Patent ID: 8787374

Claim:
A network system, comprising: a plurality of lower switches; a plurality of upper switches; and a link group for interconnecting the plurality of lower switches to the plurality of upper switches in a multipoint-to-multipoint relationship, wherein the link group comprises a sub-link for interconnecting the plurality of lower switches to the plurality of upper switches in a point-to-point relationship as one logical link and a main link for interconnecting the plurality of lower switches to the plurality of upper switches in a point-to-multipoint relationship as one logical link, wherein the plurality of lower switches are each set to conduct a broadcast transmission such that a broadcast frame with a broadcast address set as a destination is transmitted to the plurality of upper switches through the sub-link that is available for communication and connected to the plurality of lower switches, wherein the plurality of upper switches each receive the broadcast frame and register a source address contained in the broadcast frame into a forwarding database (FDB) of the plurality of upper switches, wherein said each of lower switches comprises: a plurality of first ports, each of the first ports being connected to one of the plurality of upper switches; and a second port different from the first ports, wherein, when said each of the lower switches receives the broadcast frame at the second port, the broadcast frame is transmitted to the plurality of upper switches, wherein the broadcast frame is transmitted to all of the plurality of upper switches through all sub-links that are available for communication and are connected to said each of the plurality of lower switches, wherein a first port of the plurality of first ports belongs to a link aggregation group (LAG), and wherein, for transferring a unicast frame that a destination address is set to be a specific MAC address, a frame transfer unit transfers the unicast frame by reference to the FDB and an LAG table.