Ring-coupling operating mode . In single(1) mode both coupling ports are local to the switch, switch performs master and slave functions. In dual-master-inband(2) mode second coupling port is on a remote switch, local switch is master, communication over network. In dual-master-outband(3) mode second coupling port is on a remote switch, local switch is master, communication over dedicated control port. In dual-slave-inband(4) mode second coupling port is on a remote switch, local switch is slave, communication over network. In dual-slave-outband(5) mode second coupling port is on a remote switch, local switch is slave, communication over dedicated control port. unknown(6) cannot be set,it's returned when no mode is configured.