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1. A network system in which a first device and a second device, each having a plurality of line terminals, a lower stage line terminal, and an inter-device line terminal, are arranged opposite each other, the inter-device line terminals of the first device and the second device, arranged opposite each other, are connected by an inter-device interface to thereby form a pair of devices and the pairs of devices are arranged in multiple stages, a lower stage line terminal of a first device arranged on an upper side and one line terminal of a first device arranged on a lower side, in adjacent stages, are connected by a line, and a lower stage line terminal of a second device arranged on an upper side and one line terminal of a second device arranged on a lower side, in adjacent stages, are connected by a line, wherein
each of the devices includes a link control function unit which notifies a counterpart device of a line in which a link disconnection occurs among lines terminated at line terminals or a lower stage line terminal of a self device and a cause of the occurrence of the link disconnection is not a forcible closure of a line terminal of the self device, and when a line in which a link disconnection occurs, notified from the counterpart device, is a line other than a line terminated at a lower stage line terminal of the counterpart device, forcibly closes a line terminal of the self device associated with the line in which the link disconnection occurs notified from the counterpart device, while when a line in which a link disconnection occurs, notified from the counterpart device, is the line terminated at the lower stage line terminal of the counterpart device, does not forcibly close the lower stage line terminal of the self device, and wherein each of the devices further includes a load balancing function unit which, when both the line terminated at the lower stage line terminal of the self device and an inter-device interface terminated at an inter-device line terminal of the self device are normal, outputs frames input via a line terminal of the self device to the inter-device line terminal of the self device and to the lower stage line terminal of the self device in a distributive manner at a predetermined ratio, when a link disconnection occurs in the line terminated at the lower stage line terminal of the self device, outputs all frames input via a line terminal of the self device to the inter-device line terminal of the self device, when a link disconnection occurs in the inter-device interface terminated at the inter-device line terminal of the self device, outputs all frames input via a line terminal of the self device to the lower stage line terminal of the self device, and when a line in which a link disconnection occurs, notified from the counterpart device, is the line terminated at the lower stage line terminal of the counterpart device, outputs all frames input via a line terminal of the self device to the inter-device line terminal of the self device.
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