'Expects to find
\'<address addr="00:08:54:26:A9:E5" addrtype="mac" vendor="Netronix" />\'
in the node
@return mac_address or \'unknown\''

def get_mac_address(self):

    mac_address = self.get_attrib_from_subnode("address[@addrtype='mac']", 'addr')
    return (mac_address if mac_address else 'unknown')
