A port switch of type A has a distribution mode for maintaining a backward compatibility with a port switch of type B. A distribution identifier calculating unit performs a logical operation with the same operational expression to the same bit region as the type B in a header contained in a frame, thereby calculating a k-bit (k>j) distribution identifier partly containing the same j-bit bit region as the type B. A distribution table indicates a correspondence relation between three sub-links and the k-bit distribution identifier, and has contents in which the j-bit distribution identifier in the distribution table of the type B is replaced with the k-bit distribution identifier partly containing the same j-bit value as that of the j-bit distribution identifier.