diff --git a/roles/run-tempest/defaults/main.yaml b/roles/run-tempest/defaults/main.yaml
index 52713be..5f13883 100644
--- a/roles/run-tempest/defaults/main.yaml
+++ b/roles/run-tempest/defaults/main.yaml
@@ -6,3 +6,6 @@
 stable_constraints_file: "{{ devstack_base_dir }}/requirements/upper-constraints.txt"
 target_branch: "{{ zuul.branch }}"
 tempest_tox_environment: {}
+# NOTE(gmann): external_bridge_mtu shows as undefined for run-tempest role
+# defining default value here to avoid that error.
+external_bridge_mtu: 0