I think the wording here is confusing. If you don't specify enabled_drivers it will still fail. It should be: ensure that enabled_drivers parameter is specified and does not contain pxe_ssh driver. https://access.redhat.com/documentation/en-us/red_hat_openstack_platform/13/html/fast_forward_upgrades/assembly-upgrading_the_undercloud#annotations:8e1fa15e-c5f2-45e2-a9a2-05884ab6bc20