No need for an ISO, found what I believe to be the trigger for the problem. WORKAROUND: Delete /etc/sysctl.d/70-yast.conf If cluster is already bootstrapped, reboot all nodes. Cluster communications work properly afterwards. NEXT STEP: Figure out why the heck /etc/sysctl.d/70-yast.conf's blocking of IP forwarding is taking an effect when /usr/lib/sysctl.d/90-yast.conf should be overriding it :)