A network relay device relays data in a layer 2 network. The network relay device includes first and second communication ports, a snooping module, a transfer information storage unit, a multicast sending module, a failure detector and a port adding module. The snooping module generates snooping information. The snooping information correlates the first communication port set to a multicast transfer port to a destination MAC address. The multicast sending module refers to the snooping information stored in the transfer information storage unit and sends a multicast frame received from the layer 2 network, from the correlated multicast transfer port. The failure detector detects a communication failure in the layer 2 network. The port adding module additionally, in response to detection of the communication failure by the failure detector, set the second communication port, in addition to the first communication port, to the multicast transfer port.