Disclosed are an expandable network system and a redundant method applied to the system. According to one embodiment, the network system may be a ring-type or linear-type network system. A plurality of network nodes form one or more sub-networks. Many interconnected sub-networks form another network. While the network system is initialized, the method defines the role of each network port of each network node in each sub-network. Concurrently, every network port interconnected between the sub-networks is also defined to be a blocked port or a forward port. This mechanism defining the role of each port not only prevents the possible broadcast storm occurred in the network system, but also modifies the network signaling path by re-defining the roles of the ports while the connection changes. Therefore, the expandable network system and the method achieve a fast-redundant connection scheme, and the more flexible network-expanding function.