Network system including lower and upper switches and link group interconnecting lower switches to upper switches, and method of operating the same

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.