An encapsulation address of a first switching device is set so as to be different only in a mask bit in comparison with that of a second switching device. A learning information control unit of a third switching device does not learn the correspondence relation between a source customer address and a source encapsulation address in a first case and a second case. The first case corresponds to a case in which an encapsulated frame is received at an upper-link port and an encapsulation address corresponding to the source customer address is acquired from an address table. The second case corresponds to a case in which a difference between the source encapsulation address and the encapsulation address acquired from the address table lies only in the mask bit.