A bridge apparatus or a switch apparatus (hereinafter referred to as L2 switch), which is a relay apparatus for an Ethernet (registered trademark) frame (hereinafter simply described as “frame”), includes a database called FDB (Filtering Database) for accumulating an entry including a transmission source MAC address of a received frame, a port in which the frame is received, and other information. In the FDB, an entry is added every time a frame is received anew. This operation is called MAC address learning (or address learning).
The FDB is used when a received frame is relayed. Specifically, the L2 switch that receives a frame searches through the FDB using a destination MAC address of the received frame as a search key. If the destination MAC address is a learned MAC address, a port (an output port) corresponding to the MAC address used as the search key is obtained. If the received frame is transferred to the port, because another L2 switch of a transfer destination performs the same operation, the received frame finally reaches a target Ethernet (registered trademark) node (hereinafter simply described as “node”). On the other hand, concerning a received frame having a destination MAC address without an entry in the FDB, information concerning an output port is not obtained even if the L2 switch that receives the frame searches through the FDB (it is unknown to which port the frame should be transferred). Therefore, the L2 switch broadcasts the received frame to all ports other than a port in which the frame is received. This operation is called flooding. Such a frame relay operation is described in Non Patent Literature 1.