diff --git a/python/ironic-understack/ironic_understack/inspect_hook_update_baremetal_ports.py b/python/ironic-understack/ironic_understack/inspect_hook_update_baremetal_ports.py
index fcb87a0e5..806e4b41d 100644
--- a/python/ironic-understack/ironic_understack/inspect_hook_update_baremetal_ports.py
+++ b/python/ironic-understack/ironic_understack/inspect_hook_update_baremetal_ports.py
@@ -172,12 +172,14 @@ def _set_node_traits(task, vlan_groups: set[str]):
 
     We remove pre-existing traits if the node does not have the required
     connections.
+
+    Traits other than CUSTOM_*_SWITCH are left alone.
     """
     node = task.node
     existing_traits = set(node.traits.get_trait_names())
     vlan_group_traits = {_trait_name(x) for x in vlan_groups if x}
     irrelevant_existing_traits = {x for x in existing_traits if not _is_our_trait(x)}
-    required_traits = irrelevant_existing_traits.intersection(vlan_group_traits)
+    required_traits = irrelevant_existing_traits.union(vlan_group_traits)
 
     if existing_traits == required_traits:
         LOG.debug(
diff --git a/python/ironic-understack/ironic_understack/tests/test_inspect_hook_update_baremetal_ports.py b/python/ironic-understack/ironic_understack/tests/test_inspect_hook_update_baremetal_ports.py
index 1ba9955ee..4cf5bd735 100644
--- a/python/ironic-understack/ironic_understack/tests/test_inspect_hook_update_baremetal_ports.py
+++ b/python/ironic-understack/ironic_understack/tests/test_inspect_hook_update_baremetal_ports.py
@@ -87,7 +87,7 @@ def test_with_valid_data(mocker, caplog):
         "ironic_understack.inspect_hook_update_baremetal_ports.CONF.ironic_understack.switch_name_vlan_group_mapping",
         MAPPING,
     )
-    mocker.patch(
+    trait_create = mocker.patch(
         "ironic_understack.inspect_hook_update_baremetal_ports.objects.TraitList.create"
     )
 
@@ -103,3 +103,7 @@ def test_with_valid_data(mocker, caplog):
     assert mock_port.physical_network == "f20-3-network"
     mock_port.save.assert_called()
     mock_node.save.assert_called_once()
+
+    trait_create.assert_called_once_with(
+        mock_context, 1234, {"CUSTOM_STORAGE_SWITCH", "CUSTOM_NETWORK_SWITCH", "bar"}
+    )