Patent Publication Number: US-7898942-B2

Title: Ring network system, failure recovery method, failure detection method, node and program for node

Description:
TECHNICAL FIELD 
     The present invention relates to a ring network system, and a failure recovery method, a failure detection method, a node and a program for a node which are applied to a ring network system 
     BACKGROUND ART 
     With the increase in a volume of data system packet traffic whose representative is IP, efficient transmission of data is demanded also of related art communication service providers (carriers) which provide transmission service mainly for voice. Along with the demand, a highly reliable protection method similar to SONET (Synchronous Optical Network) is required also in a data transmission network. Among failure recovery methods in a packet transmission network is an RPR (Resilient Packet Ring) disclosed in Literature 5. 
     In RPR, an RPR-MAC frame is transferred between nodes in a packet ring (hereinafter simply referred to as a ring). An RPR-MAC frame has a format including a frame as a payload, to which frame, a transmission destination RPR-MAC address, a transmission source RPR-MAC address and the like are added. Frame to be a payload in an RPR-MAC frame is called a user MAC frame. In the following, a user MAC frame will be denoted to as a U-MAC frame. When a node in a ring is to receive a frame from a terminal outside the ring, the node receives a U-MAC frame. Then, the node adds a transmission destination RPR-MAC address, a transmission source RPR-MAC address and the like to the U-MAC frame to generate an RPR-MAC frame and transfers the generated frame within the ring. In the following description, a transmission destination address in a U-MAC frame will be denoted as a transmission destination U-MAC address. Similarly, a transmission source address in a U-MAC frame will be denoted as a transmission source U-MAC address. 
     In the following, a failure protection method using a related RPR will be described. 
       FIG. 20  shows an example of an RPR network using a single ring. The ring has an inner ring  101  and an outer ring  102 . The inner ring  101  and the outer ring  102  are paths for transferring packets in reverse directions to each other. In addition, the RPR network illustrated in  FIG. 20  comprises six RPR nodes  300 - 1 ˜ 300 - 6  in the ring. To the RPR node  300 - 1 , a terminal  110  is connected and to the RPR node  300 - 4 , a terminal  111  is connected. First, description will be made of operation of transferring a U-MAC frame from the terminal  110  to the terminal  111  when a failure is yet to occur (in a normal state) in the RPR network shown in  FIG. 20 . RPR-MAC addresses of the RPR nodes  300 - 1 ˜ 300 - 6  are assumed to be  300 - 1 ˜ 300 - 6 , respectively. 
     The RPR node  300 - 1  having received a U-MAC frame transferred from the terminal  110  solves the RPR-MAC address  300 - 4  of a transmission destination RPR node from a transmission destination U-MAC address of the U-MAC frame. With a learning data base provided in which a correspondence between a U-MAC address and an RPR-MAC address is stored, each node searches an RPR-MAC address corresponding to a transmission destination U-MAC address to solve an RPR-MAC address of a transmission destination RPR node. Subsequently, the RPR node  300 - 1  adds an RPR-MAC header with a transmission destination RPR-MAC address as  300 - 4  and a transmission source RPR-MAC address as  300 - 1  to the U-MAC frame to generate an RPR-MAC frame. Furthermore, the RPR node  300 - 1  determines to which of an outer ring and an inner ring, the RPR-MAC frame should be transferred to send out the RPR-MAC frame. Assume here that the RPR-MAC frame is to be sent to the outer ring  102 . 
     The RPR node  300 - 6  having received the RPR-MAC frame refers to a transmission destination RPR-MAC address of the RPR-MAC frame. The RPR node  300 - 6 , since the transmission destination RPR-MAC address ( 300 - 4 ) fails to coincide with the address ( 300 - 6 ) of the node itself, determines that it is not a frame to be dropped by the node itself to transfer the RPR-MAC frame to the subsequent RPR node  300 - 5 . Similarly to the RPR node  300 - 6 , the RPR node  300 - 5  transfers the RPR-MAC frame to the subsequent RPR node  300 - 4 . 
     The RPR node  300 - 4 , since the transmission destination RPR-MAC address of the received RPR-MAC frame coincides with the address of the node itself, takes in the RPR-MAC frame and removes an RPR-MAC overhead to transfer the obtained frame as a U-MAC frame to the terminal  111 . Thus, the U-MAC frame is transferred from the terminal  110  to the terminal  111 . 
     Next, description will be made of operation executed when a failure occurs in the ring.  FIG. 21  is a diagram for use in explaining operation of the ring executed when a failure occurs in the ring. Assume here that the inner ring  101  and the outer ring  102  between the RPR  300 - 5  and  300 - 6  develop a fault. A path indicated by a dotted line in  FIG. 21  is the same path as a frame transfer path in a normal state shown in  FIG. 20 . 
     The RPR nodes  300 - 5  and  300 - 6  adjacent to a failure occurrence position sense occurrence of the failure. Then, the RPR nodes  300 - 5  and  300 - 6  each transfer a failure notification RPR-MAC frame including its own node RPR-MAC address to the inner ring  101  and the outer ring  102 . The RPR nodes  300 - 5  and  300 - 6  apply an identifier indicative of being a failure notification RPR-MAC frame as a transmission destination RPR-MAC address of the failure notification RPR-MAC frame. Thus, there occurs a case where to a transmission destination RPR-MAC address of an RPR-MAC frame, an identifier for control is applied. The controlling identifier may be denoted as ┌reserved RPR-MAC address┘ in some cases. Controlling identifier does not indicate an RPR-MAC address of a specific RPR node. 
     Upon receiving a failure notification RPR-MAC frame from both the inner ring  101  and the outer ring  102 , each of the RPR nodes  300 - 1 ˜ 300 - 6  locates a failure position from topology information and failure detection node information. Then, the node switches a transfer direction of an RPR-MAC frame passing through the failure position. For example, the RPR node  300 - 1  switches a sending-out ring of an RPR-MAC frame with the RPR node  300 - 4  as a transmission destination from the outer ring  102  to the inner ring  101 . The RPR node  300 - 1 , upon receiving a U-MAC frame from the terminal  110  similarly to the case described in  FIG. 20 , sends an RPR-MAC frame with a header added to the U-MAC frame to the inner ring  101 . The RPR-MAC frame is transferred to the RPR node  300 - 4  through the RPR nodes  300 - 2  and  300 - 3 . The RPR node  300 - 4  removes an RPR-MAC overhead from the received RPR-MAC frame and transfers the obtained frame as a U-MAC frame to the terminal  111 . Failure recovery is executed as described in the foregoing to transfer a frame from the terminal  110  to the terminal  111  even at failure occurrence. 
     Next, a structure of an RPR node will be described.  FIG. 22  is a block diagram showing an example of a structure of a related RPR node. The RPR node comprises a packet switch  310 , a frame conversion circuit  320 , a forwarding engine  330 , a ring topology information collection circuit  340 , a ring failure information collection circuit  350 , an inside ring path determination circuit  360  and an ADM (Add-Drop Multiplexer)  370 . The node further comprises a learning data base  321  which stores a correspondence relationship between a U-MAC address and an RPR-MAC address. The node further comprises a forwarding data base  331  which stores a correspondence relationship between a transmission destination RPR-MAC address and an output ring (either one or both of an inner ring and an outer ring). 
     The packet switch  310 , which is a UNI (User Network Interface), executes transmission/reception of a U-MAC frame to/from the terminals through UNI ports  301  and  302 . Upon receiving a U-MAC frame through the respective UNI ports  301  and  302 , the packet switch  310  concentrates U-MAC frames from the UNI ports  301  and  302  and transfers the frames to the frame conversion circuit  320 . In addition, when a U-MAC frame is transferred from the frame conversion unit  320 , the packet switch  310  outputs (transmits) the U-MAC frame through an appropriate UNI port (UNI port connected to a terminal as a transmission destination). 
     When the packet switch  310  receives a U-MAC frame from the terminal and the U-MAC frame is transferred from the packet switch  310 , the frame conversion circuit  320  refers to the learning data base  321  to specify a transmission destination RPR-MAC address in the ring based on a transmission destination U-MAC address of the U-MAC frame (i.e. solve the transmission destination RPR-MAC address). Subsequently, the frame conversion circuit  320  adds a header with the RPR-MAC address as a transmission destination RPR-MAC address and its own node address as a transmission source RPR-MAC address (RPR-MAC overhead) to the U-MAC frame to generate an RPR-MAC frame and transfers the RPR-MAC frame to the forwarding engine  330 . 
     In addition, when the ADM  370  receives an RPR-MAC frame within the ring and the RPR-MAC frame is transferred from the ADM  370 , the frame conversion circuit  320  removes the RPR-MAC overhead from the RPR-MAC frame to convert the frame into a U-MAC frame. Then, the circuit transfers the U-MAC frame to the packet switch  310 . 
     The forwarding engine  330  determines based on the transmission destination RPR-MAC address of the RPR-MAC frame transferred from the frame conversion circuit  320  whether the RPR-MAC frame is to be sent to the inner ring  101  or the outer ring  102 , or to both of the inner ring  101  and the outer ring  102 . At this time, the forwarding engine  330  only needs to refer to the forwarding data base  331  to search an output ring (either the inner ring  101  or the outer ring  102  or both) corresponding to the transmission destination RPR-MAC address. Thereafter, the forwarding engine  330  transfers the RPR-MAC frame to the ADM  370  and notifies the ADM  370  of the determined sending-out destination ring. 
     In the following, information will be referred to as output ring information which indicates that an RPR-MAC frame is to be sent out to the inner ring  101 , that an RPR-MAC frame is to be sent out to the outer ring  102 , or that an RPR-MAC frame is to be sent out to both the inner ring  101  and the outer ring  102 . 
     The ring topology information collection circuit  340  generates a topology discovery RPR-MAC frame for comprehending an RPR node structure in the ring to transfer the frame to the ADM  370  together with output ring information. When the ADM  370  receives a topology discovery RPR-MAC frame from inside the ring and the topology discovery RPR-MAC frame is transferred from the ADM  370 , the ring topology information collection circuit  340  notifies the inside ring path determination circuit  360  of topology information indicated by the topology discovery RPR-MAC frame. 
     When the ADM  370  receives a failure notification RPR-MAC frame from inside the ring and the failure notification RPR-MAC frame is transferred from the ADM  370 , the ring failure information collection circuit  350  notifies the inside ring path determination circuit  360  of failure detection node information indicated by the failure notification RPR-MAC frame. Thereafter, the ring failure information collection circuit  350  outputs the failure notification RPR-MAC frame to the ADM  370  and causes the ADM  370  to send out the failure notification RPR-MAC frame within the ring. 
     Adjacent RPR nodes transmit/receive failure notification RPR-MAC frames not including failure detection node information to/from each other. When none of failure notification RPR-MAC frames without including failure detection node information is transferred for a fixed time period, the ring failure information collection circuit  350  determines that a failure is detected. Then, the circuit generates a failure information notification RPR-MAC frame with its own node as a failure detection node and outputs the failure information notification RPR-MAC frame and the output ring information to the ADM  370  to cause the ADM  370  to send out the failure information notification RPR-MAC frame within the ring. In addition, in this case, the ring failure information collection circuit  350  transfers information that its own node is a failure detection node to the inside ring path determination unit  360 . 
     Based on topology information notified from the ring topology information collection circuit  340  and failure information notified from the ring failure information collection circuit  350 , the inside ring path determination circuit  360  correlates output ring information with a transmission destination RPR-MAC address and registers the correlation in the forwarding data base  331 . 
     The ADM  370  refers to a transmission destination RPR-MAC address of an RPR-MAC frame input from the outer ring  102  to determine whether the RPR-MAC frame should be terminated or not. When determining to terminate the frame, the ADM  370  transfers the RPR-MAC frame to any one of the forwarding engine  330 , the ring topology information collection circuit  340  and the failure information collection circuit  350  (to which circuit the transfer should be made depends on a kind of RPR-MAC frame). When determining not to terminate the frame, the ADM  370  sends out the RPR-MAC frame to the same ring as the ring to which the frame is input (the inner ring or the outer ring). 
     In addition, when an RPR-MAC frame and output ring information are transferred from the forwarding engine  330 , the ring topology information collection circuit  340  or the ring failure information collection circuit  350 , the ADM  370  sends out the RPR-MAC frame to either the inner ring  101  or the outer ring  102  (or both of them) according to the output ring information. 
     Such an RPR node realizes such failure recovery as shown in  FIG. 21 . 
     Literature reciting techniques related to a ring network include Literature 1 through 4 and the like. Recited in Literature 1 is the technique of making ring soundness known to all the nodes by using a KAB (keep alive bit). Recited in Literature 2 is the technique that when a link between a node inside a ring and a node outside the ring develops a fault, the node inside the ring sends out a frame with a flooding bit set within the ring. Recited in Literature 3 is necessity of notifying a serving node of occurrence of line cut-off. Recited in Literature 4 is deleting an unnecessary transfer table when among a plurality of contact nodes belonging to two rings, an active contact code is switched. 
     Literature 1: Japanese Patent Laying-Open No. 2003-174458 (paragraphs 0029-0048). 
     Literature 2: Japanese Patent Laying-Open No. 2004-242194 (paragraphs 0030-0031). 
     Literature 3: Japanese Patent Laying-Open No. 10-4424 (paragraph 0006). 
     Literature 4: Japanese Patent Laying-Open No. 2003-258822 (paragraphs 0022-0085). 
     Literature 5: “IEEE Draft P802.17/D3.3 Part 17: Resilient Packet Ring (RPR) Access Method &amp; Physical Layer Specifications)”, IEEE (Institute of Electrical and Electronics Engineers, Inc), “IEEE Draft P802.17/D3.3”, p. 74, Apr. 21, 2004. 
     Related RPR techniques enable extremely high-speed failure recovery to be provided for a single ring network. However, the problem is disabling high-speed failure recovery of a failure of an inter-link in a multi-ring with a plurality of rings connected. 
       FIG. 23  is a diagram for use in explaining an example of a multi-ring network having two rings connected by two inter-links. In the multi-ring network shown in  FIG. 23 , two rings  401  and  402  are connected through inter-links  403  and  404 . The inter-link  403  is used as a living link and the inter-link  404  is a spare inter-link. One ring  401  comprises the RPR nodes  300 - 1 ˜ 300 - 6 , with a terminal  410  connected to the RPR node  300 - 1 . The other ring  402  comprises RPR nodes  300 - 7 ˜ 300 - 12 , with a terminal  411  connected to the RPR node  300 - 10 . 
     A path (traffic flow)  420  illustrated in  FIG. 23  shows a path used in a case of transferring a frame from the terminal  410  to the terminal  411  in a normal state. Type of frame (U-MAC frame or RPR-MAC frame) varies with a position on the path  420 . The RPR node  300 - 1  receives a U-MAC frame from the terminal  410 . Then, the RPR node  300 - 1  adds a header with  300 - 3  as a transmission destination RPR-MAC address to convert the U-MAC frame into an RPR-MAC frame and transmits the frame directed to the RPR node  300 - 3 . The RPR node  300 - 3  removes an RPR-MAC overhead of the received RPR-MAC frame to convert the frame into a U-MAC frame. Then, the node sends out the U-MAC frame to the inter-link  403 . Upon receiving the U-MAC frame, the RPR node  300 - 7  adds a header with  300 - 10  as a transmission destination RPR-MAC address to convert the U-MAC frame into an RPR-MAC frame and sends out the frame to an outer ring  402 -outer. The RPR node  300 - 10  removes the overhead of the arriving RPR-MAC frame to convert the frame into a U-MAC frame and transfers the obtained frame to the terminal  411 . As a result, the frame is transferred along the path  420  shown in  FIG. 23  in a normal state. 
     Assume that in this multi-ring network, a failure occurs on the inter-link  403 . In a related multi-ring network, the RPR node  300 - 1  is not allowed to know occurrence of a failure on the inter-link  403  immediately. Therefore, the RPR node  300 - 1  converts the U-MAC frame from the terminal  410  into an RPR-MAC frame with  300 - 3  added as a transmission destination RPR-MAC address as in a normal state. As a result, the RPR-MAC frame is terminated at the RPR node  300 - 3  and not at the RPR node  300 - 4  at which the frame is to originally arrive. For reaching the RPR node  300 - 4 , a time is required until a correspondence between a U-MAC address of the terminal  411  and an RPR-MAC address is invalidated at a learning data base of the RPR node  300 - 1  (in general, called an aging time whose time is approximately 5 minutes), so that high-speed failure recovery is impossible. In other words, from when a correspondence relationship between latest U-MAC address and RPR-MAC address is learned until a lapse of the aging time, a correspondence relationship between new U-MAC address and RPR-MAC address can not be learned. Then, after the lapse of the aging time, a broadcast frame is transferred and at that time a correspondence relationship between a transmission source U-MAC address and a transmission source RPR-MAC address is learned. Until the learning is executed, no frame transfer from the RPR node  300 - 1  to the RPR node  300 - 4  is allowed to disable high-speed failure recovery. 
     An exemplary object of the present invention is to provide, in a multi-ring network with a plurality of ring networks connected, a ring network system, a failure recovery method, a failure detection method, a node and a program for a node which enable high-speed failure recovery when a failure occurs between ring networks. 
     SUMMARY 
     According to a first exemplary aspect of the invention, a ring network system with a first ring network and a second ring network connected by a living communication path and a spare communication path, wherein each node in said first ring network and in said second ring network includes a learning data base for storing a correspondence relationship between an address of a terminal outside the ring network and an address of a node within the ring network, a failure occurrence determination unit for determining that a failure occurs on the living communication path, and an erasure unit for erasing information stored by the learning data base when said failure occurrence determination unit determines that a failure occurs on the living communication path, each node other than a contact node arranged as an end portion of the living communication path and the spare communication path includes a broadcast unit for, when an address of a node corresponding to a transmission destination address that a user frame received from a terminal outside the ring network fails to be stored in said learning data base, broadcast-transmitting a frame with said user frame accommodated as a payload, and a contact node arranged as an end portion of the spare communication path includes a state change unit for changing a port connected to the spare communication path to a state of enabling a user frame to be sent out when said failure occurrence determination unit determines that a failure occurs on the living communication path. 
     According to another exemplary aspect of the invention, the erasure unit erases all the information stored by the learning data base when the failure occurrence determination unit determines that a failure occurs on the living communication path. 
     According to another exemplary aspect of the invention, the erasure unit erases only an address of a contact node arranged as an end portion of the living communication path and an address of a terminal corresponding to the address among the information stored by the learning data base when the failure occurrence determination unit determines that a failure occurs on the living communication path. 
     According to another exemplary aspect of the invention, a contact node arranged as an end portion of the living communication path includes an existence confirmation signal transmission unit for periodically transmitting an existence confirmation signal to other ring network through the living communication path, and an existence confirmation signal reception unit for receiving an existence confirmation signal from said other ring network. 
     According to another exemplary aspect of the invention, a contact node arranged as an end portion of the living communication path includes an existence confirmation frame transmission unit for transmitting a frame with an existence confirmation signal received by the existence confirmation signal reception unit accommodated as a payload within a ring network to which the contact node belongs, the failure occurrence determination unit that other node than said contact node includes among nodes in said ring network determines that a failure occurs on the living communication path when failing to receive said frame for a fixed time period, and the failure occurrence determination unit that said contact node includes determines that a failure occurs on the living communication path when failing to receive an existence confirmation signal for a fixed time period. 
     According to a second exemplary aspect of the invention, a ring network system with a first ring network and a second ring network connected by a plurality of communication paths correlated with network identifiers in advance, wherein among nodes in said first ring network and in said second ring network, each node other than a contact node arranged as an end portion of said plurality of communication paths includes a network identifier deriving unit for deriving a network identifier to be applied to a user frame received from a terminal outside the ring network from network identifiers corresponding to communication paths developing no fault, a network identifier applying unit for applying said network identifier to said user frame, a failure occurrence determination unit for determining that a failure occurs on a communication path, a failure occurring communication path notification unit for, when said failure occurrence determination unit determines that a failure occurs on a communication path, notifying said network identifier deriving unit of information which can specify a network identifier corresponding to the communication path developing the fault, a learning data base for storing a correspondence relationship among an address of a terminal outside the ring network, a network identifier and an address of a node within the ring network, and a broadcast unit for, when an address of a node corresponding to a combination between a transmission destination address that a user frame received from a terminal outside the ring network has and a network identifier derived by the network identifier deriving unit fails to be stored in said learning data base, broadcast-transmitting a frame with a user frame to which said network identifier is applied accommodated as a payload, and each contact node arranged as an end portion of each communication path includes a user frame sending unit for, when a network identifier corresponding to a communication path with the contact node itself as an end portion and a network identifier applied to a user frame coincide, sending said user frame to said communication path. 
     According to another exemplary aspect of the invention, each contact node arranged as an end portion of each communication path includes an existence confirmation signal transmission unit for periodically transmitting an existence confirmation signal to other ring network through a communication path with the contact node itself as an end portion, and an existence confirmation signal reception unit for receiving an existence confirmation signal from said other ring network. 
     According to another exemplary aspect of the invention, each contact node arranged as an end portion of each communication path includes an existence confirmation frame transmission unit for transmitting a frame with an existence confirmation signal received by the existence confirmation signal reception unit accommodated as a payload within a ring network to which the contact node belongs, and when failing to receive said frame for a fixed time period, the failure occurrence determination unit that other node than said contact node includes among nodes in said ring network determines that a failure occurs on a communication path with a contact node which is a transmission source of said frame as an end portion. 
     According to a third exemplary aspect of the invention, a ring network system with a first ring network and a second ring network connected by a communication path, wherein a contact node arranged as an end portion of said communication path includes an existence confirmation signal transmission unit for periodically transmitting an existence confirmation signal to other ring network through said communication path, an existence confirmation signal reception unit for receiving an existence confirmation signal from said other ring network, and an existence confirmation frame transmission unit for transmitting a frame with an existence confirmation signal received by the existence confirmation signal reception unit accommodated as a payload within a ring network to which the contact node belongs, and a node other than a contact node among nodes in the ring network includes a failure occurrence determination unit for determining that a failure occurs on said communication path when failing to receive said frame for a fixed time period. 
     According to a fourth exemplary aspect of the invention, a failure recovery method to be applied to a ring network system with a first ring network and a second ring network connected by a living communication path and a spare communication path, wherein each node in said first ring network and in said second ring network includes a learning data base for storing a correspondence relationship between an address of a terminal outside the ring network and an address of a node within the ring network, and when determining that the living communication path develops a fault, erases information stored by the learning data base, each node other than a contact node arranged as an end portion of the living communication path and the spare communication path, when an address of a node corresponding to a transmission destination address that a user frame received from a terminal outside the ring network has fails to be stored in said learning data base, broadcast-transmits a frame with said user frame accommodated as a payload, and a contact node arranged as an end portion of the spare communication path, when determining that the living communication path develops a fault, changes a port connected to the spare communication path to a state of enabling a user frame to be sent out. 
     According to another exemplary aspect of the invention, each node in the first ring network and the second ring network erases all the information stored by the learning data base when determining that a failure occurs on the living communication path. 
     According to another exemplary aspect of the invention, each node in the first ring network and the second ring network erases only an address of a contact node arranged as an end portion of the living communication path and an address of a terminal corresponding to the address among the information stored by the learning data base when determining that a failure occurs on the living communication path. 
     According to another exemplary aspect of the invention, a contact node arranged as an end portion of the living communication path periodically transmits an existence confirmation signal to other ring network through the living communication path, and receives an existence confirmation signal from said other ring network. 
     According to another exemplary aspect of the invention, a contact node arranged as an end portion of the living communication path transmits a frame with a received existence confirmation signal accommodated as a payload within a ring network to which the contact node belongs, and determines that a failure occurs on the living communication path when failing to receive an existence confirmation signal for a fixed time period, and other node than said contact node among nodes in said ring network, when failing to receive said frame for a fixed time period, determines that a failure occurs on the living communication path. 
     According to a fifth exemplary aspect of the invention, a failure recovery method to be applied to a ring network system with a first ring network and a second ring network connected by a plurality of communication paths correlated with network identifiers in advance, wherein among nodes in said first ring network and in said second ring network, each node other than a contact node arranged as an end portion of said plurality of communication paths includes a learning data base for storing a correspondence relationship among an address of a terminal outside the ring network, a network identifier and an address of a node within the ring network, derives a network identifier to be applied to a user frame received from a terminal outside the ring network from network identifiers corresponding to communication paths developing no fault, applies said network identifier to said user frame, when determining that a failure occurs on a communication path, specifies a network identifier corresponding to the communication path developing the fault, and when an address of a node corresponding to a combination between a transmission destination address that a user frame received from a terminal outside the ring network has and a network identifier derived fails to be stored in said learning data base, broadcast-transmits a frame with a user frame to which said network identifier is applied accommodated as a payload, and each contact node arranged as an end portion of each communication path, when a network identifier corresponding to a communication path with the contact node itself as an end portion and a network identifier applied to a user frame coincide, sends said user frame to said communication path. 
     According to another exemplary aspect of the invention, each contact node arranged as an end portion of each communication path periodically transmits an existence confirmation signal to other ring network through a communication path with the contact node itself as an end portion, and receives an existence confirmation signal from said other ring network. 
     According to another exemplary aspect of the invention, each contact node arranged as an end portion of each communication path transmits a frame with a received existence confirmation signal accommodated as a payload within a ring network to which the contact node belongs, and other node than said contact node among nodes in said ring network, when failing to receive said frame for a fixed time period, determines that a failure occurs on a communication path with a contact node which is a transmission source of said frame as an end portion. 
     According to a sixth exemplary aspect of the invention, a failure detection method to be applied to a ring network system with a first ring network and a second ring network connected by a communication path, wherein a contact node arranged as an end portion of said communication path periodically transmits an existence confirmation signal to other ring network through said communication path, receives an existence confirmation signal from said other ring network, and transmits a frame with a received existence confirmation signal accommodated as a payload within a ring network to which the contact node belongs, and a node other than a contact node among nodes in the ring network determines that a failure occurs on said communication path when failing to receive said frame for a fixed time period. 
     According to a seventh exemplary aspect of the invention, a node to be applied to a ring network system with a first ring network and a second ring network connected by a living communication path and a spare communication path, includes a learning data base for storing a correspondence relationship between an address of a terminal outside the ring network and an address of a node within the ring network, a failure occurrence determination unit for determining that a failure occurs on the living communication path, and an erasure unit for erasing information stored by the learning data base when said failure occurrence determination unit determines that the living communication path develops a fault. 
     According to another exemplary aspect of the invention, the erasure unit erases all the information stored by the learning data base when the failure occurrence determination unit determines that a failure occurs on the living communication path. 
     According to another exemplary aspect of the invention, the erasure unit erases only an address of a contact node arranged as an end portion of the living communication path and an address of a terminal corresponding to the address among the information stored by the learning data base when the failure occurrence determination unit determines that a failure occurs on the living communication path. 
     According to another exemplary aspect of the invention, the node including a broadcast unit for, when an address of a node corresponding to a transmission destination address that a user frame received from a terminal outside the ring network has fails to be stored in said learning data base, broadcast-transmitting a frame with said user frame accommodated as a payload. 
     According to another exemplary aspect of the invention, wherein the failure occurrence determination unit determines that a failure occurs on the living communication path when failing to receive a frame with an existence confirmation signal accommodated as a payload for a fixed time period. 
     According to another exemplary aspect of the invention, the node is arranged as an end portion of the living communication path and further includes an existence confirmation signal transmission unit for periodically transmitting an existence confirmation signal to other ring network through the living communication path, and an existence confirmation signal reception unit for receiving an existence confirmation signal from said other ring network. 
     According to another exemplary aspect of the invention, the node including an existence confirmation frame transmission unit for transmitting a frame with an existence confirmation signal received by the existence confirmation signal reception unit accommodated as a payload within a ring network to which the contact node belongs. 
     According to a eighth exemplary aspect of the invention, a node to be applied to a ring network system with a first ring network and a second ring network connected by a plurality of communication paths correlated with network identifiers in advance, includes a learning data base for storing a correspondence relationship among an address of a terminal outside the ring network, a network identifier and an address of a node within the ring network, a network identifier deriving unit for deriving a network identifier to be applied to a user frame received from a terminal outside the ring network from network identifiers corresponding to communication paths developing no fault, a network identifier applying unit for applying said network identifier to said user frame, a failure occurrence determination unit for determining that a failure occurs on a communication path, a failure occurring communication path notification unit for, when said failure occurrence determination unit determines that a failure occurs on a communication path, notifying said network identifier deriving unit of information which can specify a network identifier corresponding to the communication path developing the fault, and a broadcast unit for, when an address of a node corresponding to a combination between a transmission destination address that a user frame received from a terminal outside the ring network has and a network identifier derived by the network identifier deriving unit fails to be stored in said learning data base, broadcast-transmitting a frame with a user frame to which said network identifier is applied accommodated as a payload. 
     According to another exemplary aspect of the invention, the failure occurrence determination unit determines that a failure occurs on the living communication path when failing to receive a frame with an existence confirmation signal accommodated as a payload for a fixed time period. 
     According to a ninth exemplary aspect of the invention, a node to be applied to a ring network system with a first ring network and a second ring network connected by a communication path, which is arranged as an end portion of said living communication path and includes an existence confirmation signal transmission unit for periodically transmitting an existence confirmation signal to other ring network through said communication path, an existence confirmation signal reception unit for receiving an existence confirmation signal from said other ring network, and an existence confirmation frame transmission unit for transmitting a frame with an existence confirmation signal received by the existence confirmation signal reception unit accommodated as a payload within a ring network to which the node belongs. 
     According to a tenth exemplary aspect of the invention, a program for a node for causing a computer which a node to be applied to a ring network system with a first ring network and a second ring network connected by a living communication path and a spare communication path includes and which includes a learning data base for storing a corresponding relationship between an address of a terminal outside the ring network and an address of a node within the ring network 
     to execute failure occurrence processing of determining that a failure occurs on the living communication path and erasing processing of erasing information stored in the learning data base when determination is made that a failure occurs on the living communication path by said failure occurrence processing. 
     According to another exemplary aspect of the invention, the program for a node causes the computer to execute processing of erasing all the information stored by the learning data base in the erasing processing. 
     According to another exemplary aspect of the invention, the program for a node causes the computer to execute processing of erasing only an address of a contact node arranged as an end portion of the living communication path and an address of a terminal corresponding to the address among the information stored by the learning data base in the erasing processing. 
     According to another exemplary aspect of the invention, the program for a node causes the computer to execute broadcast processing of, when an address of a node corresponding to a transmission destination address that a user frame received from a terminal outside the ring network has fails to be stored in said learning data base, broadcast-transmitting a frame with said user frame accommodated as a payload. 
     According to another exemplary aspect of the invention, the program for a node causes the computer to, when failing to receive a frame with an existence confirmation signal accommodated as a payload for a fixed time period in the failure occurrence processing, determine that a failure occurs on the living communication path. 
     According to another exemplary aspect of the invention, the program for a node causes the computer that a node arranged as an end portion of the living communication path includes to execute existence confirmation signal transmission processing of periodically transmitting an existence confirmation signal to other ring network through the living communication path, and existence confirmation signal reception processing of receiving an existence confirmation signal from said other ring network. 
     According to another exemplary aspect of the invention, the program for a node causes the computer to execute existence confirmation frame transmission processing of transmitting a frame with an existence confirmation signal received in the existence confirmation signal reception processing accommodated as a payload within a ring network to which the node itself belongs. 
     According to a eleventh exemplary aspect of the invention, a program for a node for causing a computer which a node to be applied to a ring network system with a first ring network and a second ring network connected by a plurality of communication paths correlated with network identifiers in advance includes and which includes a learning data base for storing a corresponding relationship among an address of a terminal outside the ring network, a network identifier and an address of a node within the ring network to execute network identifier deriving processing of deriving a network identifier to be applied to a user frame received from a terminal outside the ring network from network identifiers corresponding to communication paths developing no fault, network identifier applying processing of applying said network identifier to said user frame, failure occurrence determination processing of determining that a failure occurs on a communication path, failure network identifier specifying processing of specifying, when said failure occurrence determination processing determines that a failure occurs on a communication path, a network identifier corresponding to the communication path developing the fault, and broadcasting processing of, when an address of a node corresponding to a combination between a transmission destination address that a user frame received from a terminal outside the ring network has and a network identifier derived by the network identifier deriving processing fails to be stored in said learning data base, broadcast-transmitting a frame with a user frame to which said network identifier is applied accommodated as a payload. 
     According to another exemplary aspect of the invention, the program for a node causes the computer to determine, when failing to receive a frame with an existence confirmation signal accommodated as a payload for a fixed time period in the failure occurrence processing, that a failure occurs on the living communication path. 
     According to a twelfth exemplary aspect of the invention, a program for a node for causing a computer that a node includes which is to be applied to a ring network system with a first ring network and a second ring network connected by a communication path and which is arranged as an end portion of said communication path to execute existence confirmation signal transmission processing of periodically transmitting an existence confirmation signal to other ring network through said communication path, and existence confirmation signal reception processing of receiving an existence confirmation signal from said other ring network and existence confirmation frame transmission processing of transmitting a frame with a received existence confirmation signal accommodated as a payload within a ring network to which the node belongs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram for use in explaining an example of a structure of a ring network system according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a diagram for use in explaining a situation where a living inter-link develops a fault; 
         FIG. 3  is a flow chart showing operation of a contact node connected to an inter-link which develops a fault; 
         FIG. 4  is a flow chart showing operation of other node than a contact node connected to an inter-link which develops a fault (excluding a contact node connected to a spare inter-link); 
         FIG. 5  is a flow chart showing operation of a contact node connected to a spare inter-link; 
         FIG. 6  is a diagram for use in explaining a situation where after flushing a learning data base in response to failure occurrence on an inter-link, each node transfers a frame from one terminal to another; 
         FIG. 7  is a diagram for use in explaining a situation where after broadcast-transmission, a frame is transferred from one terminal to another; 
         FIG. 8  is a flow chart showing a first manner of notifying each node in the ring of inter-link failure occurrence by using a KeepAlive signal; 
         FIG. 9  is a flow chart showing a second manner of notifying each node in the ring of inter-link failure occurrence by using a KeepAlive signal; 
         FIG. 10  is a block diagram showing an example of a structure of a contact node applied to the first exemplary embodiment; 
         FIG. 11  is a block diagram showing an example of a structure of other node than a contact node applied to the first exemplary embodiment; 
         FIG. 12  is a diagram for use in explaining an example of a structure of a ring network system according to a second exemplary embodiment; 
         FIG. 13  is a diagram for use in explaining a situation where an inter-link develops a fault; 
         FIG. 14  is a flow chart showing operation of a contact node when a failure occurs; 
         FIG. 15  is a flow chart showing operation of other node than a contact node when a failure occurs; 
         FIG. 16  is a diagram for use in explaining a situation where after changing a VLAN identifier deriving method in response to failure occurrence on an inter-link, each node transfers a frame from one terminal to another; 
         FIG. 17  is a diagram for use in explaining a situation where after broadcast-transmission, a frame is transferred from one terminal to another; 
         FIG. 18  is a block diagram showing an example of a structure of a contact node applied to the second exemplary embodiment; 
         FIG. 19  is a block diagram showing an example of a structure of other node than a contact node applied to the second exemplary embodiment; 
         FIG. 20  is a diagram for use in explaining an example of an RPR network of a single ring; 
         FIG. 21  is a diagram for use in explaining operation of a ring when a failure occurs in the ring; 
         FIG. 22  is a block diagram showing an example of a structure of a related RPR node; and 
         FIG. 23  is a diagram for use in explaining an example of a multi-ring network having two rings connected by two inter-links. 
     
    
    
     EXEMPLARY EMBODIMENT 
     In the following, best modes for implementing the present invention will be described with reference to the drawings. 
     First Exemplary Embodiment 
       FIG. 1  is a diagram for use in explaining an example of a structure of a ring network system according to the present invention. The ring network system according to the present invention is a multi-ring network system comprising a plurality of rings (packet rings with nodes connected in a ring)  501 - a  and  501 - b . The ring  501 - a  comprises RPR nodes (hereinafter simply denoted as a node)  900 - 1 ˜ 900 - 4  and nodes  700 - 1  and  700 - 2  connected to an inter-link. In the following, a node connected to other ring through an inter-link (or other communication network) will be denoted as a contact node. The ring  501 - b  comprises nodes  900 - 5 ˜ 900 - 8  and contact nodes  700 - 3  and  700 - 4 . The rings  501 - a  and  501 - b  are two-fiber rings, with the ring  501 - a  comprising an inner ring  501 - a -inner and an outer ring  501 - a -outer. Similarly, the ring  501 - b  comprises an inner ring  501 - b -inner and an outer ring  501 - b -outer. Assume here that each inner ring executes clockwise traffic transfer and each outer ring executes counterclockwise traffic transfer. Also assume that to the node  900 - 1 , a terminal  510  is connected and to the node  900 - 7 , a terminal  511  is connected. RPR-MAC addresses of the respective nodes  900 - 1 ˜ 900 - 8  are  900 - 1 ˜ 900 - 8 , respectively. Similarly, RPR-MAC addresses of the respective contact nodes  700 - 1 ˜ 700 - 4  are  700 - 1 ˜ 700 - 4 , respectively. 
     The contact nodes  700 - 1  and  700 - 3  are connected by an inter-link  505 . The contact nodes  700 - 2  and  700 - 4  are connected by an inter-link  506 . Assume here that the inter-link  505  is a living inter-link used in a normal state and the inter-link  506  is a spare inter-link. In other words, assume that when the living inter-link  505  develops a fault, the inter-link  506  is used. 
     In the following, description will be made of operation of the ring network system in a normal state with respect to a case where a U-MAC frame is transferred from the terminal  510  to the terminal  511  as an example. U-MAC frame is an Ethernet (registered trademark) frame, for example. In  FIG. 1 , paths  520 ,  521  and  522  indicated by dotted lines are U-MAC frame transfer paths. Also in  FIG. 1 , a path  530  indicated by a solid line is an RPR-MAC frame transfer path. 
     The node  900 - 1  having received a U-MAC frame output from the terminal  510  searches an RPR-MAC address corresponding to a transmission destination U-MAC address in the U-MAC frame by using a learning data base owned by the node itself to determine a transmission destination RPR-MAC address. In the present example, assume that a transmission destination U-MAC address (the address of the terminal  511 ) and the RPR-MAC address “ 700 - 1 ” are correlated with each other and learned by the learning data base in advance. Accordingly, the node  900 - 1  adds, to the U-MAC frame, an RPR-MAC overhead with “ 700 - 1 ” as a transmission destination RPR-MAC address and its own RPR-MAC address “ 900 - 1 ” as a transmission source RPR-MAC address to convert the U-MAC frame into an RPR-MAC frame. Thus adding an overhead to a U-MAC frame and accommodating the U-MAC frame as a payload is referred to as encapsulation. 
     Subsequently, the node  900 - 1  refers to its own forwarding data base to specify output ring information corresponding to the transmission destination RPR-MAC address. Output ring information is information indicative of whether an RPR-MAC frame is to be output to an inner ring, to an outer ring or to both of the inner ring and the outer ring. The node  900 - 1  refers to the output ring information of the forwarding data base to determine to which of the inner ring and the outer ring the RPR-MAC frame is to be sent out and sends the RPR-MAC frame according to the determination.  FIG. 1  shows a case where the frame is sent to the inner ring  501 - a -inner. 
     The node  900 - 2  having received the RPR-MAC frame sent from the node  900 - 1  refers to the transmission destination RPR-MAC address of the RPR-MAC frame and compares the address with its own address. Since the transmission destination RPR-MAC address and the address of the node  900 - 2  itself differ from each other, the node  900 - 2  determines that the RPR-MAC frame should not be terminated by itself to transfer the RPR-MAC frame to the inner ring  501 - a -inner. 
     The node  700 - 1  having received the RPR-MAC frame sent from the node  900 - 2  refers to the transmission destination RPR-MAC address of the RPR-MAC frame and compares the address with its own address. Since the compared two addresses coincide, the node  700 - 1  takes in the RPR-MAC address. In other words, the node takes out the RPR-MAC frame from the ring  501 - a . Then, the node  700 - 1  extracts the RPR-MAC overhead from the RPR-MAC frame to convert the RPR-MAC frame into a U-MAC frame. The node  700 - 1  sends out the U-MAC frame to the inter-link  505 . 
     The node  700 - 3  receives the U-MAC frame through the inter-link  505 . Then, the node  700 - 3  refers to its own learning data base to determine a transmission destination RPR-MAC address (“ 900 - 7 ” here) in the ring  501 - b . The node  700 - 3  adds, to the U-MAC frame, an RPR-MAC overhead with “ 900 - 7 ” as a transmission destination RPR-MAC address and its own RPR-MAC address “ 700 - 3 ” as a transmission source RPR-MAC address to convert the U-MAC frame into an RPR-MAC frame. Subsequently, the node  700 - 3  refers to its own forwarding data base to specify output ring information based on the transmission destination RPR-MAC address. Here, the node  700 - 3  sends out the RPR-MAC frame to the outer ring  502 - b -outer according to the output ring information. 
     Each of the node  700 - 4  and  900 - 8 , upon receiving the RPR-MAC frame, determines that it is not an RPR-MAC frame to be terminated by itself because the transmission destination RPR-MAC address and its own RPR-MAC address fail to coincide with each other. Then, the node sends out the RPR-MAC frame to the outer ring  502 - b -outer. As a result, the node  900 - 7  receives the RPR-MAC frame. The node  900 - 7  takes in the received RPR-MAC frame because the transmission destination RPR-MAC address of the received RPR-MAC frame coincides with its own RPR-MAC address (takes out the RPR-MAC frame from the ring  501 - b ). Subsequently, the node  900 - 7  removes the RPR-MAC overhead from the RPR-MAC frame to convert the RPR-MAC frame into a U-MAC frame. The node  900 - 7  transfers the U-MAC frame to the terminal  511 . 
     Next, operation executed at the time of occurrence of a failure will be described.  FIG. 2  is a diagram for use in explaining a situation where the living inter-link  505  develops a fault. As a result of occurrence of a failure on the living inter-link  505 , no U-MAC frame can be transferred from the terminal  510  to the terminal  511  due to the traffic flow shown in  FIG. 1 . 
       FIG. 3  is a flow chart showing operation of contact nodes (the contact nodes  700 - 1  and  700 - 3  in this example) connected to an inter-link on which a failure occurs. When the living inter-link  505  develops a fault, the contact nodes  700 - 1  and  700 - 3  connected to the inter-link  505  detect the fault (Step S 610 ). Subsequently, each of the contact nodes  700 - 1  and  700 - 3  sends out an RPR-MAC frame (inter-link failure notification RPR-MAC frame) which notifies occurrence of a failure on the inter-link  505  within a ring to which its own node belongs (Step S 611 ). The contact nodes  700 - 1  and  700 - 3  include information which can specify an inter-link developing a fault in the inter-link failure notification RPR-MAC frame. To be included in the inter-link failure notification RPR-MAC frame are, for example, an inter-link ID for identifying an individual inter-link and 
     an RPR-MAC address of a node having detected an inter-link failure (i.e. RPR-MAC addresses of the contact nodes  700 - 1  and  700 - 3  themselves). 
     In the following, description will be made, as an example, with respect to a case where as information which can specify an inter-link developing a fault, an RPR-MAC address of a node having detected the inter-link failure is included. When an inter-link ID is included as information which can specify an inter-link developing a fault, each node is allowed to specify contact nodes at the opposite ends of the inter-link from its inter-link ID. The inter-link failure notification RPR-MAC frame may be sent to an inner ring, an outer ring or both of them. Illustrated in the example shown in  FIG. 2  is a case where the contact nodes  700 - 1  and  700 - 3  send out an inter-link failure notification RPR-MAC frame to the inner ring  501 - a -inner and the inner ring  502 - b -inner, respectively. Paths  601 - a  and  601 - b  shown in  FIG. 2  indicate a transfer path of an inter-link failure notification RPR-MAC frame. After Step S 611 , the contact nodes  700 - 1  and  700 - 3  flush (erase) learning contents of their own learning data bases (Step S 612 ). 
       FIG. 4  is a flow chart showing operation of other node than a contact node connected to an inter-link developing a fault (excluding a contact node connected to a spare inter-link). The nodes  900 - 1 ˜ 900 - 8  are relevant nodes. Although description will be here made of operation of the node  900 - 1 , the nodes  900 - 2 ˜ 900 - 8  execute the same operation. The node  900 - 1  receives an inter-link failure notification RPR-MAC frame transmitted by the contact node  700 - 1  at Step  611  (Step S 620 ). Then, the node  900 - 1  forcibly flushes learning contents of its own learning data base (Step S 621 ). 
       FIG. 5  is a flow chart showing operation of a contact node (the contact nodes  700 - 2  and  700 - 4  in this example) connected to the spare inter-link  506 . Upon receiving an inter-link failure notification RPR-MAC frame, each of the contact nodes  700 - 2  and  700 - 4  forcibly flushes learning contents of its own learning data base (Steps S 630  and S 631 ). Operations at Steps S 630  and S 631  are the same as those at Steps S 620  and S 621  shown in  FIG. 4 . The contact nodes  700 - 2  and  700 - 4  set the spare inter-link  506  at a forwarding state (Step S 632 ). Either of the operations at Steps S 631  and S 632  can be executed first. 
     The forwarding state represents a state which enables packet passage. On the other hand, a state where a received frame is abandoned without sending is referred to as a blocked state. The forwarding state and the blocked states are set on the basis of a port of each node. Until execution of Step S 632 , among ports which the contact nodes  700 - 2  and  700 - 4  have, a port connected to the inter-link  506  is set at the blocked state to prevent sending of a frame to the inter-link  506 . At Step S 632 , the contact nodes  700 - 2  and  700 - 4  set the port connected to the inter-link  506  at the forwarding state to enable a frame to be sent to the inter-link  506 . 
     As shown in  FIG. 3  through  FIG. 5 , each node flushes its own learning data base. At this time, flushing is forcibly executed irrespectively whether an aging time has elapsed or not. In other words, even before a lapse of an aging time, flushing is executed. 
     In addition, each node may flush all the learning contents of the learning data base. Alternatively, among learning contents of the learning data base, each node may selectively flush only information about an RPR-MAC address of a node having detected an inter-link failure and about a U-MAC address correlated to the RPR-MAC address. Information of the node having detected an inter-link failure is included in an inter-link failure notification RPR-MAC frame. In a case, for example, of receiving an inter-link failure notification RPR-MAC frame including information that the node  700 - 1  detects an inter-link failure, each node may selectively flush the RPR-MAC address “ 700 - 1 ” and a U-MAC address correlated to the address. 
       FIG. 6  is a diagram for use in explaining a situation where in response to occurrence of a failure on the inter-link  505 , after flushing the learning data base, each of the nodes  700 - 1 ˜ 700 - 4  and  900 - 1 ˜ 900 - 8  transfers a frame from the terminal  510  to the terminal  511 . In  FIG. 6 , paths  640 ,  641  and  642  indicated by dotted lines are U-MAC frame transfer paths. Also in  FIG. 6 , paths  650  and  651  indicated by solid lines are transfer paths of a broadcast-transmitted RPR-MAC frame. 
     The node  900 - 1  having received a U-MAC frame output from the terminal  510 , by using its own learning data base, searches an RPR-MAC address corresponding to a transmission destination U-MAC address in the U-MAC frame. At this time, since learning contents of the learning data base are flushed, the search fails. Even when flushing of the learning data base is selectively executed, because the RPR-MAC address “ 700 - 1 ” of a node having detected an inter-link failure and a U-MAC address correlated to the address are erased, search of the RPR-MAC address “ 700 - 1 ” is impossible, which can be done in a normal state. When search of an RPR-MAC address fails, the node  900 - 1  adds, to the U-MAC frame, an RPR-MAC overhead with the transmission destination RPR-MAC address as a broadcast address and its own RPR-MAC address “ 900 - 1 ” as a transmission source RPR-MAC address. The node  900 - 1  converts the U-MAC frame into an RPR-MAC frame by adding the RPR-MAC overhead. The node  900 - 1  broadcast-transmits the RPR-MAC frame. At this time, the node  900 - 1  may send the RPR-MAC frame to either of an inter ring and outer ring. The frame may be sent to both of the inner ring and the outer ring. Shown in  FIG. 6  is a case where the node  900 - 1  sends an RPR-MAC frame to the outer ring  501 - a -outer and the transfer path  650  executes transfer. 
     Upon receiving the RPR-MAC frame (the RPR-MAC frame with a broadcast address as a transmission destination), the node  900 - 4  takes in the RPR-MAC frame, as well as sending a copy of the RPR-MAC frame to the ring  501 - a . At this time, the node  900 - 4  sends the copy of the RPR-MAC frame to a ringlet to which the RPR-MAC frame is transferred, either the inner ring or the outer ring. Accordingly, in the present example, the node sends out the copy of the RPR-MAC frame to the outer ring  501 - a -outer. 
     In addition, the node  900 - 4  registers, at its own learning data base, a correspondence relationship between a transmission source RPR-MAC address (“ 900 - 1 ” in this example) in the taken RPR-MAC frame and a transmission source U-MAC address (the address of the terminal  510  in this example) of a U-MAC frame accommodated in the RPR-MAC frame. Thereafter, the node  900 - 4  removes the RPR-MAC overhead from the RPR-MAC frame to convert the frame into a U-MAC frame. Then, the node  900 - 4  transfers the U-MAC frame to a terminal or an apparatus (not shown) connected to the node  900 - 4  outside the ring  501 - a.    
     Similarly to the node  900 - 4 , the nodes  900 - 3 ,  700 - 2 ,  700 - 1  and  900 - 2  take in the RPR-MAC frame, as well as sending a copy of the RPR-MAC frame to the outer ring  501 - a -outer. As a result, the RPR-MAC frame will be transferred within the ring along the transfer path  650  shown in  FIG. 6 . Also similarly to the node  900 - 4 , the nodes  900 - 3 ,  700 - 2 ,  700 - 1  and  900 - 2  register a corresponding relationship between a transmission source RPR-MAC address (“ 900 - 1 ” in this example) and a transmission source U-MAC address (the address of the terminal  510  in this example) at their own learning data bases. Then, the nodes remove the RPR-MAC overhead of the RPR-MAC frame to transfer the U-MAC frame to a terminal or an apparatus (not shown) outside the ring  501 - a.    
     In a case of a ring structure, a broadcast-transmitted RPR-MAC frame makes a round of the ring to return to a node as a transmission source. Each of the nodes  900 - 1 ˜ 900 - 8  and  700 - 1 ˜ 700 - 4  refers to a transmission source RPR-MAC address of the received RPR-MAC frame and when the address coincides with its own node RPR-MAC address, abandons the RPR-MAC frame. Accordingly, the RPR-MAC frame fails to loop. 
     The node  700 - 2  removes the RPR-MAC overhead of the RPR-MAC frame taken to convert the RPR-MAC frame into a U-MAC frame. Then, the node  700 - 2  sends the U-MAC frame to the inter-link  506 . Since the node  700 - 2  has the port connected to the inter-link  506  set at the forwarding state (see Step S 632  shown in  FIG. 5 ), the node is allowed to send the U-MAC frame to the inter-link  506 . The U-MAC frame will be transferred to the node  700 - 4  along the transfer path  641  shown in  FIG. 6 . 
     The node  700 - 4  having received the U-MAC frame through the inter-link  506  searches an RPR-MAC address corresponding to the transmission destination U-MAC address in the U-MAC frame by using its own learning data base. At this time, since the learning contents of the learning data base are flushed, the search fails. When the search of the RPR-MAC address fails, the node  700 - 4  adds, to the U-MAC frame, an RPR-MAC overhead with the transmission destination RPR-MAC address as a broadcast address and its own RPR-MAC address “ 700 - 4 ” as a transmission source RPR-MAC address. The node  700 - 4  converts the U-MAC frame into an RPR-MAC frame by adding the RPR-MAC overhead. The node  700 - 4  broadcast-transmits the RPR-MAC frame. At this time, the node  700 - 4  may send the RPR-MAC frame to either of the inner ring and the outer ring. The node may send the frame to both the inner ring and the outer ring. Shown in  FIG. 6  is a case where the node  700 - 4  sends the RPR-MAC frame to the outer ring  502 - b -outer to transfer the frame by using the transfer path  651 . 
     Upon receiving the RPR-MAC frame (the RPR-MAC frame with a broadcast address as a transmission destination), the node  900 - 8  takes in the RPR-MAC frame, as well as sending a copy of the RPR-MAC frame to the outer ring  502 - b -outer. The node  900 - 8  also registers, at its own learning data base, a correspondence relationship between a transmission source RPR-MAC address in the taken RPR-MAC frame (“ 700 - 4 ” in this example) and a transmission source U-MAC address of a U-MAC frame accommodated in the RPR-MAC frame (the address of the terminal  510  in this example). Thereafter, the node  900 - 8  removes the RPR-MAC overhead from the RPR-MAC frame to convert the frame into a U-MAC frame. Then, the node  900 - 8  transfers the U-MAC frame to a terminal or an apparatus (not shown) connected to the node  900 - 8  outside the ring  501 - b.    
     Similarly to the node  900 - 8 , the nodes  900 - 7 ,  900 - 6 ,  900 - 5  and  700 - 3  take in the RPR-MAC frame, as well as sending a copy of the RPR-MAC frame to the outer ring  502 - b -outer. As a result, the RPR-MAC frame will be transferred along the transfer path  651  in the ring shown in  FIG. 6 . Similarly to the node  900 - 8 , each of the nodes  900 - 7 ,  900 - 6 ,  900 - 5  and  700 - 3  registers, at its own learning data base, a correspondence relationship between a transmission source RPR-MAC address (“ 700 - 4 ” in this example) and a transmission source U-MAC address (the address of the terminal  510  in this example). Then, the node removes the RPR-MAC overhead of the RPR-MAC frame to transfer a U-MAC frame to a terminal or an apparatus (not shown) outside the ring  501 - b.    
     In addition, the node  700 - 4 , when an RPR-MAC frame broadcast-transmitted by itself makes a round of the ring to be transferred to the node  700 - 4  itself, abandons the RPR-MAC frame. 
     The node  900 - 7  removes the RPR-MAC overhead of the taken RPR-MAC frame to convert the RPR-MAC frame into a U-MAC frame. Then, the node  900 - 7  transfers the U-MAC frame to the terminal  511 . The U-MAC frame will be transferred from the node  900 - 7  to the terminal  511  along the transfer path  642  shown in  FIG. 6 . 
     Thus, even when the inter-link  505  develops a fault, frame transfer from the terminal  510  to the terminal  511  is enabled. Next, frame transfer from the terminal  511  to the terminal  510  will be described. 
       FIG. 7  is a diagram for use in explaining a situation where a frame is transferred from the terminal  511  to the terminal  510 . In  FIG. 7 , paths  643 ,  645  and  647  indicated by dotted lines are transfer paths for a U-MAC frame. In  FIG. 7 , paths  644  and  646  indicated by solid lines are transfer paths for an RPR-MAC frame. 
     In the course of transfer of a frame from the terminal  510  to the terminal  511  shown in  FIG. 6 , each node in the ring  501 - b  learns a correspondence relationship between an address of the terminal  510  (U-MAC address) and the RPR-MAC address “ 700 - 4 ” into its own learning data base. Similarly, each node in the ring  501 - a  learns a correspondence relationship between the address of the terminal  510  and the RPR-MAC address “ 900 - 1 ” into its own learning data base. Accordingly, unicast-communication from the terminal  511  to the terminal  510  is enabled. 
     More specifically, upon receiving a U-MAC frame with a transmission destination U-MAC address as the address of the terminal  510  from the terminal  511 , the node  900 - 7  is allowed to specify the transmission destination RPR-MAC address as “ 700 - 4 ” by referring to the learning data base. Then, the node  900 - 7  adds, to the U-MAC frame, an RPR-MAC overhead with the transmission destination RPR-MAC address as “ 700 - 4 ” and the transmission source RPR-MAC address as “ 900 - 7 ” to transfer the obtained frame within the ring  501 - b . As a result, the RPR-MAC frame will be transferred from the node  900 - 7  to the node  700 - 4  along the transfer path  644 . Upon receiving the RPR-MAC frame, each of the nodes  900 - 8  and  700 - 4  on the transfer path  644  registers, at its own learning data base, a correspondence relationship between the transmission source RPR-MAC address (“ 900 - 7 ” in this example) and a transmission source U-MAC address of the U-MAC frame accommodated in the RPR-MAC frame (the address of the terminal  511  in this example). 
     Upon receiving the RPR-MAC frame, the node  700 - 4 , because the transmission destination RPR-MAC address of the frame coincides with its own RPR-MAC address, takes in the RPR-MAC frame. Then, the node removes the RPR-MAC overhead from the taken RPR-MAC frame to convert the RPR-MAC frame into a U-MAC frame. The node  700 - 4  sends the U-MAC frame to the inter-link  506 . The U-MAC frame will be transferred to the node  700 - 2  along the transfer path  644  shown in  FIG. 7 . 
     The node  700 - 2  having received the U-MAC frame through the inter-link  506  refers to an RPR-MAC address corresponding to a transmission destination U-MAC address of the frame (the address of the terminal  510 ). At the learning data base, as an RPR-MAC address corresponding to the address of the terminal  510 , “ 900 - 1 ” is registered. Accordingly, the node  700 - 2  adds, to the U-MAC frame, an RPR-MAC overhead with a transmission destination RPR-MAC address as “ 900 - 1 ” and a transmission source RPR-MAC address as “ 700 - 2 ” to transfer the obtained frame within the ring  501 - a . As a result, the RPR-MAC frame will be transferred from the node  700 - 2  to the node  900 - 1  along the transfer path  646 . Upon receiving an RPR-MAC frame, each of the nodes  900 - 3 ,  900 - 4  and  900 - 1  on the transfer path  646  registers, at its learning data base, a correspondence relationship between a transmission source RPR-MAC address (“ 700 - 2 ” in this example) and a transmission source U-MAC address of a U-MAC frame accommodated in the RPR-MAC frame (the address of the terminal  511  in this example). 
     As a result, the nodes  900 - 1 ,  900 - 4  and  900 - 3  learn a correspondence relationship between the address of the terminal  511  and the RPR-MAC address “ 700 - 2 ” into the learning data base. In addition, the nodes  700 - 4  and  900 - 8  learn a correspondence relationship between the address of the terminal  511  and the RPR-MAC address “ 900 - 7 ” into the learning data base. Accordingly, unicast-communication from the terminal  510  to the terminal  511  is enabled. 
     For realizing such recovery operation at the occurrence of a failure as described above, it is necessary to detect a failure on an inter-link. Failure detection of the inter-link  505  may be realized, for example, by direct detection of a failure on a physical link by each node between the nodes  700 - 1  and  700 - 3  and between the nodes  700 - 2  and  700 - 4  at the opposite ends of the inter-links  505  and  506 , respectively. 
     Inter-link failure may be detected by non-arrival of a KeepAlive signal which is periodically transmitted by a contact node to a contact node of other ring through an inter-link. KeepAlive signal is, for example, a signal having the same format as that of a U-MAC frame. When a KeepAlive signal is generated as a signal having the same format as that of a U-MAC frame, the KeepAlive signal is transmitted or received as a U-MAC frame for control. There are two manners of notifying each node in a ring of inter-link failure occurrence by using a KeepAlive signal. In the first manner, a contact node itself determines that a failure occurs on an inter-link to which the contact node is connected when receiving no KeepAlive signal for a fixed time period. Then, when determining that a failure occurs, the node broadcast-transmits an inter-link failure notification RPR-MAC frame indicative of failure detection within the ring. In the inter-link failure notification RPR-MAC frame, information of a contact node which has detected occurrence of an inter-link failure is to be included. Assume, for example, that the contact node  700 - 1  fails to receive a KeepAlive signal from the contact node  700 - 3  for a fixed time period. Then, the contact node  700 - 1  only needs to broadcast-transfer, within the ring  501 - a , an inter-link failure notification RPR-MAC frame which indicates that the contact node  700 - 1  detects an inter-link failure. In the first manner, a KeepAlive signal transmitted/received through an inter-link is terminated at each contact node. In addition, the inter-link failure notification RPR-MAC frame transferred in the ring notifies failure occurrence on the inter-link. 
     In the second manner, without terminating a KeepAlive signal, a contact node transfers the same with an RPR-MAC overhead added within a ring. More specifically, the contact node may convert a KeepAlive signal into an inter-link failure notification RPR-MAC frame by adding an RPR-MAC overhead to a KeepAlive signal and broadcast-transfer the inter-link failure notification RPR-MAC frame within the ring. In this case, the inter-link failure notification RPR-MAC frame includes a KeepAlive signal, which indicates that no failure occurs on the inter-link. In the second manner, upon no transfer of an inter-link failure notification RPR-MAC frame for a fixed time, each node in the ring detects occurrence of a failure on an inter-link connected to a transmission source node of the inter-link failure notification RPR-MAC frame including a KeepAlive signal. In the first manner, a contact node detects an inter-link failure to notify failure detection to each node in the ring to which the contact node belongs, while in the second manner, each node in the ring by itself detects occurrence of an inter-link failure. 
     The contact node may set a transmission destination RPR-MAC address of an inter-link failure notification RPR-MAC frame to be a reserved RPR-MAC address (controlling identifier). More specifically, a transmission destination RPR-MAC address can be a controlling identifier indicating that a kind of RPR-MAC frame is an inter-link failure notification RPR-MAC frame which is to be broadcast-transferred. 
     In addition, the inter-link failure notification RPR-MAC frame will be abandoned after making a round of the ring to return to a transmission source node. Assume, for example, that the contact node  700 - 1  transmits an inter-link failure notification RPR-MAC frame, so that the inter-link failure notification RPR-MAC frame makes a round of the ring to return to the contact node  700 - 1 . Then, the contact node, since a transmission source RPR-MAC address of the inter-link failure notification RPR-MAC frame coincides with its own RPR-MAC address, abandons the inter-link failure notification RPR-MAC frame. 
       FIG. 8  is a flow chart showing the first manner of notifying each node in the ring of occurrence of an inter-link failure by using a KeepAlive signal. Although description will be here made of  FIG. 8  mainly with respect to operation of the contact node  700 - 1 , this is also the case with operation of the contact node  700 - 3  and the like. 
     The contact node  700 - 1 , when failing to receive a KeepAlive signal from the contact node  700 - 3  of other ring through the inter-link  505  for a fixed time period, determines that a failure occurs on the inter-link  505  (Step S 641 ). Then, the contact node  700  sends, within the ring  501 - a , an inter-link failure notification RPR-MAC frame including information which can specify an inter-link on which a failure occurs (e.g. an RPR-MAC address of the node itself which has detected the failure) (Step S 642 ). 
     Other node in the same ring as that of the contact node  700 - 1 , upon receiving the inter-link failure notification RPR-MAC frame, flushes learning contents of the learning data base (Step S 643 ). Then, the node sends the inter-link failure notification RPR-MAC frame to a subsequent node (Step S 644 ). The contact node  700 - 2  connected to the spare inter-link  506  also executes processing of brining a port of the node itself connected to the inter-link  506  into the forwarding state after Step S 643  (or before). 
     The contact node  700 - 1 , upon receiving an inter-link failure notification RPR-MAC frame transmitted by itself, abandons the inter-link failure notification RPR-MAC frame (Step S 645 ). 
       FIG. 9  is a flow chart showing the second manner of notifying each node in the ring of occurrence of an inter-link failure by using a KeepAlive signal. Although description will be made of  FIG. 9  mainly with respect to operation of the contact node  700 - 1 , this is also the case with operation of the contact node  700 - 3  and the like. 
     Upon receiving a KeepAlive signal from the contact node  700 - 3  of other ring through the inter-link  505 , the contact node  700 - 1  sends an inter-link failure notification RPR-MAC frame including the KeepAlive signal within the ring  501 - a  (Step  651 ). More specifically, without terminating a KeepAlive signal, the contact node  700 - 1  converts the signal into an inter-link failure notification RPR-MAC frame and sends the frame within the ring. The inter-link failure notification RPR-MAC frame indicates that the inter-link  505  is in a normal state. 
     Upon receiving the inter-link failure notification RPR-MAC frame, other node in the same ring as that of the contact node  700 - 1  determines that no failure occurs on the inter-link  505  to transfer the frame to a subsequent node (Step S 652 ). The contact node  700 - 1 , upon receiving the inter-link failure notification RPR-MAC frame transmitted by itself, abandons the inter-link failure notification RPR-MAC frame (Step S 653 ). 
     In addition, when failing to receive an inter-link failure notification RPR-MAC frame including a KeepAlive signal for a fixed time period, other node in the same ring as that of the contact node  700 - 1  determines that the inter-link  505  develops a fault (Step S 654 ) to flush the learning contents of the learning data base (Step S 655 ). The contact node  700 - 2  connected to the spare inter-link  506  also executes processing of brining a port of the node itself connected to the inter-link  506  into the forwarding state after step S 655  (or before). 
     Although not shown in  FIG. 9 , the contact node  700 - 1  determines that the inter-link  505  develops a fault similarly to the first manner (similarly to Step S 641  shown in  FIG. 8 ). 
     For transmitting an inter-link failure notification RPR-MAC frame for notifying each node in the ring of failure occurrence on the inter-link, the inter-link failure notification RPR-MAC frame may be broadcast-transmitted. Alternatively, the inter-link failure notification RPR-MAC frame may be transmitted as an RPR-MAC frame terminated at an adjacent node. Then, when receiving an inter-link failure notification RPR-MAC frame to terminate the inter-link failure notification RPR-MAC frame, each node may generate an inter-link failure notification RPR-MAC frame to be terminated at a subsequent node and transmit the frame to the subsequent node. Thus, after once terminating a frame at each node, transmitting the frame to a subsequent node is called hop-by-hop processing. In a case of broadcast-transmission, an inter-link failure notification RPR-MAC frame is transferred within the ring without being terminated at each node. 
     As described in the foregoing, the ring network system according to the present invention flushes the learning contents of the learning data base when an inter-link failure occurs. Flushing of the learning contents of the learning data base is executed only when the living inter-link  505  develops a fault. 
     Also as already described, when flushing the learning contents of the learning data base, all the learning contents may be flushed. Alternatively, out of the learning contents of the learning data base, only information about an RPR-MAC address of a node having detected a failure of an inter-link and a U-MAC address correlated with the RPR-MAC address may be selectively flushed from among the learning contents of the learning data base. When the learning contents are selectively flushed, for transmitting a U-MAC frame until a desired terminal without passing through the inter-link, such broadcast communication as shown in  FIG. 6  is unnecessary. Assume, for example, that the inter-link  505  develops a fault to selectively flush only the RPR-MAC address “ 700 - 1 ” and a U-MAC address corresponding to “ 700 - 1 ”. Assume that thereafter, the U-MAC frame is transmitted from the terminal  510  to a terminal (not shown) connected to the node  900 - 4 . At this time, the node  900 - 1  leaves a correspondence relationship between an address of a terminal as a transmission destination of the U-MAC frame and the RPR-MAC address “ 900 - 4 ” in the learning data base. It is accordingly possible to search the RPR-MAC address “ 900 - 4 ” from the transmission destination U-MAC address, so that it is only necessary to transmit an RPR-MAC frame converted from the U-MAC frame to the node  900 - 4 . As a result, such broadcast-transmission as shown in  FIG. 6  is unnecessary. 
     In addition, a KeepAlive signal sent from the nodes  700 - 3  and  700 - 4  connected to the inter-links  505  and  506  may be notified together with ring information to which a node as a KeepAlive signal transmission source belongs (identification information of a ring; ring identification information of the ring  501 - b  to which the node  700 - 3  belongs in this example). Thus, assume that the KeepAlive signal and identification information of the ring to which the node of the KeepAlive signal transmission source belongs are transmitted to a contact node of other ring through the inter-link and similarly to the above-described second manner, a contact node having received the KeepAlive signal transfers an inter-link failure notification RPR-MAC frame including the KeepAlive signal to each node in the ring. In this case, upon detecting a failure of a living inter-link, each node in the ring is allowed to rewrite the contents of the learning data base so as to transfer a frame between rings through a spare inter-link connecting a ring specified by ring identification information and a ring to which the node itself belongs. For example, each node of the ring  501 - a  changes the RPR-MAC address “ 700 - 1 ” in the learning data base to the RPR-MAC address “ 700 - 2 ” of a contact node connected to a spare inter-link specified by ring identification information. As a result, without executing such broadcast transmission as shown in  FIG. 6 , a U-MAC frame can be transferred to a desired terminal by using the spare inter-link  506 . 
     Each node in each ring transmits/receives a failure notification RPR-MAC frame regarding failure occurrence on a link between nodes in the ring separately from an inter-link failure notification RPR-MAC frame related to failure occurrence on a inter-link. ┌Inter-link failure notification RPR-MAC frame┘ and ┌failure notification RPR-MAC frame┘ are distinguished as different RPR-MAC frames. In a case where no failure occurs on a link between adjacent nodes, each node in the ring transmits/receives a failure notification RPR-MAC frame not including failure detection node information (information of a node which has detected a failure on a link) to/from an adjacent node. Reception of a failure notification RPR-MAC frame not including the failure detection node information represents that no failure occurs on a link between adjacent nodes. Then, each node in the ring, when failing to receive a failure notification RPR-MAC frame not including failure detection node information from an adjacent node for a fixed time, determines that a failure occurs on a link with the adjacent node. Then, the node transfers, within the ring, a failure notification RPR-MAC frame including its own information (e.g. its own RPR-MAC address) as link failure detection node information. 
     A failure notification RPR-MAC frame not including failure detection node information is transmitted to an adjacent node and terminated at the adjacent node. On the other hand, a failure notification RPR-MAC frame including failure detection node information which is transmitted when a failure is actually detected is broadcast-transmitted to make a round of the ring and abandoned when returned to a node as a transmission source of the failure notification RPR-MAC frame. As a result, it is possible to notify each node in the ring of information of a node having detected a failure on a link in the ring. 
     Next, a structure of a node to be applied to a ring network system will be described. In the following, description will be made of a node structure separately with respect to a contact node and other node than a contact node. 
       FIG. 10  is a block diagram showing an example of a structure of the contact nodes  700 - 1 ˜ 700 - 4  (these contact nodes will be denoted as a contact node  700  in the lump in some cases) to be applied to the ring network system according to the present invention. Each contact node  700  comprises a packet demultiplexing circuit  710 , a frame conversion circuit  720 , a forwarding engine  730 , a ring topology information collection circuit  740 , a ring failure information collection circuit  750 , an inter-link failure detection circuit  760 , an RPR inter-link failure information collection circuit  770 , a path determination circuit  780 , a flash circuit  781  and an ADM (Add-Drop Multiplexer)  790 . The contact node  700  further comprises a learning data base  721  for storing a correspondence relationship between a U-MAC address and an RPR-MAC address, and comprises a forwarding data base  731  for storing a correspondence relationship between an RPR-MAC address and an output ring (information indicative of either or both of an inter ring and an outer ring). 
     The learning data base  721  also stores a correspondence relationship between a reserved U-MAC address included in a U-MAC frame for control (identifier for control included in a U-MAC frame for control) and a reserved RPR-MAC address (identifier for control which will be a transmission destination RPR-MAC address of an RPR-MAC frame). It is accordingly possible to search the learning data base  721 , for example, for an identifier for control which is used at the time of adding an RPR-MAC overhead to a KeepAlive signal. 
     In the following, description will be made assuming that occurrence of an inter-link failure is detected by using a KeepAlive signal. The description will be made, as an example, of a case where an inter-link failure occurrence notification manner is the above-described first manner. 
     The packet demultiplexing circuit  710  is connected to the inter-link  505  or the inter-link  506 . In a case where a port of the packet demultiplexing circuit  710  connected to the inter-link is set at the forwarding state, the packet demultiplexing circuit  710  operates as described in the following. The packet demultiplexing circuit  710  receives a U-MAC frame from the frame conversion circuit  720 . The U-MAC frame is conversion of an RPR-MAC frame transferred within the ring by the frame conversion circuit  720 . In addition, the packet demultiplexing circuit  710  periodically receives a KeepAlive signal from the inter-link failure detection circuit  760  (as will be described later, the inter-link failure detection circuit  760  periodically generates a KeepAlive signal and sends the same to the packet demultiplexing circuit  710 ). The packet demultiplexing circuit  710  multiplexes the U-MAC frame sent from the frame conversion circuit  720  and the KeepAlive signal sent from the inter-link failure detection circuit  760  to send the result to the connected inter-link. 
     In addition, the packet demultiplexing circuit  710 , when receiving a U-MAC frame (excluding a KeepAlive signal) from a contact node of other ring through the inter-link to which the packet demultiplexing circuit  710  itself is connected, sends the U-MAC frame to the frame conversion circuit  720 . The packet demultiplexing circuit  710 , also when receiving a KeepAlive signal from a contact node of other ring through the inter-link, sends the KeepAlive signal to the inter-link failure detection circuit  760 . 
     When the port of the packet demultiplexing circuit  710  connected to the inter-link is set at the blocked state, the packet demultiplexing circuit  710  operates as described in the following. The packet demultiplexing circuit  710 , when receiving a U-MAC frame from the frame conversion circuit  720 , abandons the U-MAC frame. When receiving a U-MAC frame (excluding a KeepAlive signal) from a contact node of other ring through the inter-link, the circuit abandons the U-MAC frame as well. Even when the port connected to the inter-link is set at the blocked state, the packet demultiplexing circuit  710  refrains from abandoning the KeepAlive signal similarly to the case of the forwarding state. More specifically, when receiving a KeepAlive signal from the inter-link failure detection circuit  760 , the packet demultiplexing circuit  710  sends the KeepAlive signal to the inter-link. In addition, when receiving a KeepAlive signal from a contact node of other ring through the inter-link, the circuit sends the KeepAlive signal to the inter-link failure detection circuit  760 . 
     The frame conversion circuit  720 , in a case where the packet demultiplexing circuit  710  receives a U-MAC frame other than a KeepAlive signal from a contact node of other ring and the U-MAC frame is transferred from the packet demultiplexing circuit  710 , refers to the learning data base  721  to search an RPR-MAC address corresponding to a transmission destination U-MAC address of the U-MAC frame. Then, the frame conversion circuit  720  converts the U-MAC frame into an RPR-MAC frame by adding an RPR-MAC overhead with the searched address as a transmission destination RPR-MAC address and its own node address as a transmission source RPR-MAC address. The frame conversion circuit  720 , when failing to search an RPR-MAC address corresponding to a transmission destination U-MAC address of the U-MAC frame (i.e. a correspondence relationship between the U-MAC address and an RPR-MAC address is yet to be learned), converts the frame into an RPR-MAC frame with a broadcast address as a transmission destination RPR-MAC address. After converting the U-MAC frame into an RPR-MAC frame, the frame conversion circuit  720  sends the RPR-MAC frame to the forwarding engine  730 . 
     There is a case where a control signal other than a KeepAlive signal is sent as a U-MAC frame from the packet demultiplexing circuit  710  to the frame conversion circuit  720 . Also in this case, the frame conversion circuit  720  only needs to search the learning data base for a reserved RPR-MAC address (identifier for control) corresponding to a reserved U-MAC address of the U-MAC frame (an identifier for control which is included in a U-MAC frame for control) to convert the U-MAC frame into an RPR-MAC frame with the identifier for control as a transmission destination RPR-MAC address. 
     In addition, when the ADM  790  receives an RPR-MAC frame within the ring and the RPR-MAC frame is transferred form the ADM  790 , the frame conversion circuit  720  removes the RPR-MAC overhead to convert the RPR-MAC frame into a U-MAC frame. Then, the frame conversion circuit  720  sends the U-MAC frame to the packet demultiplexing circuit  710 . At this time, the frame conversion circuit  720  registers, at the learning data base  721 , a correspondence relationship between a transmission source RPR-MAC address in the RPR-MAC frame transferred from the ADM  790  and a transmission source U-MAC address in a U-MAC frame accommodated in the RPR-MAC frame. When it is already registered, the circuit registers the same by overwriting. 
     The forwarding engine  730  determines which of the inner ring or the outer ring the RPR-MAC frame should be sent or whether the frame should be sent to both of the inner ring and the outer ring based on a transmission destination RPR-MAC address of the RPR-MAC frame transferred from the frame conversion circuit  720 . At this time, the forwarding engine  730  only needs to refer to the forwarding data base  331  to refer to an output ring corresponding to the transmission destination RPR-MAC address and make determination. Then, the forwarding engine  730  transfers the RPR-MAC frame and the output ring information (information indicating whether the RPR-MAC frame should be sent to the inner ring or the outer ring or whether it should be sent to both) to the AMD  790 . 
     The ring topology information collection circuit  740  generates a topology discovery RPR-MAC frame for comprehending a node structure in a ring to which the contact node shown in  FIG. 10  itself belongs and transfers the frame to the ADM  790  together with output ring information. In addition, when the ADM  790  receives a topology discovery RPR-MAC frame within the ring and the topology discovery RPR-MAC frame is transferred from the ADM  790 , the ring topology information collection circuit  740  notifies the path determination circuit  780  of ring topology information indicated by the topology discovery RPR-MAC frame. Upon receiving a topology discovery RPR-MAC frame, each frame in the ring adds information of its own node (e.g. RPR-MAC address) to the frame and transfers the obtained frame to a subsequent node. It is accordingly possible for a node as a transmission source to recognize an arrangement condition (ring topology information) of a node in the ring when a topology discovery RPR-MAC frame transmitted by a certain node makes a round of the ring to return to the transmission source node. The ring topology information collection circuit  740  extracts the ring topology information from the topology discovery RPR-MAC frame transferred from the ADM  790  and notifies the path determination circuit  780  of the information. 
     Next, operation of the ring failure information collection circuit  750  will be described. As is already described, each node in the ring transmits and receives a failure notification RPR-MAC frame including none of failure detection node information to and from an adjacent node when none of failure occurs on a link with the adjacent node. When the ADM  790  receives a failure notification RPR-MAC frame including none of failure detection node information within the ring and the failure notification RPR-MAC frame is transferred from the ADM  790 , the ring failure information collection circuit  750  refrains from operating. More specifically, when none of failure occurs on a link with an adjacent node, while the ring failure information collection circuit  750  receives a failure notification RPR-MAC frame including none of failure detection node information, it executes no operation in response to the reception of the frame. 
     The ring failure information collection circuit  750 , when no failure notification RPR-MAC frame including none of failure detection node information is transferred from the ADM  790  for a fixed time, determines that a failure occurs on a link with an adjacent node. Then, the ring failure information collection circuit  750  transfers information indicating that its own node detects a failure on the link with the adjacent node to the path determination circuit  780 . The circuit further transfers a failure notification RPR-MAC frame including information of its own node as failure detection node information to the ADM  790  together with output ring information. In this case, the ADM  790  sends the failure notification RPR-MAC frame within the ring. 
     On the other hand, when the ADM  790  receives a failure notification RPR-MAC frame including failure detection node information within the ring and the failure notification RPR-MAC frame is transferred from the ADM  790 , the ring failure information collection circuit  750  extracts the failure detection node information from the failure notification RPR-MAC frame. Then, the ring failure information collection circuit  750  notifies the path determination circuit  780  of the failure detection node information. Then, the ring failure information collection circuit  750  transfers the failure notification RPR-MAC frame including the failure detection node information to the ADM  790  together with output ring information. At this time, the ring failure information collection circuit  750  defines output ring information which designates a ringlet through which the failure notification RPR-MAC frame is transferred to the ADM  790  as output ring information. In a case, for example, where the failure notification RPR-MAC frame is transferred from an inner ring, output ring information which designates an inner ring is defined. As a result, the ADM  790  sends the failure notification RPR-MAC frame to the same ringlet as that through which the failure notification RPR-MAC frame is transferred. When a failure notification RPR-MAC frame including failure detection node information is received, therefore, the same frame as the failure notification RPR-MAC frame will be transferred to a subsequent node. 
     The inter-link failure detection circuit  760  periodically generates a KeepAlive signal (KeepAlive signal for informing other ring that no failure occurs on an inter-link). At this time, the inter-link failure detection circuit  760  generates a KeepAlive signal as a U-MAC frame with a controlling identifier of the KeepAlive signal as a transmission destination U-MAC address and its own node address as a transmission source U-MAC address. The inter-link failure detection circuit  760  transmits the generated KeepAlive signal (U-MAC frame) to the packet demultiplexing circuit  710 . The KeepAlive signal is transferred to other ring by the packet demultiplexing circuit  710 . The inter-link failure detection circuit  760  may include an identifier of a ring to which its own node belongs and an identifier for unitarily deciding its own node in the KeepAlive signal. When an identifier is assigned to an inter-link to which its own node is connected, the inter-link identifier may be included in the KeepAlive signal. 
     When the packet demultiplexing circuit  710  receives a KeepAlive signal from other ring and the KeepAlive signal is transferred from the packet demultiplexing circuit  710 , the inter-link failure detection circuit  760  determines that no failure occurs on the inter-link to which its own node is connected. Then, the inter-link failure detection circuit  760 , when no KeepAlive signal is transferred from the packet demultiplexing circuit  710  for a fixed time period, determines that a failure occurs on the inter-link to which its own node is connected to notify the RPR inter-link failure information collection circuit  770  of occurrence of a failure on the inter-link. 
     Upon receiving a notification of a failure occurring on a inter-link from the inter-link failure detection circuit  760 , the RPR inter-link failure information collection circuit  770  generates an inter-link failure notification RPR-MAC frame including failure information (information which can specify an inter-link on which a failure occurs) and transfers the frame to the ADM  790  together with output ring information. As a result, the inter-link failure notification RPR-MAC frame is sent within the ring from the ADM  790 . The RPR inter-link failure information collection circuit  770  can execute broadcast-transmission such that the inter-link failure notification RPR-MAC frame makes a round of the ring to return to its own node. In this case, a transmission destination RPR-MAC address of the inter-link failure notification RPR-MAC frame is set to be a controlling identifier according to broadcast-transmission. In addition, the RPR inter-link failure information collection circuit  770  can define a transmission destination RPR-MAC address such that the inter-link failure notification RPR-MAC frame is sequentially transferred to each node by hop-by-hop processing. The RPR inter-link failure information collection circuit  770  also notifies the flash circuit  781  of information that a failure occurs on the inter-link to which its own node is connected. The flash circuit  781  flushes learning contents of the learning data base  721  in response to the notification. 
     There is also a case where to the contact node shown in  FIG. 10 , an inter-link failure notification RPR-MAC frame sent by other contact node is transferred. In this case, first, the ADM  790  receives an inter-link failure notification RPR-MAC frame sent by other contact node. Then, the unit transfers the inter-link failure notification RPR-MAC frame to the RPR inter-link failure information collection circuit  770 . In this case, the RPR inter-link failure information collection circuit  770  notifies the flash circuit  781  of failure information (information which can specify an inter-link developing a fault) included in the inter-link failure notification RPR-MAC frame. 
     In a case where an inter-link failure notification RPR-MAC frame is set to be broadcast-transmitted, the ADM  790  only needs to transfer a copy of a received inter-link failure notification RPR-MAC frame to a subsequent node without processing the same. 
     On the other hand, in a case where an inter-link failure notification RPR-MAC frame is set to be sequentially transferred to each node by hop-by-hop processing, the RPR inter-link failure information collection circuit  770  generates an inter-link failure notification RPR-MAC frame to be terminated at a subsequent node which is an inter-link failure notification RPR-MAC frame including failure information included in the inter-link failure notification RPR-MAC frame received from the ADM  790  and transferring the frame to the ADM  790  together with output ring information. At this time, as output ring information, the RPR inter-link failure information collection circuit  770  defines output ring information which designates a ringlet through which the inter-link failure notification RPR-MAC frame is transferred to the ADM  790 . In a case, for example, where an inter-link failure notification RPR-MAC frame is transferred from an inner ring, output ring information which designates the inner ring is defined. As a result, the ADM  790  sends the inter-link failure notification RPR-MAC frame to the same ringlet as the ringlet through which the inter-link failure notification RPR-MAC frame is transferred. 
     The path determination circuit  780  determines an output ring corresponding to a transmission destination RPR-MAC address based on ring topology information notified by the ring topology information collection circuit  740  and information notified by the ring failure information collection circuit  750  (failure detection node information extracted from the failure notification RPR-MAC frame) to register the ring at the forwarding data base  731 . When a failure occurs in the ring, the path determination circuit  780  changes an output ring which is to pass the failure occurrence position to an output ring which fails to pass the failure occurrence position. 
     Upon receiving information notified by the RPR inter-link failure information collection circuit  770  (information that a failure occurs on an inter-link to which its own node is connected or failure information included in the inter-link failure notification RPR-MAC frame), the flash circuit  781  flushes learning contents registered at the learning data base  721 . At this time, the flash circuit  781  may flush all the learning contents or selectively a part of the learning contents. When selectively flushing a part of the learning contents, the circuit flushes an entry including an RPR-MAC address of a node specified by the information which is notified by the RPR inter-link failure information collection circuit  770 . 
     Upon receiving information that a failure occurs on an inter-link to which its own node is connected, the flash circuit  781  brings a port of the packet demultiplexing circuit  710  connected to the inter-link into the blocked state. Upon receiving information that other contact node detects failure occurrence on the living inter-link  505 , the flash circuit  781  brings the port of the packet demultiplexing circuit  710  connected to the inter-link into the forwarding state. 
     The ADM  790  sends various kinds of RPR-MAC frames transferred from the forwarding engine  730 , the ring topology information collection circuit  740 , the ring failure information collection circuit  750  and the RPR inter-link failure information collection circuit  770  to the inner ring or the outer ring, or to both of them. The ADM  790  determines whether the RPR-MAC frame should be sent to the inner ring or the outer ring, or to both of them based on output ring information transferred together with the RPR-MAC frame. 
     In addition, when receiving various kinds of RPR-MAC frames from the inner ring or the outer ring, the ADM  790  refers to a transmission destination RPR-MAC address of the RPR-MAC frame. Then, based on the transmission destination RPR-MAC address, the unit determines whether to drop (take in from the ring) the RPR-MAC frame or not. As to an RPR-MAC frame not to be dropped, the ADM  790  sends the frame to a subsequent node without processing. On the other hand, when determining to drop a frame, the ADM  790  transfers a received RPR-MAC frame to the frame conversion circuit  720 , the ring topology information collection circuit  740 , the ring failure information collection circuit  750  or the RPR inter-link failure information collection circuit  770 . To which circuit transfer is made depends on a kind of RPR-MAC frame. When a received RPR-MAC frame is a topology discovery RPR-MAC frame, for example, the unit transfers the frame to the ring topology information collection circuit  740 . When the frame is a failure notification RPR-MAC frame, the unit transfers the frame to the ring failure information collection circuit  750 . When the frame is an inter-link failure notification RPR-MAC frame, the unit transfers it to the RPR inter-link failure information collection circuit  770 . When a transmission destination RPR-MAC address is set to be an RPR-MAC address of its own node, the unit transfers the frame to the frame conversion circuit  720 . 
       FIG. 11  is a block diagram showing an example of a structure of each of nodes  900 - 1 ˜ 900 - 8  (these nodes will be denoted as a node  900  in the lump in some cases) other than a contact node which are to be applied to the ring network system according to the present invention. The same components as those of the contact node  700  are given the same reference numerals as those of  FIG. 10  to omit their description. Each node  900  comprises a packet switch  910 , the frame conversion circuit  720 , the forwarding engine  730 , the ring topology information collection circuit  740 , the ring failure information collection circuit  750 , the RPR inter-link failure information collection circuit  770 , the path determination circuit  780 , the flash circuit  781  and the ADM  790 . The node  900  further comprises the learning data base  721  and the forwarding data base  731 . 
     More specifically, the node  900  comprises the packet switch  910  in place of the packet demultiplexing circuit  710  and eliminates the need of the inter-link failure detection circuit  760 . 
     Packet switch  910 , which is a UNI (User Network Interface), transmits and receives a U-MAC frame to/from a terminal through UNI ports  901  and  902 . Upon receiving a U-MAC frame through the respective UNI ports  901  and  902 , the packet switch  910  concentrates U-MAC frames from the UNI ports  901  and  902  and transfers the frames to the frame conversion circuit  720 . When a U-MAC frame is transferred from the frame conversion circuit  720 , the packet switch  910  outputs the U-MAC frame through an appropriate UNI port (UNI port connected to a terminal as a transmission destination of the U-MAC frame). 
     The frame conversion circuit  720  of the node  900  transmits and receives a U-MAC frame to/from not the packet demultiplexing circuit  710  but the packet switch  910 . The reminder is the same as that of the frame conversion circuit of the contact node  700  (see  FIG. 10 ). 
     The RPR inter-link failure information collection circuit  770  of the node  900  operates in the same manner as that of the RPR inter-link failure information collection circuit (see  FIG. 10 ) of the contact node  700 . The node  900  fails to comprise such inter-link failure detection circuit  760  as shown in  FIG. 10 . Accordingly, the node refrains from executing operation of receiving a notification of failure occurrence on the inter-link and responsively generating an inter-link failure notification RPR-MAC frame including failure information to transfer the frame to the ADM  790 . Except for this point, operation of the RPR inter-link failure information collection circuit  770  of the node  900  is the same as the operation of the RPR inter-link failure information collection circuit of the contact node  700 . 
     Using other node than the contact node shown in  FIG. 10  and the contact node shown in  FIG. 11  realizes the already described operation of the ring network system. 
     With reference to  FIG. 10  and  FIG. 11 , the description has been made with respect to a case of detecting occurrence of an inter-link failure by using a KeepAlive signal to notify inter-link failure occurrence by the above-described first manner as an example. Inter-link failure occurrence may be notified by the above-described second manner. More specifically, without terminating a KeepAlive signal, the contact node may transfer the same with an RPR-MAC overhead added within the ring. In the following, operation executed in this case will be described. While structure of each node is the same as those shown in  FIG. 10  and  FIG. 11 , operation of a part of the circuits differs from that in a case of the first manner. 
     Operation of the packet demultiplexing circuit  710  of the contact node  700  which is executed when receiving a U-MAC frame other than a KeepAlive signal through an inter-link is the same as that already described. The packet demultiplexing circuit  710  of the contact node  700 , when receiving a KeepAlive signal through the inter-link, outputs the KeepAlive signal to both the inter-link failure detection circuit  760  and the frame conversion circuit  720  in the second manner. 
     Upon receiving the KeepAlive signal from the packet demultiplexing circuit  710 , the frame conversion circuit  720  generates an inter-link failure notification RPR-MAC frame as encapsulation of the KeepAlive signal by adding an RPR-MAC overhead to the KeepAlive signal. At this time, the frame conversion circuit  720  sets a transmission destination RPR-MAC address reserved in advance (controlling identifier) for a KeepAlive signal. Set, for example, a transmission destination RPR-MAC address to be a controlling identifier indicating that a kind of RPR-MAC frame is an inter-link failure notification RPR-MAC frame which is to be broadcast-transferred. Alternatively, a transmission destination RPR-MAC address may be defined such that an inter-link failure notification RPR-MAC frame including KeepAlive signal is subjected to hop-by-hop processing (i.e. so as to be once terminated at a subsequent node). Since the inter-link failure notification RPR-MAC frame generated by the frame conversion circuit  720  includes a KeepAlive signal, it indicates that no failure occurs on the inter-link. The frame conversion circuit  720  outputs the generated inter-link failure notification RPR-MAC frame to the forwarding engine  730 . The forwarding engine  730  specifies an output ring corresponding to the transmission destination RPR-MAC address of the inter-link failure notification RPR-MAC frame to transfer the inter-link failure notification RPR-MAC frame and the output ring information to the ADM  790 . Then, according to the output ring information, the ADM  790  sends out the inter-link failure notification RPR-MAC frame within the ring. 
     Similarly to the case shown in the first manner, when no KeepAlive signal is transferred from the packet demultiplexing circuit  710  for a fixed time period, the inter-link failure detection circuit  760  determines that a failure occurs on the inter-link to which its own node is connected to notify the RPR inter-link failure information collection circuit  770  of failure occurrence on the inter-link. In the second manner, the RPR inter-link failure information collection circuit  770  generates none of inter-link failure notification RPR-MAC frames even when receiving the notification from the inter-link failure detection circuit  760 . Processing of notifying the flash circuit  781  of information that a failure occurs on the inter-link to which its own node is connected is executed similarly to the case of the first manner. 
     The KeepAlive signal is periodically received by the contact node  700  through the inter-link. Then, in the second manner, the frame conversion circuit  720  generates an inter-link failure notification RPR-MAC frame including a KeepAlive signal, which is sent out into the ring by the ADM  790 . Accordingly, each node in the ring which is not a transmission source of the inter-link failure notification RPR-MAC frame periodically receives the inter-link failure notification RPR-MAC frame. When an inter-link failure notification RPR-MAC frame is transferred, the ADM  790  of each node (see  FIG. 10  and  FIG. 11 ) transfers the inter-link failure notification RPR-MAC frame to the RPR inter-link failure information collection circuit  770 . When none of inter-link failure notification RPR-MAC frames including a KeepAlive signal is transferred from the ADM  790  for a fixed time, the RPR inter-link failure information collection circuit  770  determines that a failure occurs on the inter-link to notify the flash circuit  781  that a failure occurs on an inter-link connected to a transmission source node of the being received inter-link failure notification RPR-MAC frame. 
     The other operation is the same as that of first manner. 
     In a case of notifying each node in the ring of inter-link failure occurrence in the second manner, a contact node having received a KeepAlive signal transfers the KeepAlive signal within the ring without once terminating the signal and each node is allowed to detect failure occurrence on an inter-link, so that inter-link failure occurrence can be detected at a high speed. On the other hand, in a case of notifying each node in the ring of inter-link failure occurrence in the second manner, operation of the node can be simplified. 
     Moreover, according to the present invention, when the RPR inter-link failure information collection circuit  770  notifies the flash circuit  781  of information that a failure occurs on the inter-link, the flash circuit  781  immediately flushes the storage contents of the learning data base  721  without waiting for a lapse of an aging time. Timing is accordingly sped up of setting a broadcast address as a transmission destination RPR-MAC address by the frame conversion circuit  720  to enable such broadcast-transmission as shown in  FIG. 6 . As a result, an inter-link failure can be recovered at a high speed. 
     In a case of selectively flushing the storage contents of the learning data base  721 , learning contents related to a communication path which fails to pass the inter-link are left as they are. At the time of frame transfer from a terminal to a terminal without passing through the inter-link, broadcast-transmission is accordingly unnecessary to enable frame transfer similarly to a case where no inter-link failure occurs. More specifically, when a frame is transferred without passing through the inter-link, lack of broadcast-transmission prevents an increase in the volume of traffic to realize effective use of a band. 
     Moreover, since each node comprises the ring failure information collection circuit  750  and the path determination circuit  780 , even when a failure occurs on a link between nodes of the ring, the failure can be recovered. 
     Even when the respective rings are connected through other communication network by transmission/reception of a KeepAlive signal to/from contact nodes, it is possible to detect a failure on a path connecting the rings and recover from the failure. More specifically, the contact nodes  700 - 1  and  700 - 3  may be connected not through the inter-link  505  but through other communication network. Similarly, the contact nodes  700 - 2  and  700 - 4  may be connected not through the inter-link  506  but through other communication network. 
     When the contact nodes are connected not through the inter-link but through other communication network, the inter-link failure detection circuit  760  of each contact node adds information of a contact node as a transmission destination of a KeepAlive signal to the KeepAlive signal as information which can be recognized by the communication network. Assume, for example, that the contact nodes are connected through an Ethernet (registered trademark) network. In this case, the inter-link failure detection circuit  760  only needs to designate a MAC address of a transmission destination contact node within a KeepAlive signal as information of the contact node as a transmission destination of the KeepAlive signal. 
     While in  FIG. 10 , the description has been made with respect to a structure in which the contact node  700  comprises each circuit, the contact node may comprise a computer, which computer operates in the same manner as those of the packet demultiplexing circuit  710 , the frame conversion circuit  720 , the forwarding engine  730 , the ring topology information collection circuit  740 , the ring failure information collection circuit  750 , the inter-link failure detection circuit  760 , the RPR inter-link failure information collection circuit  770 , the path determination circuit  780 , the flash circuit  781  and the ADM  790  according to a program. The program only needs to be stored in a storage device in advance that the contact node  700  comprises. 
     Similarly, the node  900  other than the contact node may comprise a computer, which computer operates in the same manner as those of the packet switch  910 , the frame conversion circuit  720 , the forwarding engine  730 , the ring topology information collection circuit  740 , the ring failure information collection circuit  750 , the RPR inter-link failure information collection circuit  770 , the path determination circuit  780 , the flash circuit  781  and the ADM  790  according to a program. The program only needs to be stored in a storage device in advance that the node  900  comprises. 
     In the first exemplary embodiment, the inter-link or a communication network which connects rings is equivalent to the communication path recited in the claims. The inter-link failure detection unit  760  of the contact node and the RPR inter-link failure information collection circuit  770  of other node than the contact node are equivalent to the failure occurrence determination unit. The flash circuit  781  is equivalent to the erasure unit. The frame conversion circuit  720 , the forwarding engine  730  and the ADM  790  are equivalent to the broadcast unit. The packet demultiplexing circuit  710  is equivalent to the state change unit. KeepAlive signal is equivalent to an existence confirmation signal. The inter-link failure detection unit  760  and the packet demultiplexing circuit  710  are equivalent to the existence confirmation signal transmission unit. The packet demultiplexing circuit  710  is equivalent to the existence confirmation signal reception unit. The frame conversion circuit  720 , the forwarding engine  730  and the ADM  790  are equivalent to the existence confirmation frame transmission unit. 
     Second Exemplary Embodiment 
     Shown in the first exemplary embodiment is a case of executing broadcast-transmission early to realize high-speed failure recovery by immediately flushing storage contents of the learning data base when an inter-link failure occurs. On the other hand, in a second exemplary embodiment of the present invention, the learning data base stores a correspondence relationship among a U-MAC address, a network identifier (VLAN identifier in this example) and an RPR-MAC address. Then, at the occurrence of an inter-link failure, a VLAN identifier deriving method is changed. As a result, when search of an RPR-MAC address corresponding to a pair of a U-MAC address and a VLAN identifier fails at the occurrence of an inter-link failure, executing broadcast-transmission realizes high-speed failure recovery. 
       FIG. 12  is a diagram for use in explaining an example of a structure of a ring network system according to the second exemplary embodiment. The ring network system according to the second exemplary embodiment is a multi-ring network system comprising a plurality of rings (packet rings)  1101 - a  and  1101 - b . The ring  1101 - a  comprises RPR nodes (hereinafter simply denoted as a node)  1500 - 1 ˜ 1500 - 4  and contact nodes  1300 - 1  and  1300 - 2 . The ring  1101 - b  comprises nodes  1500 - 5 ˜ 1500 - 8  and contact nodes  1300 - 3  and  1300 - 4 . The rings  1101 - a  and  1101   b  are two-fiber rings, with the ring  1101 - a  comprising an inner ring  1101 - a -inner and an outer ring  1101 - a -outer. Similarly, the ring  1101 - b  comprises an inner ring  1101 - b -inner and an outer ring  1101 - b -outer. Assume here that each inner ring executes traffic transfer clockwise and each outer ring executes traffic transfer counterclockwise. Assume that to the node  1500 - 1 , a terminal  1110  is connected and to the node  1500 - 7 , a terminal  1111  is connected. Also assume that RPR-MAC addresses of the nodes  1500 - 1 ˜ 1500 - 8  are  1500 - 1 ˜ 1500 - 8 , respectively. Similarly, assume that RPR-MAC addresses of the contact nodes  1300 - 1 ˜ 1300 - 4  are  1300 - 1 ˜ 1300 - 4 , respectively. 
     The contact nodes  1300 - 1  and  1300 - 3  are connected by an inter-link  1105 . The contact nodes  1300 - 2  and  1300 - 4  are connected by an inter-link  1106 . At each inter-link, a VLAN (Virtual LAN) is set. To one inter-link, a plurality of VLANs may be set. For simplicity of explanation, assume here that a VLAN 1  is set at the inter-link  1105  and a VLAN 2  is set at the inter-link  1106 . 
     Each contact node refers to a VLAN identifier of a U-MAC frame. Then, when a VLAN set at an inter-link connected to the node itself and its VLAN identifier correspond to each other, the node transfers the U-MAC frame to the inter-link. Accordingly, a U-MAC frame having the VLAN identifier “VLAN 1 ” is transferred to the inter-link  1105  and not to the inter-link  1106 . Similarly, a U-MAC frame having the VLAN identifier “VLAN 2 ” is transferred to the inter-link  1106  and not to the inter-link  1105 . In addition, while broadcast-transmission is first executed for making a learning data table learn a correspondence relationship among a U-MAC address, a VLAN identifier and an RPR-MAC address, each contact node also at this time compares a VLAN identifier that a U-MAC frame has and a VLAN set at an inter-link and when they do not correspond to each other, refrains from executing transfer to the inter-link. 
     In the present embodiment, the inter-links  1105  and  1106  are both used in the forwarding state indiscriminatingly between a living inter-link and a spare inter-link. Each inter-link passes only a U-MAC frame having a VLAN identifier according to a VLAN set at itself as described above. 
     In the following, description will be made of operation of a ring network system in a normal state with respect to a case where U-MAC frame transfer is made from the terminal  1110  to the terminal  1111 . U-MAC frame is an Ethernet (registered trademark) frame, for example. In  FIG. 12 , paths  1120 ,  1121  and  1122  indicated by dotted lines are U-MAC frame transfer paths. Paths  1130  and  1131  indicated by solid lines in  FIG. 12  are RPR-MAC frame transfer paths. 
     The node  1500 - 1  receives a U-MAC frame output by the terminal  1110 . Here, the terminal  1110  may output a U-MAC frame with a VLAN identifier added. Alternatively, the terminal  1110  may output a U-MAC frame with no VLAN identifier added. 
     Upon receiving a U-MAC frame output from the terminal  1110 , the node  1500 - 1  adds a VLAN identifier to the U-MAC frame or changes a VLAN identifier added in advance. When with a VLAN identifier already added to the received U-MAC frame, the node  1500 - 1  newly adds a VLAN identifier, an addition region of a VLAN identifier is provided in the U-MAC frame to newly add the VLAN identifier to the region. At this time, a VLAN identifier originally added to the U-MAC frame is preserved in the U-MAC frame as it is. Such VLAN identifier addition manner is called VLAN tag stack. The node  1500 - 1  may change a VLAN identifier originally added to the U-MAC frame without stacking a VLAN tag. When no VLAN identifier is originally added to the U-MAC frame, the node  1500 - 1  adds a VLAN identifier to the U-MAC frame. 
     Each node other than a contact node derives a VLAN identifier by executing predetermined operation and adds the VLAN identifier to a U-MAC frame. Alternatively, the node changes a VLAN identifier originally added to a derived VLAN identifier. Accordingly, depending on an operation result, “VLAN 1 ” may be derived or “VLAN 2 ” may be derived as an identifier. 
     When notified occurrence of a failure on an inter-link, each node other than a contact node changes a method of operating a VLAN identifier such that only an identifier of a VLAN set at an inter-link developing no fault is derived. 
     In the following, description will be made of a case where the node  1500 - 1  derives the “VLAN 1 ” as a VLAN identifier and adds the “VLAN 1 ” to a U-MAC frame. In the present exemplary embodiment, each node comprises a learning data base in which a correspondence relationship among a U-MAC address, a VLAN identifier and an RPR-MAC address is stored. 
     Upon newly adding a VLAN identifier to a U-MAC frame received from the terminal  1110 , the node  1500 - 1  refers to its own learning data base. Then, the node searches an RPR-MAC address corresponding to a combination between a transmission destination U-MAC address in the U-MAC frame and an added VLAN identifier to determine a transmission destination RPR-MAC address. In this example, assume that a transmission destination U-MAC address (the address of the terminal  1111 ), the “VLAN 1 ” and the RPR-MAC address  1300 - 1  are correlated in advance and learned by the learning data base. The node  1500 - 1  accordingly adds an RPR-MAC overhead with “ 1300 - 1 ” as a transmission destination RPR-MAC address and its own RPR-MAC address “ 1500 - 1 ” as a transmission source RPR-MAC address and converts the U-MAC frame into an RPR-MAC frame. 
     Subsequently, the node  1500 - 1  refers to its own forwarding data base to specify output ring information. Then, according to the output ring information, the node sends the RPR-MAC frame to the inner ring or the outer ring or both of them. Shown in  FIG. 12  is a case where the frame is sent out to the inner-ring  1101 - a -inner. 
     The node  1500 - 2  having received an RPR-MAC frame sent from the node  1500 - 1  refers to the transmission destination RPR-MAC address of the RPR-MAC frame to compare the address with its own address. Since the transmission destination RPR-MAC address differs from the address of the node  1500 - 2  itself, the node  1500 - 2  determines that itself should not terminate the RPR-MAC frame to transfer the RPR-MAC frame to the inner link  1101 - a -inner. 
     The node  1300 - 1  having received the RPR-MAC frame sent from the node  1500 - 2  refers to the transmission destination RPR-MAC address of the RPR-MAC frame to compare the address with its own address. Since the compared two addresses coincide with each other, the node  1300 - 1  takes in the RPR-MAC address. In other words, the node takes out the RPR-MAC frame from the ring  1101 - a . Then, the node  1300 - 1  removes the RPR-MAC overhead from the RPR-MAC frame to convert the RPR-MAC frame into the U-MAC frame. To the U-MAC frame, the VLAN identifier “VLAN 1 ” is added. In addition, since the VLAN 1  is set at the inter-link  1105 , the node  1300 - 1  is allowed to send the U-MAC frame to the inter-link  1105 , so that the node sends the U-MAC frame to the inter-link  1105 . 
     The node  1300 - 3  receives the U-MAC frame through the inter-link  1105 . Then, the node  1300 - 3  refers to its own learning data base to search an RPR-MAC address corresponding to a combination between a transmission destination U-MAC address in the U-MAC frame and an added VLAN identifier to determine a transmission destination RPR-MAC address (“ 1500 - 7 ” here). The node  1300 - 3  adds an RPR-MAC overhead with “ 1500 - 7 ” as the transmission destination RPR-MAC address and its own RPR-MAC address “ 1300 - 3 ” as the transmission source RPR-MAC address to the U-MAC frame to convert the U-MAC frame into an RPR-MAC frame. Subsequently, the node  1300 - 3  refers to its own forwarding data base to specify output ring information based on the transmission destination RPR-MAC address. In this example, the node  1300 - 3  sends the RPR-MAC frame to the outer ring  1101 - b -outer according to the output ring information. 
     Upon receiving the RPR-MAC frame, either of the nodes  1300 - 4  and  1500 - 8  determines that the frame is not an RPR-MAC frame to be terminated by the node itself because the transmission destination RPR-MAC address and its own RPR-MAC address fail to coincide. Then, the node sends out the RPR-MAC frame to the outer rig  1101 - b -outer. As a result, the node  1500 - 7  receives the RPR-MAC frame. The node  1500 - 7  takes in the received RPR-MAC frame because the transmission destination RPR-MAC address of the received RPR-MAC frame is coincident with its own RPR-MAC address (takes out the RPR-MAC frame from the ring  1101 - b ). Subsequently, the node  1500 - 7  removes the RPR-MAC overhead from the RPR-MAC frame to convert the RPR-MAC frame into a U-MAC frame. 
     The node  1500 - 7  deletes the VLAN identifier “VLAN 1 ” added by the node  1500 - 1  from the converted U-MAC frame. As a result, the U-MAC frame returns to a state of being output by the terminal  1110 . Although the description has been made here of a case where the node  1500 - 1  adds the VLAN identifier “VLAN 1 ”, when the VLAN identifier added to the U-MAC frame is changed to the “VLAN 1 ”, the node  1500 - 7  only needs to restore the “VLAN 1 ” to the original VLAN identifier. The node  1500 - 7  transfers the U-MAC frame with the VLAN identifier “VLAN 1 ” deleted to the terminal  1111 . 
     Next, operation executed when a failure occurs will be described.  FIG. 13  is a diagram for use in explaining a condition where the inter-link  1105  develops a fault. When the inter-link  1105  develops a fault, traffic flow shown in  FIG. 12  prevents transfer of a U-MAC frame from the terminal  1110  to the terminal  1111 . 
       FIG. 14  is a flow chart showing operation of a contact node when a failure occurs. Description will be here made of the contact nodes  1300 - 1  and  1300 - 3  connected to the inter-link  1105  as an example. When the inter-link  1105  develops a fault, the contact nodes  1300 - 1  and  1300 - 3  detect the failure (Step S 1210 ). Subsequently, the contact nodes  1300 - 1  and  1300 - 3  send an inter-link failure notification RPR-MAC frame notifying that the inter-link develops a fault within a ring to which the node itself belongs (Step S 1211 ). The contact nodes  1300 - 1  and  1300 - 3  include information which can specify an inter-link developing a fault in the inter-link failure notification RPR-MAC frame. The inter-link failure notification RPR-MAC frame may be sent out to the inner ring or the outer ring, or to both of them. In the example shown in  FIG. 13 , illustrated is a case where the contact nodes  1300 - 1  and  1300 - 3  send an inter-ink failure notification RPR-MAC frame to the inner ring  1101 - a -inner and the inner ring  1101 - b -inner, respectively. In addition, the paths  1201 - a  and  1201 - b  shown in  FIG. 13  show transfer paths of an inter-link failure notification RPR-MAC frame. 
       FIG. 15 . is a flow chart showing operation of a node other than a contact node at failure occurrence. The nodes  1500 - 1 ˜ 1500 - 8  are relevant nodes. Although description will be here made of operation of the node  1500 - 1 , the nodes  1500 - 2 ˜ 1500 - 8  execute the same operation. The node  1500 - 1  receives an inter-link failure notification RPR-MAC frame transmitted to the contact node  1300 - 1  at Step S 1210  (Step S 1220 ). The node  1500 - 1  then changes a method of deriving a VLAN identifier (Step S 1221 ). At Step S 1221 , the node  1500 - 1  specifies an inter-link on which a failure occurs based on the inter-link failure notification RPR-MAC frame to change the method of deriving a VLAN identifier so as to prevent a VLAN identifier corresponding to a VLAN set at the inter-link from being derived. In other words, the method of deriving a VLAN identifier is changed such that only a VLAN identifier corresponding to a VLAN set at an inter-link developing no failure is derived. While in a normal state, the “VLAN 1 ” is derived as a VLAN identifier in some cases, changing the VLAN identifier deriving method at Step S 1221  results in excluding the “VLAN 1 ” from targets to be derived. Then, as a VLAN identifier, a VLAN identifier corresponding to the VLAN set at the inter-link  1106  (in this example, “VLAN 2 ”) will be derived. Then, the node  1500 - 1  adds the “VLAN 2 ” to the U-MAC frame. Alternatively, the node changes an originally added VLAN identifier to the “VLAN 2 ”. 
       FIG. 16  is a diagram for use in explaining a condition where a frame is transferred from the terminal  1110  to the terminal  1111  after each of the nodes  1500 - 1 ˜ 1500 - 8  changes a VLAN identifier deriving method in response to failure occurrence on the inter-link  1105 . In  FIG. 16 , paths  1230 ,  1231  and  1232  indicated by dotted lines are transfer paths of a U-MAC frame. Paths  1250  and  1251  indicated by solid lines in  FIG. 16  are transfer paths of an RPR-MAC frame to be broadcast-transmitted. 
     With reference to  FIG. 16 , description will be made of a case where a derived VLAN identifier is added to a U-MAC frame. 
     Upon receiving a U-MAC frame from the terminal  1110 , the node  1500 - 1  derives a VLAN identifier. At this time, since a VLAN identifier deriving method has been changed at Step S 1221 , even in a case where the “VLAN 1 ” is supposed to be derived in a normal state, the node derives the “VLAN 2 ”. Then, the node  1500 - 1  adds the VLAN identifier “VLAN 2 ” to the U-MAC frame. 
     Subsequently, the node  1500 - 1  searches its own learning data base for an RPR-MAC address corresponding to a combination between a transmission destination U-MAC address (the address of the terminal  1111 ) in the received U-MAC frame and the VLAN identifier “VLAN 2 ”. While the address of the terminal  1111  (U-MAC address) and the RPR-MAC address corresponding to the “VLAN 1 ” are stored in the learning data base, the address of the terminal  1111  (U-MAC address) and the RPR-MAC address corresponding to the “VLAN 2 ” are not stored in the learning data base. Therefore, the node  1500 - 1  fails in searching an RPR-MAC address. When search of the RPR-MAC address fails, the node  1500 - 1  adds an RPR-MAC overhead with the transmission destination RPR-MAC address as a broadcast address and its own RPR-MAC address “ 1500 - 1 ” as a transmission source RPR-MAC address to the U-MAC frame. The node  1500 - 1  converts the U-MAC frame into an RPR-MAC frame by adding the RPR-MAC overhead. The node  1500 - 1  broadcast-transmits the RPR-MAC frame. At this time, the node  1500 - 1  may transmit the RPR-MAC frame to either of the inner ring and the outer ring. The node may send the same to both of the inner ring and the outer ring.  FIG. 16  shows a case where the node  1500 - 1  sends an RPR-MAC frame to the outer ring  1101 - a -outer to transfer the same through the transfer path  1250 . 
     Upon receiving the RPR-MAC frame (an RPR-MAC frame with a broadcast address as a transmission destination), the node  1500 - 4  takes in the RPR-MAC frame, as well as sending a copy of the RPR-MAC frame to the ring  1101 - a . At this time, the node  1500 - 4  sends the copy of the RPR-MAC frame to a ringlet through which the RPR-MAC frame is transferred out of the inner ring and the outer ring. In this example, accordingly, a copy of the RPR-MAC frame is sent out to the outer ring  1101 - a -outer. 
     In addition, the node  1500 - 4  registers, at its own learning data base, a correspondence relationship among a transmission source RPR-MAC address in the taken RPR-MAC frame (in this example, “ 1500 - 1 ”), a transmission source U-MAC address of a U-MAC frame accommodated in the RPR-MAC frame (in this example, the address of the terminal  1110 ) and a VLAN identifier added to the U-MAC frame (“VLAN 2 ” in this example). Thereafter, the node  1500 - 4  removes the RPR-MAC overhead from the RPR-MAC frame to convert the frame into a U-MAC frame. The node also removes the VLAN identifier “VLAN 2 ” added to the U-MAC frame by the node  1500 - 1 . As a result, the U-MAC frame returns to a state of being output by the terminal  1110 . While the description has been here made with respect to a case where the node  1500 - 1  adds the VLAN identifier “VLAN 2 ”, when a VLAN identifier added to the U-MAC frame is changed to the “VLAN 2 ”, the node  1500 - 4  only needs to restore the “VLAN 2 ” to an original VLAN identifier. The node  1500 - 4  transfers the U-MAC frame with the VLAN identifier removed to a terminal or an apparatus (not shown) connected to the node  1500 - 4  outside the ring  1101 - a -outer. 
     Similarly to the node  1500 - 4 , the nodes  1500 - 3 ,  1300 - 2 ,  1300 - 1  and  1500 - 2  take in the RPR-MAC frame, as well as sending a copy of the RPR-MAC frame to the outer ring  1101 - a -outer. As a result, the RPR-MAC frame will be transferred within the ring along the transfer path  1250  shown in  FIG. 16 . Also similarly to the node  1500 - 4 , each of the nodes  1500 - 3 ,  1300 - 2 ,  1300 - 1  and  1500 - 2  registers, at its own learning data base, a correspondence relationship among a transmission source RPR-MAC address (in this example, “ 1500 - 1 ”), a transmission source U-MAC address (in this example, the address of the terminal  1110 ) and the VLAN identifier “VLAN 2 ”. Then, each of the nodes  1500 - 3  and  1500 - 2  removes the RPR-MAC overhead of the RPR-MAC frame to remove the VLAN identifier “VLAN 2 ” from the U-MAC frame. As a result, the U-MAC frame returns to a state of being output by the terminal  1110 . Thereafter, each node transfers the U-MAC frame to a terminal or an apparatus (not shown) outside the ring  1101 - a . After conversion to a U-MAC frame, a contact node connected to the inter-link sends the U-MAC frame to the inter-link without removing a VLAN identifier. 
     In a case of a ring structure, a broadcast-transmitted RPR-MAC frame makes a round of the ring to return to a node as its transmission source. Each of the nodes  1500 - 1 ˜ 1500 - 8  and  1300 - 1 ˜ 1300 - 4 , when referring to a transmission source RPR-MAC address of the received RPR-MAC frame to find that the address coincides with an RPR-MAC address of its own node, abandons the RPR-MAC frame. No RPR-MAC frame accordingly loops. 
     The node  1300 - 2  removes the RPR-MAC overhead from the taken RPR-MAC frame to convert the RPR-MAC frame into a U-MAC frame. Since the VLAN identifier “VLAN 2 ” added to the converted U-MAC frame and the VLAN 2  set at the inter-link  1106  correspond, the node  1300 - 2  determines to send the U-MAC frame to the inter-link  1106  to send the U-MAC frame to the inter-link  1106 . The node  1300 - 2  sends the frame to the inter-link  1106  without deleting the VLAN identifier “VLAN 2 ” added to the U-MAC frame. The U-MAC frame will be transferred to the node  1300 - 4  along the transfer path  1231  shown in  FIG. 16 . 
     The node  1300 - 4  having received the U-MAC frame through the inter-link  1106  searches its own learning data base for an RPR-MAC address corresponding to a combination between a transmission destination U-MAC address in the received U-MAC frame (the address of the terminal  1111 ) and the “VLAN 2 ” added to the U-MAC frame. In the learning data base of the node  1300 - 4 , neither the address of the terminal  1111  (a U-MAC address) nor an RPR-MAC address corresponding to the “VLAN 2 ” is stored. Therefore, the node  1300 - 4  fails in searching an RPR-MAC address. When the RPR-MAC address search fails, the node  1300 - 4  adds to the U-MAC frame an RPR-MAC overhead with a transmission destination RPR-MAC address as a broadcast address and its own RPR-MAC address “ 1300 - 4 ” as a transmission source RPR-MAC address. The node  1300 - 4  converts the U-MAC frame into an RPR-MAC frame by adding an RPR-MAC overhead. The node  1300 - 4  broadcast-transmits the RPR-MAC frame. At this time, the node  1300 - 4  is allowed to send the RPR-MAC frame to either of the inner ring and the outer ring. Alternatively, the frame may be sent to both of the inner ring and the outer ring.  FIG. 16  shows a case where the node  1300 - 4  sends the RPR-MAC frame to the outer ring  1101 - b -outer and transfers the same through the transfer path  1251 . 
     Upon receiving the RPR-MAC frame (the RPR-MAC frame with a broadcast address as a transmission source), the node  1500 - 8  takes in the RPR-MAC frame, as well as sending a copy of the RPR-MAC frame to the outer ring  1101 - b -outer. The node  1500 - 8  also registers, at its own learning data base, a correspondence relationship among a transmission source RPR-MAC address in the taken RPR-MAC frame (in this example, “ 1300 - 4 ”), a transmission source U-MAC address of a U-MAC frame accommodated in the RPR-MAC frame (in this example, the address of terminal  1110 ) and a VLAN identifier added to the U-MAC frame (in this example, “VLAN 2 ”). Thereafter, the node  1500 - 8  removes the RPR-MAC overhead from the RPR-MAC frame to convert the frame into a U-MAC frame. The node further removes the VLAN identifier “VLAN 2 ” added to the U-MAC frame. Then, the node  1500 - 8  transfers the U-MAC frame to a terminal or an apparatus (not shown) connected to the node  1500 - 8  outside the ring  1101 - b . While the description has been made with respect to a case where the node  1500 - 1  adds the VLAN identifier “VLAN 2 ” as an example, when a VLAN identifier added to a U-MAC frame is changed to the “VLAN 2 ”, the node  1500 - 8  only needs to restore the “VLAN 2 ” to the original VLAN identifier. 
     Similarly to the node  1500 - 8 , the nodes  1500 - 7 ,  1500 - 6 ,  1500 - 5  and  1300 - 3  take in an RPR-MAC frame, as well as sending a copy of the RPR-MAC frame to the outer ring  1101 - b -outer. As a result, the RPR-MAC frame will be transferred within the ring along the transfer path  1251  shown in  FIG. 16 . In addition, each of the nodes  1500 - 7 ,  1500 - 6 ,  1500 - 5  and  1300 - 3  registers at its own learning data base a correspondence relationship among a transmission source RPR-MAC address (in this example, “ 700 - 4 ”), a transmission source U-MAC address (in this example, the address of the terminal  510 ) and a VLAN identifier (in this example, “VLAN 2 ”) similarly to the node  900 - 8 . Then, the node removes the RPR-MAC overhead of the RPR-MAC frame and removes the VLAN identifier “VLAN 2 ” added to the U-MAC frame to transfer the obtained U-MAC frame to a terminal or an apparatus (not shown) outside the ring  1101 - b . A contact node, however, removes no VLAN identifier. 
     On the other hand, the node  1300 - 4 , when an RPR-MAC frame broadcast-transmitted by itself makes a round of the ring and is transferred to the node  1300 - 4 , abandons the RPR-MAC frame. 
     The node  1500 - 7  removes the RPR-MAC overhead of the taken RPR-MAC frame to convert the RPR-MAC frame into a U-MAC frame. Furthermore, the node removes the VLAN identifier “VLAN 2 ” added to the U-MAC frame. As a result, the U-MAC frame returns to a state of being output by the terminal  1110 . Then, the node  1500 - 7  transfers the U-MAC frame to the terminal  1111 . The U-MAC frame will be transferred from the node  1500 - 7  to the terminal  1111  along the transfer path  1232  shown in  FIG. 16 . 
     Thus, even when the inter-link  1105  develops a fault, frame transfer from the terminal  1110  to the terminal  1111  is enabled. Next, frame transfer from the terminal  1111  to the terminal  1110  will be described. 
       FIG. 17  is a diagram for use in explaining a condition where a frame is transferred from the terminal  1111  to the terminal  1110 . In  FIG. 17 , paths  1233 ,  1235  and  1237  indicated by dotted lines are U-MAC frame transfer paths. Also in  FIG. 17 , paths  1234  and  1236  indicated by solid lines are RPR-MAC frame transfer paths. 
     In the course of frame transfer from the terminal  1110  to the terminal  1111  shown in  FIG. 17 , each node in the ring  1101 - b  learns into its own learning data table a correspondence relationship among the address of the terminal  1110  (the U-MAC address), the VLAN identifier “VLAN 2 ” and the RPR-MAC address “ 1300 - 4 ”. Similarly, each node in the ring  1101 - a  learns into its own learning data base a correspondence relationship among the address of the terminal  1110 , the VLAN identifier “VLAN 2 ” and the RPR-MAC address “ 1500 - 1 ”. Accordingly, unicast communication from the terminal  1111  to the terminal  1110  is enabled. In the following, the unicast communication will be described. 
     The node  1500 - 7  receives a U-MAC frame whose transmission destination U-MAC address is the address of the terminal  1110  from the terminal  1111 . Then, the node  1500 - 7  derives a VLAN identifier to be added to the U-MAC frame and adds the same to the U-MAC frame. Since the processing at Step S 1221  (see  FIG. 15 ) is executed in response to failure occurrence on the inter-link  1105  here, the node  1500 - 7  derives the identifier “VLAN 2 ” set at the inter-link  1106  and adds the “VLAN 2 ” to the U-MAC frame. 
     Subsequently, the node  1500 - 7  refers to the learning data base to search the RPR-MAC address  1300 - 4  corresponding to a combination between the transmission destination U-MAC address and the “VLAN 2 ”. Then, the node  1500 - 7  adds to the U-MAC frame an RPR-MAC overhead with “ 1300 - 4 ” as a transmission destination RPR-MAC address and “ 1500 - 7 ” as a transmission source RPR-MAC address and transfers the obtained frame within the ring  1101 - b . As a result, the RPR-MAC frame will be transferred from the node  1500 - 7  to the node  1300 - 4  along the transfer path  1234 . 
     Upon receiving an RPR-MAC frame, each of the nodes  1500 - 8  and  1300 - 4  on the transfer path  1234  registers at its own learning data base a correspondence relationship among the transmission source RPR-MAC address (“ 1500 - 7 ” in this example), a transmission source U-MAC address of a U-MAC frame accommodated in the RPR-MAC frame (the address of the terminal  1111  in this example) and a VLAN identifier added to the U-MAC frame (“VLAN 2 ” in this example). 
     Upon receiving the RPR-MAC frame, the node  1300 - 4  also takes in the RPR-MAC frame because the transmission destination RPR-MAC address coincides with its own RPR-MAC address. Then, the node removes the RPR-MAC overhead of the taken RPR-MAC frame to convert the RPR-MAC frame into a U-MAC frame. Without removing the VLAN identifier from the U-MAC frame, the node  1300 - 4  sends the U-MAC frame to the inter-link  1106 . The U-MAC frame will be transferred to the node  1300 - 2  along the transfer path  1235  shown in  FIG. 17 . 
     The node  1300 - 2  having received the U-MAC frame through the inter-link  1106  refers to an RPR-MAC address corresponding to a combination between the transmission destination U-MAC address (the address of the terminal  1110 ) and the “VLAN 2 ” added to the U-MAC frame. Registered at the learning data base is “ 1500 - 1 ” as an RPR-MAC address corresponding to the combination between the address of the terminal  1110  and the “VLAN 2 ”. The node  1300 - 2  accordingly adds to the U-MAC frame an RPR-MAC overhead with “ 1500 - 1 ” as a transmission destination RPR-MAC address and “ 1300 - 2 ” as a transmission source RPR-MAC address and transfers the obtained frame within the ring  1101 - a . As a result, the RPR-MAC frame will be transferred from the node  1300 - 2  to the node  1500 - 1  along the transfer path  1236 . 
     Upon receiving the RPR-MAC frame, each of the nodes  1500 - 3 ,  1500 - 4  and  1500 - 1  on the transfer path  1236  registers at its own learning data base a correspondence relationship among a transmission source RPR-MAC address (“ 1300 - 2 ” in this example), a transmission source U-MAC address of a U-MAC frame accommodated in the RPR-MAC frame (the address of the terminal  1111  in this example) and a VLAN identifier added to the U-MAC frame (“VLAN 2 ” in this example). 
     As a result, the nodes  1500 - 1 ,  1500 - 4  and  1500 - 3  learn into the learning data base a correspondence relationship among the address of the terminal  1111 , the “VLAN 2 ” and the RPR-MAC address “ 1300 - 2 ”. The nodes  1300 - 4  and  1500 - 8  learn into the learning data base a correspondence relationship among the address of the terminal  1111 , the “VLAN 2 ” and the RPR-MAC address “ 1500 - 7 ”. Accordingly, unicast-communication from the terminal  1110  to the terminal  1111  is enabled. 
     In the second exemplary embodiment, each contact node only needs to detect a failure of an inter-link similarly to the first exemplary embodiment. More specifically, by directly detecting a failure of a physical link, the contact node can detect a failure of an inter-link. Alternatively, the contact node may transmit and receive a KeepAlive signal to/from each other to detect a failure on an inter-link upon non-arrival of the KeepAlive signal. While the two manners have been described as a method of notifying each node in the ring of inter-link failure occurrence by using a KeepAlive signal, any of the two manners can be applied to the present exemplary embodiment. 
       FIG. 18  is a block diagram showing an example of a structure of the contact nodes  1300 - 1 ˜ 1300 - 8  (these contact nodes will be denoted as a node  1300  in the lump in some cases) applied to the ring network system according to the second exemplary embodiment. The same components as those of the contact node  700  according to the first exemplary embodiment are given the same reference numerals as those in  FIG. 10  to omit their description. Each contact node  1300  comprises a packet demultiplexing circuit  710 , a frame conversion circuit  720 , a forwarding engine  730 , a ring topology information collection circuit  740 , a ring failure information collection circuit  750 , an inter-link failure detection circuit  760 , an RPR inter-link failure information collection circuit  770 , a path determination circuit  780 , an ADM  790  and a VLAN filtering circuit  1301 . The node  1300  further comprises a learning data base  741  and a forwarding data base  731 . 
     More specifically, the contact node  1300  according to the present exemplary embodiment differs from the contact node  700  according to the first exemplary embodiment in failing to comprise a flash circuit and in comprising the VLAN filtering circuit  1301 . Also, in the second exemplary embodiment, the learning data base  741  stores a correspondence relationship among a U-MAC address, a VLAN identifier and an RPR-MAC address. 
     The VLAN filtering circuit  1301  has a U-MAC frame transferred from the frame conversion circuit  720 . The U-MAC frame is a conversion from an RPR-MAC frame received by the ADM  790  which is transferred within the ring into a U-MAC frame by the frame conversion circuit  720 . The VLAN filtering circuit  1301  refers to a VLAN identifier that the U-MAC frame has (not an originally existing VLAN identifier but a newly added VLAN identifier when a VLAN tag is stacked). Then, the VLAN filtering circuit  1301  determines whether the VLAN identifier is an identifier corresponding to a VLAN set at an inter-link connected to its own node or not. That the VLAN identifier of the U-MAC frame is not an identifier corresponding to a VLAN set at the inter-link denotes that the U-MAC frame will not be transferred to the inter-link. Conversely, that the VLAN identifier of the U-MAC frame is an identifier corresponding to a VLAN set at the inter-link denotes that the U-MAC frame can be transferred to the inter-link. Accordingly, the VLAN filtering circuit  1301 , when a VLAN identifier of a U-MAC frame received from the frame conversion circuit  720  is not an identifier corresponding to a VLAN set at the inter-link, abandons the U-MAC frame. On the other hand, when a VLAN identifier of a U-MAC frame received from the frame conversion circuit  720  is an identifier corresponding to a VLAN set at the inter-link, the VLAN filtering circuit  1301  transfers the U-MAC frame to the packet demultiplexing circuit  710 . The VLAN filtering circuit  1301  only needs to store information of a VLAN set at the inter-link connected to its own node in advance. 
     The VLAN filtering circuit  1301  executes the same processing in a case where a U-MAC frame is transferred from the packet demultiplexing circuit  710 . More specifically, the VLAN filtering circuit  1301 , when a VLAN identifier of a U-MAC frame received from the packet demultiplexing circuit  710  is not an identifier corresponding to a VLAN set at the inter-link, abandons the U-MAC frame. On the other hand, when a VLAN identifier of a U-MAC frame received from the packet demultiplexing circuit  710  is an identifier corresponding to a VLAN set at the inter-link, the VLAN filtering circuit  1301  transfers the U-MAC frame to the frame conversion circuit  720 . 
     When a transmission destination U-MAC address of the U-MAC frame is a predetermined address reserved in advance, the VLAN filtering circuit  1301  is allowed to transfer the frame to a subsequent circuit irrespective of a VLAN identifier. When an inter-link failure is notified by the second manner using a KeepAlive signal, for example, the packet demultiplexing circuit  710  transfers a KeepAlive signal received from other ring through the inter-link to the VLAN filtering circuit  1301 . As a transmission destination U-MAC address of a U-MAC frame as a KeepAlive signal, a controlling identifier according to the KeepAlive signal is set. In this case, the VLAN filtering circuit  1301  transfers the U-MAC frame to the frame conversion circuit  720  irrespective of a VLAN identifier. 
     Also in the present exemplary embodiment, the frame conversion circuit  720  and the packet demultiplexing circuit  710  transmit and receive a U-MAC frame to/from each other through the VLAN filtering circuit  1301 . At this time, the U-MAC frame might be abandoned by the VLAN filtering circuit  1301  in some cases as described above. 
     In addition, according to the present exemplary embodiment, when determining a transmission destination RPR-MAC address, the frame conversion circuit  720  searches the learning data base  741  for an RPR-MAC frame corresponding to a combination between a transmission destination U-MAC address of a U-MAC frame and a VLAN identifier to determine a transmission destination RPR-MAC address. Other operation of the frame conversion circuit  720  and the packet demultiplexing  710  is the same as that of the first exemplary embodiment. 
     In the first exemplary embodiment, the RPR inter-link failure information collection circuit  770 , when notified of failure occurrence on an inter-link by the inter-link failure detection circuit  760 , notifies the flash circuit  781  that an inter-link develops a fault (see  FIG. 10 ). In the present exemplary embodiment, since the contact node  1300  fails to comprise a flash circuit, no processing of notification to the flash circuit is executed. Other operation of the RPR inter-link failure information collection circuit  770  is the same as that of the first exemplary embodiment. 
     The inter-link failure detection circuit  760  periodically generates a KeepAlive signal and transmits the same to the packet demultiplexing circuit  710  similarly to the first exemplary embodiment. The inter-link failure detection circuit  760  may include a VLAN identifier according to a VLAN set at an inter-link to which its own node is connected in a KeepAlive signal. The circuit also may include an inter-link identifier of the inter-link to which its own node is connected in a KeepAlive signal. Other operation of the inter-link failure detection circuit  760  is the same as that of the first exemplary embodiment. 
     In addition, operation of the other respective circuits (the forwarding engine  730 , the ring topology information collection circuit  740 , the ring failure information collection circuit  750 , the path determination circuit  780  and the ADM  790 ) is the same as that of the first exemplary embodiment. 
       FIG. 19  is a block diagram showing an example of a structure of the nodes  1500 - 1 ˜ 1500 - 8  (these nodes will be denoted as the node  1500  in the lump in some cases) other than the contact nodes applied to the ring network according to the second exemplary embodiment. The same components as those of the contact node  1500  shown in  FIG. 18  are given the same reference numerals as those in  FIG. 18  to omit their description. Each node  1500  comprises a packet switch  1510 , the frame conversion circuit  720 , the forwarding engine  730 , the ring topology information collection circuit  740 , the ring failure information collection circuit  750 , the RPR inter-link failure information collection circuit  770 , the path determination circuit  780 , the ADM  790 , a VLAN addition/change circuit  1310  and a VLAN addition/change control circuit  1381 . The node  1500  further comprises the learning data base  741  and the forwarding data base  731 . The learning data base  741  and the forwarding data base  731  of the node  1500  are the same as the learning data base  741  and the forwarding data base  731  of the contact node  1300 . 
     More specifically, the node  1500  comprises the packet switch  1510  in place of the packet demultiplexing circuit  710  and eliminates the need of the inter-link failure detection circuit  760 . The node  1500  differs from the contact node  1300  in comprising the VLAN addition/change circuit  1310  and the VLAN addition/change control circuit  1381 . 
     The VLAN addition/change circuit  1310  has a U-MAC frame that the packet switch  1510  receives from a terminal (not shown in  FIG. 19 ) transferred from the packet switch  1510 . Then, the VLAN addition/change circuit  1310  derives a VLAN identifier and adds the same to the U-MAC frame or change a VLAN identifier originally added to the U-MAC frame to the derived VLAN identifier. 
     U-MAC frame output by a terminal may have a VLAN identifier added or no VLAN identifier added. When a VLAN identifier is added to the U-MAC frame output by the terminal, the VLAN addition/change circuit  1310  is allowed to leave the VLAN identifier as it is and newly provide a VLAN identifier addition region to add a derived VLAN identifier to the region (VLAN tag stack). Alternatively, the VLAN addition/change circuit  1310  may change a VLAN identifier originally added to the U-MAC frame to a derived VLAN identifier. When no VLAN identifier is added to a U-MAC frame output by the terminal, the VLAN addition/change circuit  1310  adds a derived VLAN identifier to the U-MAC frame. 
     The VLAN addition/change circuit  1310  transfers the U-MAC frame with the VLAN identifier added (changed) to the frame conversion circuit  720 . 
     Next, description will be made of a method of deriving a VLAN identifier by the VLAN addition/change circuit  1310 . The VLAN addition/change circuit  1310  executes operation of determining one from VLAN identifiers of a VLAN which can transfer a U-MAC frame based on information included in the U-MAC frame to derive a VLAN identifier. Information included in a U-MAC frame is, for example, a transmission destination U-MAC address, a transmission source U-MAC address, a port number or the like and when a VLAN identifier is originally added to a U-MAC frame output by the terminal, the VLAN identifier is also included in the information included in the U-MAC frame. When newly deriving a VLAN identifier, however, it is unnecessary to use all the information included in the U-MAC frame and only a part of it may be used. It is possible, for example, to use all of a transmission destination U-MAC address, a transmission source U-MAC address, a port number and an originally added VLAN identifier or use a part of them. Description will be here made of a case where a transmission destination U-MAC address and a transmission source U-MAC address are used as an example. 
     The VLAN addition/change circuit  1310  determines a VLAN identifier according to a residual obtained by dividing a total value of a transmission destination U-MAC address and a transmission source U-MAC address by the number of assigned VLANs. The number of assigned VLANs is a total value of VLANs set at the respective inter-links developing no fault. Assume, for example, that only the VLAN 1  is set at the inter-link  1105  and only the VLAN 2  is set at the inter-link  1106 . Then, assume that each inter-link develops no fault. The number of assigned VLANs will be accordingly  2 . In this case, the VLAN addition/change circuit  1310  divides a total value of the transmission destination U-MAC address and the transmission source U-MAC address by the number of assigned VLANs “ 2 ” to determine, for example, that the VLAN identifier is the “VLAN 1 ” when the residual is 1 and determine that the VLAN identifier is the “VLAN 2 ” when the residual is 0. On the other hand, when one inter-link (assumed to be the inter-link  1105  here) develops a fault to enable only the inter-link  1106  to be used, the number of assigned VLANs will be 1. In this case, the VLAN addition/change circuit  1310  changes a method of deriving a VLAN identifier such that when dividing a total value of the transmission destination U-MAC address and the transmission source U-MAC address by the number of assigned VLANs “ 1 ” results in having the residual of 0, determination is made that the VLAN identifier is the “VLAN 2 ”. Thus changing the deriving method results in deriving the identifier “VLAN 2 ” of the VLAN set at the inter-link  1106  developing no fault as a VLAN identifier without fail. 
     In addition, VLAN set at each inter-link is not necessarily one. Assume, for example, that five VLANs are set at each of the inter-links  1105  and  1106  shown in  FIG. 16 . Then, the number of assigned VLANs in a normal state will be ten. In this case, the VLAN addition/change circuit  1310  only needs to divide a total value of the transmission destination U-MAC address and the transmission source U-MAC address by the number of assigned VLANs “ 10 ” to determine one VLAN identifier among ten kinds of VLANs according to the residual (any of 0˜9). On the other hand, when one inter-link (assumed to be the inter-link  1105 ) develops a fault to enable only the inter-link  1106  to be used, the number of assigned VLANs will be five. In this case, the VLAN addition/change circuit  1310  divides a total value of the transmission destination U-MAC address and the transmission source U-MAC address by the number of assigned VLANs “ 5 ” to determine one VLAN identifier among five kinds of VLANs set at the inter-link  1106  according to the residual (any of 0˜4). Thus changing the deriving method results in deriving an identifier of a VLAN set at the inter-link  1106  on which no failure occurs as a VLAN identifier without fail. 
     Although shown here is a case where a total value of the transmission destination U-MAC address and the transmission source U-MAC address is used, other information may be used. A total value of a transmission destination U-MAC address, a transmission source U-MAC address and a port number, for example, may be used. Alternatively, a value of a VLAN identifier originally added to a U-MAC frame may be divided by the number of assigned VLANs. 
     The VLAN addition/change circuit  1310  has information of a VLAN identifier of a VLAN set at an inter-link developing no fault transferred from the VLAN addition/change control circuit  1381 . The VLAN addition/change control circuit  1381 , when the inter-link develops a fault, notifies the VLAN addition/change circuit  1310  of the information of the VLAN identifier of the VLAN set at the inter-link developing no fault. The VLAN addition/change circuit  1310  changes the VLAN identifier deriving method according to the notification to derive only the VLAN identifier of the VLAN set at the inter-link developing no fault. 
     In addition, the VLAN addition/change circuit  1310  needs not to add (or change) a VLAN identifier to all the U-MAC frames transferred from the packet switch  1510 . For example, when searching the learning data base  741  for an RPR-MAC address corresponding to a transmission destination U-MAC frame finds that the RPR-MAC address is not an RPR-MAC address of a contact node, the VLAN addition/change circuit  1310  needs no addition (or change) of a VLAN identifier. This is because when an RPR-MAC address corresponding to a transmission destination U-MAC frame is not an RPR-MAC address of a contact node, the U-MAC frame will not be transferred to other ring through the inter-link. 
     When the ADM  790  receives an RPR-MAC frame transferred within the ring and the frame conversion circuit  720  converts the RPR-MAC frame into a U-MAC frame, the VLAN addition/conversion circuit  1310  has the U-MAC frame transferred from the frame conversion circuit  720 . The U-MAC frame is a frame with a VLAN identifier added (changed) when transferred to a node in the ring from the terminal and further converted into an RPR-MAC frame and transferred within the ring network system. The VLAN addition/change circuit  1310  restores the U-MAC frame transferred from the frame conversion circuit  720  to a state of being first output from the terminal. In a case of an exemplary embodiment in which a VLAN identifier is added when a frame is transferred from the terminal to a node in the ring, the VLAN addition/change circuit  1310  only needs to remove an added VLAN identifier from a U-MAC frame transferred from the frame conversion circuit  720 . As a result, the U-MAC frame returns to a state of being first output from the terminal. In a case of an exemplary embodiment in which a VLAN identifier is changed when a frame is transferred from the terminal to a node in the ring, the VLAN addition/change circuit  1310  only needs to change a VLAN identifier of a U-MAC frame transferred from the frame conversion circuit  720  to restore the same to an original VLAN identifier. The VLAN addition/change circuit  1310  only needs to specify an original VLAN identifier to change the identifier to the original VLAN identifier based on, for example, a transmission source U-MAC address, a transmission destination U-MAC address, an IP address included in a U-MAC frame and the like. 
     After restoring the U-MAC frame transferred from the frame conversion circuit  720  to a state of being first output from the terminal, the VLAN addition/change circuit  1310  transfers the obtained frame to the packet switch  110 . 
     When an inter-link develops a fault, the VLAN addition/change control circuit  1381  is notified of which inter-link develops a fault by the RPR inter-link failure information collection circuit  770 . Based on the notification, the VLAN addition/change control circuit  1381  finds a VLAN identifier of a usable VLAN to output information of the VLAN identifier to the VLAN addition/change circuit  1310 . As a result, the VLAN addition/change circuit  1310  changes the VLAN identifier deriving method as shown in the above-described example such that only a VLAN identifier of a usable VLAN can be derived. 
     The VLAN addition/change control circuit  1381  only needs to store, for example, information about each inter-link in an initial state (a state where no failure occurs) and about a VLAN identifier of a VLAN set at each inter-link. Then, the VLAN addition/change control circuit  1381  first only needs to notify the VLAN addition/change circuit  1310  of all the information of the VLAN identifiers. Then, when notified of which inter-link develops a fault from the RPR inter-link failure information collection circuit  770 , the circuit only needs to notify the VLAN addition/change circuit  1310  of a VLAN identifier of a VLAN set at an inter-link on which no failure occurs. In addition, the VLAN addition/change control circuit  1381  may be notified of information about a VLAN identifier of a VLAN set at an inter-link developing a fault by the RPR inter-link failure information collection circuit  770 . In this case, the VLAN addition/change control circuit  1381  only needs to notify the VLAN addition/change circuit  1310  of other VLAN identifier than the VLAN identifier notified from the RPR inter-link failure information collection circuit  770 . 
     The packet switch  1510 , which is a UNI (User Network Interface), transmits and receives a U-MAC frame to/from a terminal through UNI ports  1501  and  1502 . Upon receiving a U-MAC frame through the respective UNI ports  1501  and  1502 , the packet switch  1510  concentrates U-MAC frames from the UNI ports  1501  and  1502  and transfers the same to the VLAN addition/change circuit  1310 . When a U-MAC frame is transferred from the VLAN addition/change circuit  1310 , the packet switch  1510  outputs the U-MAC frame through an appropriate UNI port (a UNI port connected to a terminal as a transmission destination of the U-MAC frame). 
     The frame conversion circuit  720  of the node  1500  transmits and receives a U-MAC frame to/from the VLAN addition/change circuit  1310 . Other operation of the frame conversion circuit  720  is the same as the operation of the frame conversion circuit of the contact node  1300 . 
     The node  1500  fails to comprise such inter-link failure detection circuit  760  as shown in  FIG. 18 . The RPR inter-link failure information collection circuit  770  of the node  1500  refrains from executing operation of generating an inter-link failure notification RPR-MAC frame including failure information upon reception of a notification that an inter-link develops a fault and transferring the same to the ADM  790 . In addition, upon detecting a failure on an inter-link based on an inter-link failure notification RPR-MAC frame transferred from the ADM  790 , the RPR inter-link failure information collection circuit  770  of the node  1500  notifies the VLAN addition/change control circuit  1381  of information of the inter-link (or information of a VLAN identifier of a VLAN set at the inter-link). Other operation of the RPR inter-link failure information collection circuit  770  is the same as that of the RPR inter-link failure information collection circuit of the contact node  1300 . 
     By using a node other than the contact node shown in  FIG. 18  and the contact node shown in  FIG. 19 , the already described operation of the ring network system according to the second exemplary embodiment can be realized. 
     In a case of detecting occurrence of an inter-link failure by using a KeepAlive signal to notify the failure occurrence, detection and notification may be made by the first manner as described in the first exemplary embodiment or made by the second manner. In any case, operation of the components of a contact node and a node other than a contact node is the same as the operation described in the first exemplary embodiment. As is already described, application of the second manner enables inter-link failure occurrence to be detected at a high-speed. Application of the first manner enables node operation to be simplified. 
     Also according to the second exemplary embodiment, when the RPR inter-link failure information collection circuit  770  notifies the VLAN addition/change control circuit  1381  of information about an inter-link on which a failure occurs, the VLAN addition/change control circuit  1381  notifies the VLAN addition/change circuit  1310  of information of a VLAN identifier of a usable VLAN. Then, the VLAN addition/change circuit  1310  changes the method of deriving a VLAN identifier to be added (or changed) to a U-MAC frame to derive only a VLAN identifier of a usable VLAN. As a result, search of an RPR-MAC address corresponding to a combination between a transmission source U-MAC address and a VLAN identifier fails to speed up timing at which such broadcast-transmission as shown in  FIG. 16  can be executed. It is accordingly possible to recover from a failure on an inter-link at a high speed. 
     The present exemplary embodiment has no discrimination between living and spare inter-links. Then, even when a failure is yet to occur, each contact node sends a U-MAC frame having a VLAN identifier of a VLAN set at an inter-link to an inter-link to which its own node is connected. Accordingly, even when a failure is yet to occur, each node is allowed to transfer a U-MAC frame to other ring through each inter-link to distribute loads on inter-links. 
     In addition, since each node comprises the ring failure information collection circuit  750  and the path determination circuit  780 , even when a failure occurs on a link between nodes of the ring, the failure can be recovered. 
     Moreover, even when rings are connected to each other through other communication network, transmission and reception of a KeepAlive signal to/from contact nodes with each other enables detection of a failure on a path connecting the rings to recover from the failure. In other words, the contact nodes  1300 - 1  and  1300 - 3  may be connected not through the inter-link  1105  but through other communication networks. Similarly, the contact nodes  1300 - 2  and  1300 - 4  may be connected not through the inter-link  1106  but through other communication networks. 
     When the contact nodes are connected not through an inter-link but through other communication network, the inter-link failure detection circuit  760  of each contact node adds information of a contact node as a transmission destination of a KeepAlive signal to the KeepAlive signal as information which can be recognized by the communication network. Assume, for example, that contact nodes are connected through an Ethernet (registered trademark) network, for example. In this case, the inter-link failure detection circuit  760  only needs to designate a MAC address of the transmission destination contact node in the KeepAlive signal as information of the contact node as the transmission destination of the KeepAlive signal. 
     While with reference to  FIG. 18 , the description has been made of the structure in which the contact node  1300  comprises each circuit, the contact node can be structured to comprise a computer, which computer operates in the same manner as that of the packet demultiplexing circuit  710 , the frame conversion circuit  720 , the forwarding engine  730 , the ring topology information collection circuit  740 , the ring failure information collection circuit  750 , the inter-link failure detection circuit  760 , the RPR inter-link failure information collection circuit  770 , the path determination circuit  780 , the ADM  790  and the VLAN filtering circuit  1301  according to a program. The program only needs to be stored in advance in a storage device that the contact node  1300  comprises. 
     Similarly, also the node  1500  other than the contact node can be structured to comprise a computer, which computer operates in the same manner as that of the packet switch  1510 , the frame conversion circuit  720 , the forwarding engine  730 , the ring topology information collection circuit  740 , the ring failure information collection circuit  750 , the RPR inter-link failure information collection circuit  770 , the path determination circuit  780 , the ADM  790 , the VLAN addition/change circuit  1310  and the VLAN addition/change control circuit  1381  according to a program. The program only needs to be stored in advance in a storage device that the contact node  1500  comprises. 
     In the second exemplary embodiment, an inter-link or a communication network connecting the rings to each other is equivalent to a communication path recited in the claims. VLAN identifier is equivalent to a network identifier. The VLAN addition/change circuit  1310  is equivalent to the network identifier deriving unit and the network identifier applying unit. The RPR inter-link failure information collection circuit  770  is equivalent to the failure occurrence determination unit of a node other than a contact node. The VLAN addition/change control circuit  1381  is equivalent to the failure occurring communication path notification unit. The frame conversion circuit  720 , the forwarding engine  730  and the ADM  790  are equivalent to the broadcast unit. The VLAN filtering circuit  1301  and the packet demultiplexing circuit  710  are equivalent to the user frame sending unit. KeepAlive signal is equivalent to an existence confirmation signal. The inter-link failure detection unit  760  and the packet demultiplexing circuit  710  are equivalent to the existence confirmation signal transmission unit. The packet demultiplexing circuit  710  is equivalent to the existence confirmation signal reception unit. The frame conversion circuit  720 , the forwarding engine  730  and the ADM  790  are equivalent to the existence confirmation frame transmission unit. 
     According to the exemplary embodiment of the invention, each node comprises an erasure unit and the erasure unit erases information stored by a learning data base when a failure occurrence determination unit determines that a failure occurs on a living communication path. Then, each node other than a contact node comprises a broadcast unit, and when an address of a node corresponding to a transmission destination address which a user frame received from a terminal outside a ring network has is not stored in the learning data base, the broadcast unit broadcast-transmits a frame with the user frame accommodated as a payload. Accordingly, when a failure occurs on a living communication path, the information stored by the learning data base is erased and as a result, the broadcast unit immediately broadcast-transmits a frame with a user frame accommodated as a payload. Therefore, a failure can be recovered at a high speed when a failure occurs between ring networks. 
     In addition, in a case where when the failure occurrence determination unit determines that a failure occurs on a living communication path, among information stored in the learning data base, the erasure unit erases only an address of a contact node arranged as an end portion of the living communication path and an address of a terminal corresponding to the address, an address of a node other than the contact node and an address of a terminal corresponding to the address are left in the learning data base. Therefore, unless a contact node exists on a path to a terminal as a transmission destination of a user frame, the broadcast unit executes no broadcast transmission. Accordingly, occasions of broadcast transmission are reduced to suppress an increase in the volume of traffic, thereby realizing effective use of a band. 
     In a case where the contact node arranged as an end portion of the living communication path comprises an existence confirmation signal transmission unit for periodically transmitting an existence confirmation signal to other ring network through the living communication path and an existence confirmation signal reception unit for receiving an existence confirmation signal from the other ring network, determination can be made based on the existence confirmation signal whether in a ring to which the contact node having received the existence confirmation signal belongs, there occurs a failure on a communication path. Then, the existence confirmation signal can reach other ring even when the communication path includes a communication network. It is accordingly possible to detect failure occurrence on a communication path even when the communication path includes a communication network. 
     In a case of a structure in which when the failure occurrence determination unit that other node than a contact node comprises among nodes in the ring network fails to receive a frame with an existence confirmation signal accommodated as a payload for a fixed time period, the unit determines that a living communication path develops a fault, each node is individually allowed to detect failure occurrence on a communication path. In this case, as compared with a case where only a contact node determines that a failure occurs on a communication path and after the determination, transfers the determination result to other node, each node is allowed to detect failure occurrence more quickly. Accordingly, it is possible to contribute to high-speed failure recovery. 
     In addition, according to the exemplary embodiment of the invention, a network identifier deriving unit of each node other than a contact node derives a network identifier to be applied to a user frame which is received from a terminal outside the ring network from among network identifiers corresponding to communication paths developing no fault and a network identifier applying unit applies the network identifier to the user frame. Then, when the failure occurrence determination unit determines that a failure occurs on a communication path, a failure occurring communication path notification unit notifies the network identifier deriving unit of information which can specify a network identifier corresponding to the communication path developing the fault. Accordingly, a different network identifier will be derived in a case where a communication path develops a fault and otherwise. When no address of a node corresponding to a combination between a transmission destination address that a user frame has which is received from a terminal outside the ring network and a network identifier derived by the network identifier deriving unit is stored in the learning data base, the broadcast unit broadcast-transmits a frame with a user frame to which the network identifier is applied accommodated as a payload. Accordingly, when a failure occurs to derive a network identifier different from that derived in a normal state, the broadcast unit immediately executes broadcast-transmission. As a result, high-speed failure recovery is possible when a failure occurs between ring networks. 
     A user frame sending unit that each contact node comprises, when a network identifier corresponding to a communication path with itself as an end portion and a network identifier applied to a user frame coincide with each other, sends out the user frame to the communication path. Accordingly, when the network identifiers coincide with each other, the user frame can be sent through each communication path to distribute loads on communication paths. 
     In addition, in a case of a structure in which each contact node arranged as an end portion of each communication path comprises the existence confirmation signal transmission unit for periodically transmitting an existence confirmation signal to other ring network through a communication path with itself as an end portion and the existence confirmation signal reception unit for receiving an existence confirmation signal from the other ring network, it is possible to determine whether a failure occurs on a communication path within a ring to which a contact node having received the existence confirmation signal belongs based on the existence confirmation signal. Then, the existence confirmation signal is allowed to reach other ring even when the communication path includes a communication network. Therefore, even in a case where a communication path includes a communication network, failure occurrence on the communication path can be detected. 
     In addition, in a case where the failure occurrence determination unit that other node than a contact node comprises is structured to determine, when failing to receive a frame with an existence confirmation signal accommodated as a payload for a fixed time period, that a failure occurs on a communication path with a contact node as a transmission source of the frame as an end portion, each node is individually allowed to detect failure occurrence on a communication path. In this case, each node can detect failure occurrence earlier than a case where only a contact node determines that a communication path develops a fault and after the determination, transfers the determination result to other node. It is accordingly possible to contribute to high-speed failure recovery. 
     An exemplary advantage according to the invention is that enables high-speed failure recovery when a failure occurs between ring networks. Another exemplary advantage according to the invention is that suppresses an increase in the volume of traffic to enable efficient use of a band. Another exemplary advantage according to the invention is that enables detection of failure occurrence on a communication path even when the communication path includes the communication network. Another exemplary advantage according to the invention is that enables failure occurrence to be detected at a high-speed to contribute to high-speed failure recovery. Another exemplary advantage according to the invention is that enables high-speed failure recovery when a failure occurs between ring networks. Load on a communication path can be also scattered. 
     According to an exemplary aspect of the invention, each node includes the learning data base for storing a correspondence relationship between an address of a terminal outside a ring network and an address of a node inside the ring network, the failure occurrence determination unit for determining that a failure occurs on a living communication path, and the erasure unit for erasing information stored in the learning data base when the failure occurrence determination unit determines that a living communication path develops a fault, and each node other than a contact node arranged as an end portion of a living communication path and a spare communication path includes the broadcast unit for, when no address of a node corresponding to a transmission destination address that a user frame received from a terminal outside the ring network has is stored in the learning data base, broadcast-transmitting a frame accommodating the user frame as a payload, so that failure occurrence between ring networks can be recovered at a high speed. 
     Moreover, each node other than a contact node includes the network identifier deriving unit for deriving a network identifier to be applied to a user frame received from a terminal outside the ring network from among network identifiers corresponding to communication paths developing no fault, the network identifier applying unit for applying a network identifier to a user frame, the failure occurrence determination unit for determining that a communication path develops a fault, the failure occurring communication path notification unit for, when the failure occurrence determination unit determines that a failure occurs on a communication path, notifying the network identifier deriving unit of information by which a network identifier corresponding to the communication path developing the fault can be specified, the learning data base for storing a correspondence relationship among an address of a terminal outside the ring network, a network identifier and an address of a node inside the ring network, and the broadcast unit for, when no address of a node corresponding to a combination between a transmission destination address that a user frame received from a terminal outside the ring network has and a network identifier derived by the network identifier deriving unit is stored in the learning data base, broadcast-transmitting a frame with a user frame to which the network identifier is applied accommodated as a payload, so that a failure occurring between the ring networks can be recovered at a high speed. 
     Furthermore, a contact node includes the existence confirmation signal transmission unit for periodically transmitting an existence confirmation signal to other ring network through a communication circuit, the existence confirmation signal reception unit for receiving an existence confirmation signal from the other ring network, and an existence confirmation frame transmission unit for transmitting a frame with an existence confirmation signal received by the existence confirmation signal reception unit accommodated as a payload within a ring network to which the contact node belongs, and other node than a contact node among nodes in the ring network includes the failure occurrence determination unit for, when failing to receive a frame with an existence confirmation signal accommodated as a payload for a fixed time, determining that a communication path develops a fault, so that detection of failure occurrence can be made at a high speed to contribute to high-speed failure recovery. 
     While the invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be made therein without departing from the spirit and scope of the present invention as defined by the claims. 
     INCORPORATION BY REFERENCE 
     This application is based upon and claims the benefit of priority from Japanese patent application No. 2005-101254, filed on Mar. 31, 2005, the disclosure of which is incorporated herein in its entirety by reference.