.py

@@ -0,0 +1,56 @@
+"""Compatibility helpers for the EVPN router extension.
+
+The EVPN router feature (the ``evpn_vni`` attribute, its API extension and its
+policies) was upstreamed into neutron. On a neutron that carries it (e.g. the
+understack/2026.1 branch) core provides all of:
+
+* ``neutron_lib.api.definitions.evpn`` -- the router ``evpn_vni`` attribute
+* ``neutron.extensions.evpn``          -- the API extension
+* ``neutron.conf.policies.evpn``       -- the create/get ``router:evpn_vni`` policies
+
+If ``neutron_understack`` also registers any of these, neutron-server fails at
+startup -- most visibly ``DuplicatePolicyError`` raised from
+``neutron.policy.register_rules()``, which aborts policy initialization for the
+whole service. When core owns EVPN, Understack must register none of them and
+contribute only its runtime VNI allocation (the service-plugin callbacks).
+
+Every surface that would otherwise register an EVPN artifact keys off
+``core_provides_evpn()`` so the decision is made in exactly one place.
+"""
+
+from neutron.conf import policies as core_policies
+
+from neutron_understack.api.definitions import understack_vni
+
+try:
+    from neutron_lib.api.definitions import evpn as core_evpn_apidef
+except ImportError:
+    core_evpn_apidef = None
+
+# The router EVPN policy name core registers when it owns the feature. This is
+# also the exact rule neutron_understack would otherwise re-register, so its
+# presence in core's policy list is what triggers the DuplicatePolicyError.
+_CORE_EVPN_POLICY = "create_router:evpn_vni"
+
+
+def core_provides_evpn():
+    """Return True when neutron core registers the EVPN router feature itself.
+
+    Checks the actual policy list neutron.policy.register_rules() consumes
+    (``neutron.conf.policies.list_rules()``), rather than merely whether the
+    extension module is importable -- core registers these policies
+    unconditionally at startup, independent of which extensions are loaded.
+    """
+    return any(rule.name == _CORE_EVPN_POLICY for rule in core_policies.list_rules())
+
+
+def api_definition():
+    """The api-definition to use for the router VNI extension.
+
+    Prefer core's ``evpn`` api-definition when core owns the feature so the
+    ``evpn_vni`` attribute is defined exactly once; otherwise fall back to
+    Understack's own definition.
+    """
+    if core_provides_evpn() and core_evpn_apidef is not None:
+        return core_evpn_apidef
+    return understack_vni

python/neutron-understack/neutron_understack/extensions/understack_vni.py

@@ -1,23 +1,11 @@
-from importlib import util as importlib_util
-
 from neutron_lib.api import extensions
 
-try:
-    from neutron_lib.api.definitions import evpn as evpn_apidef
-except ImportError:
-    evpn_apidef = None
-
 from neutron_understack.api.definitions import understack_vni as apidef
 
 
-def _api_definition():
-    if (
-        evpn_apidef is not None
-        and importlib_util.find_spec("neutron.extensions.evpn") is None
-    ):
-        return evpn_apidef
-    return apidef
-
-
+# This descriptor carries Understack's own ``understack_vni`` alias. It is only
+# applied to routers when the UnderstackVniPlugin advertises that alias (see
+# vrf._supported_extension_aliases); when core owns EVPN the plugin advertises
+# core's ``evpn`` alias instead, so this descriptor is loaded but never applied.
 class Understack_vni(extensions.APIExtensionDescriptor):
-    api_definition = _api_definition()
+    api_definition = apidef

python/neutron-understack/neutron_understack/l3_router/vrf.py

@@ -6,18 +6,14 @@
 from neutron_lib.callbacks import registry
 from neutron_lib.callbacks import resources
 from neutron_lib.db import resource_extend
-
-try:
-    from neutron_lib.api.definitions import evpn as evpn_apidef
-except ImportError:
-    evpn_apidef = None
 from neutron_lib.plugins import constants as plugin_constants
 from neutron_lib.plugins import directory
 from neutron_lib.services import base as service_base
 from oslo_config import cfg
 from oslo_log import log as logging
 
 from neutron_understack import config
+from neutron_understack import evpn_compat
 from neutron_understack.api.definitions import understack_vni as apidef
 from neutron_understack.l3_router import understack_vni_db
 
@@ -44,9 +40,9 @@ def _is_vrf_router(context, router):
 
 
 def _supported_extension_aliases():
-    if evpn_apidef is not None:
-        return [evpn_apidef.ALIAS]
-    return [apidef.ALIAS]
+    # Advertise whichever extension provides the router evpn_vni attribute:
+    # core's ``evpn`` when core owns it, otherwise Understack's own.
+    return [evpn_compat.api_definition().ALIAS]
 
 
 @resource_extend.has_resource_extenders