Patent Publication Number: US-2023155924-A1

Title: Route switching method, transfer device, and communication system

Description:
TECHNICAL FIELD 
     The present disclosure relates to a route switching method, a transfer device, and a communication system in a ring network. 
     BACKGROUND ART 
     There is a communication system in which a communication route is formed in a ring shape and the communication route is made redundant by setting a blocked port in a transfer device (see, for example, PTL 1 and NPL 1). 
     CITATION LIST 
     Patent Literature 
     [PTL 1] Japanese Patent Application Publication No. 2009-189070 
     Non Patent Literature 
     [NPL 1] JT-G8032 Ethernet Ring Protection Switching, Established Feb. 23, 2012 
     SUMMARY OF THE INVENTION 
     Technical Problem 
       FIG.  1    is a figure illustrating route switching work in a conventional ring network. In the conventional ring network, when a failure occurs in the ring network, the following operation is performed. It should be noted that a transporting device may be referred to as a “node” in this specification. 
       FIG.  1 (A)  is a figure illustrating a packet transfer route before route switching. 
       FIG.  1 (B)  is a figure illustrating the case in which a failure occurs in a link between nodes A and B. The nodes A and B detect the failure of the link. 
       FIG.  1 (C)  is a figure illustrating the operations of the nodes A and B after the detection of the failure. The nodes A and B block ports a 2  and b 1  connected to the failed link and transmit control packets for route switching from ports on the opposite side of the failed link. 
       FIG.  1 (D)  is a figure illustrating a packet transfer route after the route switching. After receiving the control packets, a node E releases the blocked ports (ring protection link end points) and performs switching the route of the packet from the side of a node F to the side of a node D. 
     The route switching work as illustrated in  FIG.  1    takes time. That is, the conventional ring network has a problem in that, if a failure occurs in the ring network, communication cannot be easily continued during the process from the occurrence (detection) of the failure in  FIG.  1 (B)  to the completion of the route switching in  FIG.  1 (D) . In addition, even if route switching in the ring network is performed in a planned manner, there is a problem in that communication cannot be easily continued during the process from the route switching processing in  FIG.  1 (C)  to the completion of the route switching in  FIG.  1 (D) . 
     Accordingly, the present invention addresses the problems described above with an object of providing a route switching method, a transfer device, and a communication system that can continue communication even during route switching work. 
     Means for Solving the Problem 
     In order to achieve the object described above, the route switching method according to the present invention causes a failure detection node to make a bypass determination of a normal packet in the ring when a failure occurs in the ring network so as to temporarily bypass the packet in parallel with the route switching processing. 
     Specifically, the route switching method according to the present invention is a route switching method in a ring network, the method including: detecting a non-transferable route through which packet transfer is disabled in the ring network; performing route switching work that changes a position of a blocked port set in a transfer device in the ring network and performs switching to a route that avoids the non-transferable route; and performing bypass transfer that transfers a packet while bypassing the non-transferable route during the route switching work, in which, in the bypass transfer, the transfer device having detected the non-transferable route attaches a bypass packet flag to a packet that passes through the non-transferable route and specifies the packet as a bypass packet, the transfer device having detected the non-transferable route returns the bypass packet and transfers the bypass packet in a direction opposite to that of the packet, and the transfer device for which the blocked port is set in the ring network transfers the bypass packet from the blocked port before the route switching work. 
     It should be noted that the blocked port is released after the route switching work, so the packet is transferred through a route that avoids the non-transferable route as a normal packet without being given the bypass packet flag. 
     In addition, a transfer device according to the present invention for achieving the route switching method is a transfer device included in a ring network, the transfer device, including: a detection unit that detects a non-transferable route through which packet transfer is disabled in the ring network; and a transfer control unit that performs route switching work for setting or releasing a blocked port by communicating with another transfer device in the ring network and performs switching a route of packet to a route that avoids the non-transferable route, in which the transfer control unit has a packet processing function that attaches a bypass packet flag to a packet that passes through the non-transferable route and specifies the packet as a bypass packet when the non-transferable route is detected, a turning function that turns the bypass packet in a direction opposite to that of the packet in the ring network, and a blocked port transfer function that transfers the bypass packet from the blocked port if the blocked port is set when the bypass packet is received before the route switching work. 
       FIG.  2    is a figure illustrating the route switching method. 
       FIG.  2 (A)  is a figure illustrating a packet transfer route before route switching. 
       FIG.  2 ( b )  is a figure illustrating the case in which a failure occurs in the link between the nodes A and B. The node A detects a failure of the link. 
       FIG.  2 (C)  is a figure illustrating the operations of the nodes A and B after detection of the failure. The nodes A and B block ports a 2  and b 1  connected to the failed link and transmit control packets for route switching from ports on the opposite side of the failed link. Furthermore, the node A returns the packet transferred from a node F as a bypass packet. A node E transfers the bypass packet from the blocked port toward a node D. The transferred bypass packet is restored to the original packet at the node B and output to the outside of the ring network. 
       FIG.  2 (D)  is a figure illustrating the packet transfer route after the route switching. After receiving the control packets, the node E releases the blocked ports (ring protection link end points) and switches the route of packet from the side of the node F to the side of the node D. Since the packet does not reach the node A at this time, the node A terminates the turning of the packet. 
     By using the bypass packet, this route switching method can reduce the communication interruption time to the time (which depends on the transfer device) from the occurrence of a failure to the detection of the failure even if the failure occurs in the ring network. In addition, the route switching method can perform route switching without a communication interruption even when route switching in a ring network is performed in a planned manner. Accordingly, the present invention can provide the route switching method and the transfer device that can continue communication even during route switching work. 
     The route switching method attaches a blocked port pass flag that indicates whether to pass through the blocked port to the bypass packet. The route before passing through the blocked port is a turning section and, when the packet is output to the outside of the ring network, double transfer occurs. Accordingly, double transfer can be prevented by causing the packet to indicate “before passing through the blocked port” and “after passing through the blocked port”. 
     In this route switching method, the transfer device that transfers the packet to the outside of the ring network determines whether the packet is identical to a past packet and, when the packet is identical to the past packet, discards the packet. 
     When the packet is a multicast packet, both the normal packet and the bypass packet arrive depending on the node. Accordingly, double transfer can be prevented by checking the identity between the normal packet and the bypass packet and discarding one of these packets. 
     The communication system according to the present invention is a communication system for a ring network that includes the transfer device described above. Since this communication system includes the transfer device described above, the communication system can achieve the route switching method described above. Accordingly, the present invention can provide the communication system that can continue communication even during route switching work. 
     It should be noted that the inventions described above can be combined as much as possible. 
     Effects of the Invention 
     The present invention can provide the route switching method, the transfer device, and the communication system that can continue communication even during route switching work. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG.  1    is a figure for describing a route switching method regarding the present invention. 
         FIG.  2    is a figure for describing a route switching method according to the present invention. 
         FIG.  3    is a figure for describing a ring network. 
         FIG.  4    is a figure for describing the effect of a blocked port pass flag of the route switching method according to the present invention. 
         FIG.  5    is a figure for describing a transfer device according to the present invention. 
         FIG.  6    is a figure for describing the route switching method according to the present invention. 
         FIG.  7    is a figure for describing the route switching method according to the present invention. 
         FIG.  8    is a figure for describing the route switching method according to the present invention. 
         FIG.  9    is a table summarizing the operations of the transfer device according to the present invention. 
         FIG.  10    is a figure for describing a problem of the present invention. 
         FIG.  11    is a figure for describing the transfer device according to the present invention. 
         FIG.  12    is a figure for describing the route switching method according to the present invention. 
         FIG.  13    is a figure for describing the route switching method according to the present invention. 
         FIG.  14    is a figure for describing the transfer device according to the present invention. 
         FIG.  15    is a figure for describing the route switching method according to the present invention. 
         FIG.  16    is a figure for describing the route switching method according to the present invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of the present invention will be described with reference to the attached drawings. The embodiments described below are examples of the present invention and the present invention is not limited to the following embodiments. It should be noted that components having the same reference numeral in this specification and the drawings are assumed to be identical to each other. 
       FIG.  3    is a figure for describing a ring network. One ring network includes a plurality of transfer devices  11  connected in a ring shape. For example, transfer devices ( 11 - 1  to  11 - 4 ) constitute one ring network R 1 . In addition, the transfer devices may be shared with another ring network. For example, the transfer device  11 - 3  is shared between the ring network R 1  and a ring network R 3 . In addition, the transfer devices are also connected to a network outside the ring networks. For example, a transfer device  11 - 8  is also connected to an external network NW 1 . In the structure example illustrated in  FIG.  3   , the packet from the external network NW 1  enters the ring network R 3  via the transfer device  11 - 8 , transferred to the ring network R 1 , the ring network R 2 , and the ring network R 4  in this order, and output to an external network NW 2  through a transfer device  11 - 11 . 
     Embodiment 1 
     In the embodiment, description is given focusing on one ring network.  FIG.  2    is a figure for describing the route switching method in one ring network that the embodiment focuses on. 
     The route switching method include: detecting a non-transferable route (link directly connecting the node A and node B to each other) through which packet transfer is disabled in the ring network ( FIG.  2 (B) ); performing route switching work that changes (from a port e 1  to ports (a 2  and b 1 )) the position of a blocked port set in a transfer device in the ring network and performs switching to a route that avoids the non-transferable route; and performing bypass transfer that transfers a packet while bypassing the non-transferable route during the route switching work ( FIG.  2 (C) ). 
     It should be noted that the non-transferable route may be caused by a failure or a planned route switching work. 
     In the bypass transfer in  FIG.  2 (C) , the transfer device (node A) having detected the non-transferable route attaches a bypass packet flag to the packet that passes through the non-transferable route and specifies the packet as a bypass packet, the transfer device having detected the non-transferable route returns the bypass packet and transfers the bypass packet in a direction opposite to that of the packet in the ring network, and the transfer device for which the blocked port e 1  is set in the ring network transfers the bypass packet through the blocked port e 1  before the route switching work. 
     It should be noted that, since the blocking of the port e 1  is released as illustrated in  FIG.  2 (D)  after the route switching work, the packet is transferred as a normal packet in a route (the order of E, D, C, and B) that avoids the non-transferable route without being given a bypass packet flag. 
     When transfer by the transfer device is flooding, a blocked port pass flag indicating whether to pass through the blocked port is preferably attached to the bypass packet. FIG.  4  is a figure for describing the effect of the blocked port pass flag.  FIG.  4 (A)  is a figure illustrating the transfer of the bypass packet when the blocked port pass flag is not present and  FIG.  4 (B)  is a figure illustrating the transfer of the bypass packet when the blocked port pass flag is present. 
     First, the case in which the blocked port pass flag in  FIG.  4 (A)  is not present will be described. It is assumed that a packet is input to the node F from a terminal  1 . The node F transfers the packet to the node A and the node E. The node A transfers the packet to the outside of the ring and turns the packet (transfers the packet to the node F) as a bypass packet because a port a 2  is blocked. The node F transfers the bypass packet to the node E and also outputs the bypass packet to the terminal  1 , which is the transmission source (symbol g 1 ). Furthermore, the node E transfers both the packet transferred from the node F and the bypass packet to the outside of the ring (symbol g 2 ). That is, double transfer occurs at the node E. 
     In addition, the node E transfers the bypass packet from the blocked port e 1  to the node D. The bypass packet is transferred to the node D, the node C, and the node B in this order, restored to the original packet in these nodes, and transferred to the outside of the ring. In other words, in an area Ar 2  (area including the nodes B, C, and D) in which the bypass packet exceeds the blocked port e 1 , the packet from terminal  1  can be output even if the link between the node A and the node B fails, and the terminal  2  can receive the packet. 
     In contrast, in an area An (area including the nodes A, F, and E) in which the bypass packet does not exceed the blocked port e 1 , double transfer of the packet occurs as indicated by the symbols g 1  and g 2 . 
     The blocked port pass flag is used to prevent this double transfer of the packet in the embodiment. The individual nodes determine whether the bypass packet is output to the outside of the ring by checking the value of the blocked port pass flag. For example, the individual nodes determine that the bypass packet is not output to the outside of the ring when the blocked port pass flag is “0” or the bypass packet is output to the outside of the ring when the blocked port pass flag is “1”. 
     A specific example will be described with reference to  FIG.  4 (B) . As in the case in  FIG.  4 (A) , the node A receives the packet from the terminal  1  transferred by the node F. Then, the node A transfers the packet to the outside of the ring and returns the packet (transfers the packet to the node F) as the bypass packet because the port a 2  is blocked. At this time, the node A attaches the blocked port pass flag “0” to the bypass packet. The nodes F and E do not output the bypass packet to the outside of the ring because the blocked port pass flag of the packet is “0”. Accordingly, double transfer of the packet can be prevented in the area Ar 1 . 
     In addition, the node E changes the blocked port pass flag from “0” to “1” when transferring the bypass packet from the blocked port e 1 . Accordingly, the nodes (B, C, and D) in the area Ar 2  can output the bypass packet and the terminal  2  can receive the packet as in the case in  FIG.  4 (A) . 
     Then, the transfer device  11  that can achieve the route switching method in the ring network described above will be described.  FIG.  5    is a functional block diagram illustrating the transfer device  11 . The transfer device  11  is a transfer device included in the ring network and includes: a detection unit  15  that detects a non-transferable route through which packet transfer is disabled in the ring network; and a transfer control unit  16  that performs route switching work for setting or releasing a blocked port by communicating with another transfer device in the ring network and performs switching a route of packet to a route that avoids the non-transferable route, in which the transfer control unit  16  has a packet processing function that attaches a bypass packet flag to the packet that passes through the non-transferable route and specifies the packet as a bypass packet when the non-transferable route is detected, a turning function that turns the bypass packet in a direction opposite to that of the packet in the ring network, and a blocked port transfer function that transfers the bypass packet from the blocked port if the blocked port is set when the bypass packet is received before the route switching work. 
     It should be noted that the packet processing function preferably attaches the blocked port pass flag indicating whether to pass through the blocked port to the bypass packet. 
     The transfer device  11  will be described in more detail. The transfer device  11  includes specific ring ports ( 21 - 1  and  21 - 2 ), a non-specific ring port  22 , a packet transfer processing unit  23 , a normal packet turn processing unit  24 , a bypass packet flag attachment processing unit  25 , blocked port pass flag change processing units ( 26 - 1  and  26 - 2 ), a blocked port pass flag control processing unit  27 , and a bypass packet flag deletion processing unit  28 . 
     The specific ring ports ( 21 - 1  and  21 - 2 ) are ports constituting the ring network and transmit and receive packets. The non-specific ring port  22  is the port other than the specific ring ports ( 21 - 1  and  21 - 2 ) and sends and receives the packet to and from the outside of the ring network. The packet transfer processing unit  23  performs the transfer processing of packets. When the detection unit  15  detects a failure in the ring, the normal packet turn processing unit  24  performs turn processing of packets within the transfer device by using failure information and network information held by the transfer device  11 . It is assumed that, for example, when a packet to be transferred from the specific ring port  21 - 2  to the specific ring port  21 - 1  or from the non-specific ring port  22  to the specific ring port  21 - 1  arrives, the link of the specific ring port  21 - 1  fails and the packet cannot be transferred. The normal packet turn processing unit  24  changes the header of the packet so that the packet is transferred in the opposite direction (output from the specific ring port  21 - 2 ) using the retained information. 
     The bypass packet flag attachment processing unit  25  attaches the bypass packet flag to the packet subjected to the turn processing by the normal packet turn processing unit  24  and converts the packet to an emergency bypass packet that is concluded within the ring. It should be noted that the bypass packet flag attachment processing unit  25  preferably attaches the blocked port pass flag (for example, “0”) too when the attaching the bypass packet flag to the bypass packet. 
     Here, it is assumed that the specific ring port  21 - 2  of the transfer device  11  is blocked. When the specific ring port  21 - 1  receives a bypass packet from the outside, the packet transfer processing unit  23  outputs the packet from the blocked specific ring port  21 - 2  by using the bypass packet information, and the failure information and the network information held by the transfer device. At this time, the blocked port pass flag change processing unit  26 - 2  changes (changes the blocked port pass flag from “0” to “1”) the flag indicating that the bypass packet has passed the blocked port when the bypass packet is output from the blocked port. 
     The blocked port pass flag control processing unit  27  determines the transfer and disposal of the bypass packet based on the blocked port pass flag of the bypass packet to be output from the non-specific ring port  22 . For example, the blocked port pass flag control processing unit  27  instructs the packet transfer unit  23  to discard the bypass packet to be output from the non-specific ring port  22  because the blocked port pass flag control processing unit  27  allows the bypass packet with a blocked port pass flag of 0 to be output to the specific ring port ( 21 - 1  or  21 - 2 ) and disallows the bypass packet to be output from the non-specific ring port  22 . 
     In contrast, the blocked port pass flag control processing unit  27  allows the bypass packet with a blocked port pass flag of 1 to be output to the specific ring port ( 21 - 1  or  21 - 2 ) and to be output from the non-specific ring port  22 . Accordingly, the bypass packet flag deletion processing unit  28  deletes the bypass packet flag and the blocked port pass flag from the bypass packet to be output from the non-specific ring port  22  and restores the packet format thereof to the normal packet format. The non-specific ring port  22  outputs the packet with the packet format restored to the normal packet format. 
       FIGS.  6  to  8    are a flowchart illustrating the operation of the transfer device  11 . When receiving a packet (step S 01 ), the transfer device  11  reads the information (presence or absence of the bypass packet flag) of the packet (step S 02 ). When the packet is a normal packet (“No” in step S 03 ), the processing for a normal packet in  FIG.  7    is performed. When the packet is a bypass packet (“Yes” in step S 03 ), the processing for the bypass packet in  FIG.  8    is performed. 
     The processing for a normal packet in  FIG.  7    will be described. 
     The packet transfer processing unit  23  determines whether the transmission port that outputs the packet is in a untransmittable state (the transmission destination fails) or ring switching is not performed (ring switching report is not received yet) (step S 11 ). Here, the transmission port includes both the specific ring port  21  and the non-specific ring port  22 . In the case of “Yes” in step S 11 , a check is made as to whether the reception port that has received the packet is the specific ring port  21  (step S 12 ). In the case of “No” in step S 12 , a check is made as to whether the transmission port that outputs the packet is the specific ring port  21  (step S 13 ). In the case of “No” in step S 11  or “No” in step S 13 , packet transfer based on the header of the packet is performed (step S 14 ). 
     In the case of “Yes” in step S 12 , a check is made as to whether the transmission port that outputs the packet and the reception port that has received the packet are the specific ring ports ( 21 - 1  and  21 - 2 ) in the same ring network (step S 15 ). In the case of “No” in step S 15 , step S 14  is executed. In contrast, in the case of “Yes” in step S 13  or “Yes” in step S 15 , a check is made as to whether the blocked port pass flag is attached (step S 16 ). Specifically, a check is made as to whether the specific ring port in the same ring network as own specific ring port or the specific ring port in a untransmittable state in the transfer destination node is a blocked port. For example, in step S 16 , when a failure occurs in the link between the node A and the node B in the state as illustrated in the node A in  FIG.  2   , the specific ring port in a untransmittable state is the port a 2  and the specific ring port in the same ring network is a port a 1 . 
     In the case of “No” in step S 16  (for example, in the case of the node A in  FIG.  2 (B) ), the turn processing is performed. Specifically, the bypass packet flag and the blocked port pass flag (=“0”) are attached to the packet to specify a bypass packet and the packet is transmitted to the specific ring port (port a 1 ) in the same ring network as the specific ring port (port a 2 ) in a untransmittable state (step S 17 ). Step S 17  can prevent the double transfer of the packet to the outside of the ring network. 
     In contrast, in the case of “Yes” in step S 16  (for example, when a failure occurs in the link between the node F and the node E in  FIG.  2   ), the bypass packet flag and the blocked port pass flag (=“1”) are attached to the packet to be transferred toward the node F from the outside of the ring network to specify a bypass packet. Then, the packet is transmitted to the specific ring port (port e 1 ) in the same ring network as the specific ring port (port e 2 ) in a untransmittable state (step S 18 ). In this step, the bypass packet passes through the port e 1  even if the port e 1  is a blocked port. In step S 18 , the packet can be transferred to the outside of the ring network via a bypass route of the bypass packet. 
     It should be noted that the blocked port in step S 16  also includes the port (the ring protection link end point for which blocking has been released, for example, the port e 1  in  FIG.  2 (D) ) that was once blocked. That is, the operation of the processing described above does not change regardless of whether the port which was once the ring protection link end point is in the blocked state (initial state) or in the blocking-released state (after the control packet passes) (the packet is transferred to the adjacent node). 
     Next, the processing of the bypass packet in  FIG.  8    will be described. 
     The packet transfer processing unit  23  checks whether the bypass packet has made one turn in the ring network (step S 21 ). Specifically, a check is made as to whether the node ID of the header portion of the received bypass packet is inconsistent with that of own node. In the case of “Yes” in step S 21 , a check is made as to whether the specific ring port that transmits the bypass packet is enabled (step S 22 ). In the case of “No” in step S 21  or “No” in step S 22 , the bypass packet is discarded due to double failure (step S 23 ). 
     In contrast, in the case of “Yes” in step S 22 , a check is made as to whether the specific ring port transmits the bypass packet and the specific ring port that receives the bypass packet are present in the same ring network (step S 24 ). In the case of “No” in step S 24 , a check is made as to whether the specific ring port that transmits the bypass packet is a blocked port (step S 25 ). In the case of “Yes” in step S 25 , the blocked port pass flag of the bypass packet is changed from “0” to “1” and the bypass packet is transferred from the blocked specific ring port (step S 26 ). In contrast, in the case of “No” in step S 25 , the blocked port pass flag of the bypass packet is not changed and the bypass packet is transferred from the specific ring port that transmits the bypass packet (step S 27 ). 
     In the case of “Yes” in step S 24 , since the packet is transferred to the outside of the ring network, the blocked port pass flag is checked to prevent double transfer (step S 28 ). When the blocked port pass flag is “0” (“No” in step S 28 ), double transfer is assumed, so the bypass packet is discarded (step S 29 ). In contrast, when the blocked port pass flag is “1” (“Yes” in step S 28 ), double transfer is not assumed, so the bypass packet flag is removed and the packet is transferred from the non-specific ring port to the outside of the ring network (step S 30 ). 
     It should be noted that the blocked port in step S 25  also includes the port (the ring protection end point for which blocking has been released, for example, the port e 1  in  FIG.  2 (D) ) that was once blocked. That is, the operation of the processing described above does not change regardless of whether the port which was once the ring protection link end point is in the blocked state (initial state) or in the blocking-released state (after the control packet passes) (the packet is transferred to the adjacent node). 
       FIG.  9    is a table listing the transfer patterns performed by the transfer device  11  described in the flowchart in  FIGS.  6  to  8   . It should be noted that the description “PACKET IS TRANSFERRED AS BEFORE” in the table means that the transfer method specified in NPL 1 is followed. 
     The transfer device  11  reads the received packet and makes a bypass determination in the ring network. The determination depends on the reception packet, the attributes of the transmission and reception ports, and the state of the transmission port as illustrated in the table in  FIG.  9   . Since the transfer device  11  operates as described above, the following effects are obtained. 
     As described above, when a failure occurs in the ring network, the communication interruption time due to route switching can be reduced to the time from the occurrence to the detection of the failure by using the bypass packet. In addition, even when route switching in the ring network is performed in a planned manner, the route switching can be performed without causing a communication interruption. 
     Embodiment 2 
     The case in which a multicast packet is transferred in the ring network will be described in the embodiment.  FIG.  10    is a figure illustrating the problem with the case in which a multicast packet is transferred via a ring network. It is assumed that six nodes A to F are present and a multicast packet is transmitted from the node A. A port e 1  of the node E is blocked at an initial state. In  FIG.  10   , dashed lines represent a multicast packet transferred from the node A, long dashed lines represent a control packet that reports the occurrence or recovery of a failure between nodes, and solid lines represent a bypass packet obtained by returning the multicast packet at the node (node C in  FIG.  10   ) that has detected a failure. 
     It is assumed that a failure has occurred in the link between the node C and the node D. In this case, the packet 0 is not affected by the failure and reaches all the nodes. However, the node C blocks a port c 2  and the node D blocks a port d 1  after the failure is detected by the node C and the node D, so a packet 1 and subsequent packets are returned as bypass packets at the node C. 
     In contrast, since the packets are multicast packets, the packets are also transferred in the opposite direction in the ring network. The packets 0 that turn in the opposite direction are transferred from the node A to the node E, and the transfer of the packet is stopped at the blocked port e 1 . 
     The node C and the node D detect this failure and transmit the control packets in the direction away from the failed link, and the node E releases the blocking of the port e 1  by receiving this control packets. Accordingly, the node D can receive the packets that turn in the opposite direction even after the occurrence of the failure. However, the bypass packets returned by the node C also reach the node D in the opposite direction. That is, although the node D has received the packets  1  and  2 , the node D also receive these bypass packets  1  and  2 , thereby causing packet duplication. 
     It should be noted that the node C stops transmitting the bypass packet after the control packet transmitted by the node D reaches the node C via the nodes E, F, A, and B, so the packet duplication at the node D is resolved (packet duplication does not occur at the packet  3  and subsequent packets). 
     When the multicast packet is transferred via the ring network as illustrated in  FIG.  10   , there is a problem in that packet duplication occurs. Accordingly, the embodiment discloses a route switching method and a transfer device that can avoid packet duplication. 
       FIG.  11    is a figure illustrating a transfer device  11   a  according to the embodiment. In addition to the components of the transfer device  11  described in embodiment 1, the transfer device  11   a  further includes a determination unit that determines whether the packet is identical to a past packet when receiving the packet to be transferred to the outside of the ring network and a discarding unit that discards the packet when the packet is identical to the past packet. Specifically, in addition to the components of the transfer device  11 , the transfer device  11   a  further includes an identity determination information deletion processing unit  31  and an identity determination information giving processing unit  32  as the determination unit and a duplicate packet discard processing unit  29  as the discarding unit. 
     The duplicate packet discard processing unit  29  determines the duplication between the normal packet and the bypass packet and performs transfer or discarding. The identity determination information giving unit  32  gives information for determining the identity of the packets to the normal packet. The identity determination information deletion processing unit  31  deletes the identity determination information of the packet. The duplicate packet discard processing unit  29  compares the packet to be transferred to the outside of the ring network with the packet transferred to the outside of the ring network in the past using information given by the identity determination information giving unit  32 . As a result of the comparison, when the identity determination information is consistent with that of the packet transferred to the outside of the ring network in the past, the duplicate packet discard processing unit  29  determines packet duplication and discards the packet to be transferred to the outside of the ring network. In contrast, when the identity determination information is inconsistent with that of the packet transferred to the outside of the ring network in the past, the duplicate packet discard processing unit  29  does not discard the packet and transfers the packet to the outside of the ring network after the identity determination information deletion processing unit  31  deletes the identity determination information. 
       FIG.  6   ,  FIG.  12   , and  FIG.  13    are a flowchart illustrating the operation of the transfer device  11   a.  A determination as to whether the received packet is a normal packet or a bypass packet is the same as the operation of the transfer device  11  described in  FIG.  6   . 
     The processing of the normal packet in  FIG.  12    will be described. Here, only the difference from the processing in  FIG.  7    will be described. 
     In the case of “No” in step S 13 , the identity determination information of the received packet is compared with that of the past packet (step S 19 ). When the identity determination information of the received packet is different from that of the past packet (“No” in step S 19 ), packet transfer based on the header of the packet is performed (step S 14 ). In contrast, when the identity determination information of the received packet is the same as that of the past packet (“Yes” in step S 19 ), packet duplication has occurred, so the received packet is discarded (step S 20 ). Alternatively, in the case of “Yes” in step S 13  or “Yes” in step S 15 , the identity determination information is added to the normal packet (step S 19   a ) and then step S 16  is executed. 
     The processing of the bypass packet in  FIG.  13    will be described. Here, only the difference from the processing in  FIG.  8    will be described. 
     When the blocked port pass flag is “1” (“Yes” in step S 28 ), the identity determination information of the received bypass packet is compared with that of the past packet (step S 31 ). When the identity determination information of the received bypass packet is consistent with that of the past packet (“Yes” in step S 31 ), the received bypass packet is discarded to prevent packet duplication (step S 32 ). In contrast, when the identity determination information of the received bypass packet is inconsistent with that of the past packet (“No” in step S 31 ), step S 30  is executed. 
     The transfer device  11   a  according to the embodiment adds information for determining the identity of the packet to the normal packet, checks the identity determination information in the transfer devices. Then, when the identity determination information is inconsistent with that of the packet transferred in the past, the transfer device  11   a  deletes the identity determination information and transfer the packet to the outside of the ring network. When the identity determination information is consistent with that of the packet transferred in the past, the transfer device  11   a  discards the packet without transferring the packet to the outside of the ring network. The transfer device  11   a  according to the embodiment can prevent duplication of the same packet caused by delivery of both the normal packet and the bypass packet to the destination. 
     Embodiment 3 
     The case in which a multicast packet is transferred via a ring network will be also described in the embodiment. The problem with the case in which a multicast packet is transferred via the ring network is as illustrated in  FIG.  10   . In the embodiment, a route switching method and a transfer device that avoid packet duplication in a different way from the embodiment 2 will be disclosed. 
       FIG.  14    is a figure illustrating a transfer device  11   b  according to the embodiment. In addition to the components of the transfer device  11  described in embodiment 1, the transfer device  11   b  further includes a duplicate packet discard processing unit  29  that determines whether the packet is identical to the past packet when receiving a packet to be transferred to the outside of the ring network and, if the packet is identical to the past packet, discards the packet. 
     The duplicate packet discard processing unit  29  according to the embodiment also determines the duplication between the normal packet and the bypass packet and then performs transferring or discarding, but is different from the duplicate packet discard processing unit  29  according to the embodiment 2 that makes a determination based on the identity determination information added to the packet. The duplicate packet discard processing unit  29  according to the embodiment performs specific calculation based on packet information and compares the result with the calculation result of the packet transferred to the outside of the ring network in the past. An example of the specific calculation will be indicated. 
     The CRC (cyclic redundancy check) value of the generating polynomial G(x) below is calculated using, for example, an FCS (frame check sequence, four octets). 
         G ( x )= x   32   +x   26   +x   23   +x   22   +x   16   +x   12   +x   11   +x   10   +x   8   +x   7   +x   5     30  x   5   +x   2   +x+ 1 
     As a result of the comparison, when the calculation result is consistent with the packet transferred to the outside of the ring network in the past, the duplicate packet discard processing unit  29  determines packet duplication and discards the packet to be transferred to the outside of the ring network. In contrast, when the calculation result is inconsistent with the packet transferred to the outside of the ring network in the past, the duplicate packet discard processing unit  29  transfers the packet to the outside of the ring network without discarding the packet. 
       FIG.  6   ,  FIG.  15   , and  FIG.  16    are a flowchart illustrating the operation of the transfer device  11   b.  A determination as to whether the received packet is a normal packet or a bypass packet is the same as the operation of the transfer device  11  described in  FIG.  6   . 
     The processing of the normal packet in  FIG.  15    will be described. Here, only the difference from the processing in  FIG.  7    will be described. 
     In the case of “No” in step S 13 , the specific calculation as described above is performed (step S 19   b ). Then, the calculation result of the received packet is compared with the calculation result of the past packet (step S 19   c ). When the calculation result of the received packet is different from the calculation result of the past packet (“No” in step S 19   c ), packet transfer based on the header of the packet is performed (step S 14 ). In contrast, when the calculation result of the received packet is the same as that of the past packet (“Yes” in step S 19   c ), the received packet is discarded because packet duplication occurs (step S 20 ). 
     The processing of the bypass packet in  FIG.  16    will be described. Here, only the difference from the processing in  FIG.  8    will be described. 
     When the blocked port pass flag is “1” (“Yes” in step S 28 ), the specific calculation described above is performed on the received bypass packet (step S 31   b ). Then, the calculation result of the received bypass packet is compared with the calculation result of the past packet (step S 31   c ). When the calculated result of the received bypass packet is consistent with the calculated result of the past packet (“Yes” in step S 31   c ), the received bypass packet is discarded because packet duplication occurs (step S 32 ). In contrast, when the calculated result of the received bypass packet is inconsistent with the calculated result of the past packet (“No” in step S 31   c ), step S 30  is executed. 
     The transfer device  11   b  according to the embodiment performs the specific calculation based on the packet information and compares the calculation result with the calculation result of the packet transferred in the past. The transfer device  11   b  transfers the packet as it is when the calculation result is inconsistent with the calculation result of the packet transferred in the past or discards the packet when these calculation results are consistent with each other. The transfer device  11   b  according to the embodiment can prevent duplication of the same packet caused by delivery of both the normal packet and the bypass packet to the destination. Furthermore, the transfer device  11   b  can prevent duplication without requiring a special header or the like of the packet as compared with the transfer device  11   a  according to the second embodiment. 
     REFERENCE SIGNS LIST 
     
         
           11 ,  11   a,    11   b  Transfer device 
           15  Detection unit 
           16  Transfer control unit 
           21 - 1 ,  21 - 2  Specific ring port 
           22  Non-specific ring port 
           23  Packet transfer processing unit 
           24  Normal packet turn processing unit 
           25  Bypass packet flag attachment processing unit 
           26 - 1 ,  26 - 2  Blocked port pass flag change processing unit 
           27  Blocked port pass flag control processing unit 
           28  Bypass packet flag deletion processing unit 
           29  Duplicate packet discard processing unit 
           31  Identity determination information deletion processing unit 
           32  Identity determination information giving processing unit