Patent ID: 7911941

Claim:
A router that is interposed between a subnet and a backbone network, the subnet includes a plurality of layer 2 switches and a plurality of terminals connected to the layer 2 switches, the terminals are communicating with each other via the layer 2 switches and a master router, wherein the router connects to the master router, the subnet and the backbone network, and originally operates as a backup router, but which can be transitioned to operate as a redundant master router, the router comprising: a severance detection unit which detects a severance between the layer 2 switches within the subnet; a redundant function control unit which, in the event that a severance within the subnet has been detected by the severance detection unit: (i) resumes interrupted communication between the backbone network and the subnet via the router, (ii) receives, from the master router, a copy of an address resolution protocol (ARP) table, and stores the copy of the ARP table in the router, wherein the ARP table includes internet protocol (IP) addresses correlated with MAC addresses of the plurality of terminals, and (iii) transitions the router from operating as a backup router to operating as a redundant master router; a connection control unit which: (i) sets up a virtual local area network (VLAN), the VLAN includes intra-subnet routing in the subnet, and backbone network routing that is at least partially outside of the subnet through the backbone network, and (ii) connects the master router and the router, when the router operates as the redundant master router, via the backbone network routing; an ARP transmission/reception unit, which in the event that a severance within the subnet has been detected by the severance detection unit: (i) transmits an ARP request packet to each terminal along either the intra-subnet routing or backbone network routing, wherein the ARP request packet requests an MAC address for each IP address included in the ARP table, and (ii) receives an ARP response packet from each of the terminals via either the intra-subnet routing of the backbone network routing, wherein each ARP response packet includes the MAC address of the respective terminal; and an ARP response forwarding unit which generates a learning packet for each ARP response packet, wherein the learning packet has the MAC address extracted from the ARP response packet, and forwards the learning packet to the terminal associated with the MAC address in the learning packet along the same routing from which the respective ARP response packet was received, except in the reverse direction.