Abstract:
Arrangements dealing with multiple link failures occurring in a communication network including a first ring and a second ring connected to the first ring by sharing part of a link in the first ring as a common link. Included is stored link failure information included in the failure detection frame, and where a controller configured to control the communication unit to stop transmission of at least one auxiliary monitor frame in the event of multiple failures including a failure on the common link, but to allow transmission under certain specific circumstances.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a technique of dealing with multiple link failures occurring in a communication network. 
     2. Description of the Related Art 
     A transmission control procedure called a ring protocol is generally adopted in topology of a ring structure (hereafter referred to as ring topology) in an Ethernet (registered trademark) communication network. The ring protocol is a layer 2 network redundant protocol. Multiple devices relaying user frames in a ring (hereafter these devices are referred to as relay devices) are interlinked in the ring structure. At least one of the relay devices is assigned to a device sending a frame (monitor frame) for checking the state of the ring (hereafter this device is referred to as monitor device). This arrangement of the ring protocol continuously monitors the state of the ring and controls ports of the respective devices to detect the occurrence of any failure and immediately change the communication route in response to the detection. The ring protocol enables the prompt change of the communication route in the event of the occurrence of a failure. Another advantage of the ring topology reduces the total number of transmission paths and interfaces, compared with mesh topology of connecting the respective relay devices in a net structure. 
     Since one single ring can not deal with plural failures occurring in the network, the ring protocol generally adopts a structure of using multiple rings interlinked via a linkage (hereafter referred to as common link) to ensure the redundancy (hereafter referred to as multi-ring structure with a common link). 
     In the multi-ring structure with a common link, however, in the event of a failure occurring on the common link, the interlinked rings may cause a large loop (super-loop). Various systems have been proposed to solve this problem in the ring protocol. 
     One proposed system controls a device located at a terminal end of a common link (hereafter this device is referred to as common link terminal device) shared by two interlinked rings to send an auxiliary monitor frame to a monitor device of one ring (see Japanese Patent Laid-Open No. 2004-201009). This assigns the ring to a non-working ring of monitoring the occurrence of a failure on the common link. Even in the event of some failure on the common link, the monitor device receiving the auxiliary monitor frame (hereafter this device is referred to as auxiliary monitor device) keeps one of ports blocked to allow transmission of monitor frames but prohibit transmission of user frames, with a view to avoid the occurrence of the super-loop. 
     Another proposed system sets a preferential order to multiple rings and, in the event of a failure occurring on a common link shared by the multiple rings, causes a monitor frame in a ring of the higher priority to be introduced to a ring of the lower priority. Circling the monitor frame in these rings controls the blocking (see ‘Foundry Biglron RX Series Configuration Guide’, Chapter 15, Metro Ring Protocol (MRP)). 
     The multi-ring structure with the common link can deal with plural failures with some restriction in the ring topology. In the prior art system of the former cited reference, however, in a multi-failure state with one failure on the common link and another failure on another link in the ring assigned to the working ring of monitoring the common link, the auxiliary monitor frame sent from the common link terminal device reaches the monitor device in the ring assigned to the non-working ring of monitoring the common link. The reception of the auxiliary monitor frame interferes with release of the blocked port in the monitor device. This leads to failed recovery of communication even in the presence of the physical connection. 
     In the prior art system of the latter cited reference, on the other hand, in a multi-failure state with one failure on the common link and another failure on another link in the ring assigned to the working ring of monitoring the common link, the monitor frame does not reach the monitor device in the ring assigned to the non-working ring of monitoring the common link. This allows release of the blocked port in the monitor device and is free from the above problem arising in the system of the former cited reference. The system of the latter cited reference, however, still has some disadvantages. The occurrence of each failure on the common link increases the number of relay devices as passing points of a monitor frame. This causes the delay of the monitor frame and increases the potential for wasting the frame due to the convergence. 
     SUMMARY OF THE INVENTION 
     In a network of a multi-ring structure, even in a multi-failure state with a failure on a common link and another failure on a link other than the common link, there would be a demand for detecting the occurrence of multiple failures and enable continuous communication without increasing the number of relay devices as passing points of a monitor frame. 
     The present invention accomplishes at least part of the above and other related demands by the following configurations applied to a counter multi-failure system and a common link terminal device used in the system. 
     One aspect of the invention pertains to a counter multi-failure system of dealing with multiple link failures occurring in a communication network including a first ring and a second ring connected to the first ring by sharing part of a link in the first ring as a common link. The counter multi-failure system includes: a common link terminal device located at a terminal end of the common link and configured to send an auxiliary monitor frame to the second ring; and a monitor device located on the second ring and configured to receive a monitor frame sent by the monitor device and circled through the second ring, as well as the auxiliary monitor frame and block at least one specific port among ports connecting with the second ring. The common link terminal device has: a communication unit configured to send the auxiliary monitor frame and receive a failure detection frame sent from a device located on a ring in the occurrence of a link failure in the ring; a storage unit configured to store link failure information included in the failure detection frame; and a controller configured to identify occurrence or non-occurrence of multiple failures including a failure on the common link based on the stored link failure information and, upon identification of the occurrence of multiple failures, to control the communication unit to stop transmission of the auxiliary monitor frame. The monitor device cancels the blocking of the specific port in the case of non-reception of the monitor frame for a preset time period and non-reception of the auxiliary monitor frame. 
     In the counter multi-failure system according to one aspect of the invention, upon detection of the occurrence of multiple failures including a failure of the common link, the common link terminal device is controlled to stop transmission of the auxiliary monitor frame. The monitor device receives neither the monitor frame for the preset time period nor the auxiliary monitor frame and thereby cancels the blocking of the specific port. Cancellation of the blocking of the specific port in this manner enables continuous communication without increasing the number of relay devices as the passing points of the monitor frame. This arrangement allows continuous communication even in the occurrence of multiple failures in the communication network of the multi-ring structure. 
     In one preferable application of the counter multi-failure system according to the above aspect of the invention, in response to cancellation of a recovery deterrent state after recovery of the link from the multiple failures, the monitor device blocks the specific port and sends a specific control frame to the second ring. 
     In the counter multi-failure system of this application, in the case of recovery of the link from the multiple failures, the monitor device cancels the recovery deterrent state to block the specific port again. Transmission of the specific control frame informs another device of the recovery of the link from the multiple failures. 
     In one preferable embodiment of the counter multi-failure system of this application, when the communication unit of the common link terminal device receives the specific control frame, the controller of the common link terminal device deletes at least part of the link failure information stored in the storage unit and controls the communication unit to start transmission of the auxiliary monitor frame. 
     In the counter multi-failure system of this embodiment, the common link terminal device receives the specific control frame and recognizes recovery of the link from the multiple failures. The common link terminal device deletes non-required information, such as link failure information correlated to the recovered link, from the storage unit and starts transmission of the auxiliary monitor frame. This recovers the normal state before the occurrence of the multiple failures. 
     In another preferable application of the counter multi-failure system according to the above aspect of the invention, upon dissatisfaction of a predetermined condition based on the link failure information, the controller of the common link terminal device controls the communication unit to continue transmission of the auxiliary monitor frame even in the event of detection of the multiple failures. 
     In the counter multi-failure system of this application, even in the event of detection of multiple failures, the multi-ring structure may allow continuous communication without cancelling the blocking of the specific port in the monitor device located on the second ring. In such cases, there is no need of stopping transmission of the auxiliary monitor frame. 
     In a communication network including a first ring and a second ring connected to the first ring by sharing part of a link in the first ring as a common link, another aspect of the invention pertains to a common link terminal device located at a terminal end of the common link. The common link terminal device includes: a communication unit configured to send an auxiliary monitor frame to a monitor device located on the second ring and receive a failure detection frame sent from a device located on a ring in the occurrence of a link failure in the ring; a storage unit configured to store link failure information included in the failure detection frame; and a controller configured to identify occurrence or non-occurrence of multiple failures including a failure on the common link based on the stored link failure information and, upon identification of the occurrence of multiple failures, to control the communication unit to stop transmission of the auxiliary monitor frame. 
     Upon detection of the occurrence of multiple failures including a failure of the common link, the common link terminal device according to this aspect of the invention stops transmission of the auxiliary monitor frame to the monitor device. The monitor device does not receive the auxiliary monitor frame and may cancel the blocking of the specific port under a predetermined condition. Cancellation of the blocking of the specific port in this manner enables continuous communication without increasing the number of relay devices as the passing points of the monitor frame. This arrangement allows continuous communication even in the occurrence of multiple failures in the communication network of the multi-ring structure. 
     In one preferable application of the common link terminal device according to this aspect of the invention, when the communication unit receives the specific control frame from the monitor device, the controller deletes at least part of the link failure information stored in the storage unit and controls the communication unit to start transmission of the auxiliary monitor frame. 
     The common link terminal device of this application receives the specific control frame from the monitor device and recognizes recovery of the link from the multiple failures. The common link terminal device deletes non-required information, such as link failure information correlated to the recovered link, from the storage unit and starts transmission of the auxiliary monitor frame. This enables the common link terminal device to be recovered to the state before the occurrence of the multiple failures. 
     In another preferable application of the common link terminal device according to the above aspect of the invention, upon dissatisfaction of a predetermined condition based on the link failure information, the controller controls the communication unit to continue transmission of the auxiliary monitor frame even in the event of detection of the multiple failures. 
     Even in the event of detection of multiple failures, the multi-ring structure may allow continuous communication without cancelling the blocking of the specific port in the monitor device located on the second ring. In such cases, there is no need of stopping transmission of the auxiliary monitor frame. 
     The present invention is not restricted to the counter multi-failure system or the common link terminal device described above, but the technique of the invention may be actualized by a diversity of other applications, for example, a method of dealing with multiple failures, computer programs to attain any of the counter multi-failure system, the common link terminal device, and the method of dealing with multiple failures, and recording media in which such computer programs are recorded. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an Ethernet communication network in a multi-ring structure according to a first embodiment of the invention; 
         FIG. 2  is a block diagram schematically showing the structure of common link terminal devices  1102  and  1108  shown in  FIG. 1 ; 
         FIG. 3  is a block diagram showing the occurrence of a failure on a common link in the Ethernet communication network of  FIG. 1 ; 
         FIG. 4  shows the contents of a link failure information management database stored in the common link terminal devices  1102  and  1108  in the state of  FIG. 3 ; 
         FIG. 5  is a flowchart showing a multi-failure detection routine executed by the common link terminal device shown in  FIG. 1 ; 
         FIG. 6  is a block diagram showing the occurrence of multiple failures in the Ethernet communication network of  FIG. 1 ; 
         FIG. 7  is a block diagram showing recovery of one failed link from the multi-failure state in the Ethernet communication network of  FIG. 1 ; 
         FIG. 8  is a block diagram showing recovery of all links from multiple failures in the Ethernet communication network of  FIG. 1 ; 
         FIG. 9  is a block diagram showing an Ethernet communication network in a multi-ring structure according to a second embodiment of the invention; 
         FIG. 10  is a block diagram showing the occurrence of multiple failures in the Ethernet communication network of  FIG. 9 ; 
         FIG. 11  shows the contents of a link failure information management database stored in common link terminal devices  2103  and  2110  in the state of  FIG. 10 ; 
         FIG. 12  is a block diagram showing an Ethernet communication network in a multi-ring structure according to a third embodiment of the invention; 
         FIG. 13  is a block diagram showing the occurrence of multiple failures in the Ethernet communication network of  FIG. 12 ; 
         FIG. 14  shows the contents of a link failure information management database stored in a common link terminal device  3102  in the state of  FIG. 13 ; 
         FIG. 15  shows the contents of a link failure information management database stored in a common link terminal device  3105  in the state of  FIG. 13 ; 
         FIG. 16  shows the contents of a link failure information management database stored in a common link terminal device  3108  in the state of  FIG. 13 ; 
         FIG. 17  shows the contents of a link failure information management database stored in a common link terminal device  3142  in the state of  FIG. 13 ; 
         FIG. 18  shows the contents of a link failure information management database stored in common link terminal devices  3109  and  3143  in the state of  FIG. 13 ; 
         FIG. 19  is a block diagram showing an Ethernet communication network in a multi-ring structure according to a fourth embodiment of the invention; 
         FIG. 20  is a block diagram showing the occurrence of multiple failures in the Ethernet communication network of  FIG. 19 ; 
         FIG. 21  shows the contents of a link failure information management database stored in a common link terminal device  4104  in the state of  FIG. 20 ; 
         FIG. 22  shows the contents of a link failure information management database stored in a common link terminal device  4105  in the state of  FIG. 20 ; 
         FIG. 23  shows the contents of a link failure information management database stored in a common link terminal device  4106  in the state of  FIG. 20 ; and 
         FIG. 24  shows the contents of a link failure information management database stored in a common link terminal device  4108  in the state of  FIG. 20 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Some modes of carrying out the invention are described below in the following sequence with reference to the accompanied drawings: 
     A. First Embodiment 
     B. Second Embodiment 
     C. Third Embodiment 
     D. Fourth Embodiment 
     E. General Flow of Embodiments 
     A. First Embodiment 
       FIG. 1  is a block diagram showing an Ethernet communication network in a multi-ring structure according to a first embodiment of the invention. In the Ethernet communication network, a ring  10  includes a monitor device  1103 , relay devices  1106  and  1109 , a relay device  1105 , and common link terminal devices  1102  and  1108 . A ring  20  includes an auxiliary monitor device  1101 , relay devices  1104  and  1107 , the relay device  1105 , and the common link terminal devices  1102  and  1108 . IDs ‘ 10 ’ and ‘ 20 ’ are allocated respectively to the ring  10  and to the ring  20 . 
     A link connecting the common link terminal device  1102  with the relay device  1105  and the common link terminal device  1108  is a common link  15  shared by the ring  10  and the ring  20 . 
     The common link terminal devices  1102  and  1108  respectively send auxiliary monitor frames  1113  and  1114  to the auxiliary monitor device  1101  in the ring  20 . This assigns the ring  20  to a non-working ring of monitoring the occurrence of a failure on the common link  15 . The auxiliary monitor frames  1113  and  1114  sent from the common link terminal devices  1102  and  1108  include the ID ‘ 20 ’ representing the ring  20  as ID information. 
     The monitor device  1103  sends a monitor frame  1112  from one port connected to the ring  10  and receives the monitor frame  1112  returned after circling the ring  10 , with a view to confirming the normal state of the ring  10 . The monitor device  1103  keeps a specific port  1122  blocked, as long as normal reception of the monitor frame  1112  continues. In response to non-reception of the monitor frame  1112  for a preset time period, the monitor device  1103  recognizes the occurrence of a failure on the ring  10 , cancels the blocking of the specific port  1122 , and sends an FDB (forwarding database) flush frame as a control frame for deleting an FDB stored in each of the other devices as described later in detail. 
     The auxiliary monitor device  1101  sends a monitor frame  1111  from one port connected to the ring  20  and receives the monitor frame  1111  returned after circling the ring  20 , as well as the auxiliary monitor frames  1113  and  1114  sent by the common link terminal devices  1102  and  1108 , with a view to confirming the normal state of the ring  20 . The auxiliary monitor device  1101  keeps a specific port  1121  blocked, as long as normal reception of the monitor frame  1111  and reception of the auxiliary monitor frames  1113  and  1114  continue. In response to non-reception of the monitor frame  1111  for a preset time period and non-reception of the auxiliary monitor frames  1113  and  1114 , the auxiliary monitor device  1101  recognizes the occurrence of a failure on the ring  20 , cancels the blocking of the specific port  1121 , and sends the FDB flush frame. The blocking of the specific port  1121  is not canceled but is kept as long as reception of the auxiliary monitor frames  1113  and  1114  continues even in the case of non-reception of the monitor frame  1111  or as long as reception of the monitor frame  1111  continues even in the case of non-reception of the auxiliary monitor frames  1113  and  1114 . 
     The monitor frame may be sent from only one port connected to the ring as in this embodiment or may be sent individually from both ports connected to the ring. 
     The relay devices  1104 ,  1105 ,  1106 ,  1107 , and  1109  simply work to transfer the frames and have no other operations in the state of no failure. 
     Each device as a constituent of a communication network monitors, via its port, the state of a ring connecting with the port. In response to detection of the occurrence of any failure on a link connected to the port, the device sends a failure detection frame to the ring connecting with the port, that is, the ring as the object of monitoring via the port. When the link with detection of the occurrence of a failure is other than a common link, there is only one ring connecting with the port. The failure detection frame is thus sent to only this ring. When the link with detection of the occurrence of a failure is a common link shared by multiple rings, on the other hand, the failure detection frame is sent to all the rings connecting with a common port, that is, all the multiple rings sharing the common link. 
     If a device detecting the occurrence of a link failure and sending a failure detection frame receives the failure detection frame sent by itself, the received failure detection frame is to be discarded to avoid the loop. The failure detection frame is sent to and received from even the blocked specific port in the monitor device. 
     Each failure detection frame includes an MAC address allocated to the device sending the failure detection frame, an ID allocated to the ring connecting with the port detecting the occurrence of a link failure (that is, the ring as the object of monitoring via the port) (hereafter referred to as monitoring ring ID), and when a common port detects the occurrence of a link failure (that is, when a common link has a failure), IDs allocated to the rings sharing the common link (hereafter referred to as common ring IDs). 
       FIG. 2  is a block diagram schematically showing the structure of the common link terminal devices  1102  and  1108  shown in  FIG. 1 . As shown in  FIG. 2 , each of the common link terminal devices  1102  and  1108  mainly includes a CPU  110  controlling the operations of the respective constituents, a memory  120  storing required pieces of information, and a communication unit  130  making communication with external devices. The memory  120  stores a link failure information management database  122  (described later) and a forwarding database (FDB)  124  as an information table used for controlling the forwarding destinations of each frame. An FDB is similarly incorporated in each of the other devices, as well as the common link terminal devices  1102  and  1108 . The CPU  110  and the memory  120  in the structure of  FIG. 2  respectively correspond to the controller and the storage unit in the claims of the invention. 
       FIG. 3  is a block diagram showing the occurrence of a failure on the common link  15  in the Ethernet communication network of  FIG. 1 . In the state of  FIG. 3 , a failure  1131  occurs on the common link  15  between the common link terminal device  1102  and the relay device  1105 . In this case, the relay device  1105  detects the failure  1131  at its common port and sends a failure detection frame  1116  to all the rings  10  and  20  sharing the common link  15 . The common link terminal device  1102  similarly detects the failure  1131  at its common port and sends a failure detection frame  1117  to all the rings  10  and  20  sharing the common link  15 . 
     The failure detection frames  1116  and  1117  are sent through the specific port  1121  blocked in the auxiliary monitor device  1101  and the specific port  1122  blocked in the monitor device  1103  and eventually reach the common link terminal devices  1102  and  1108 . 
     In response to reception of the failure detection frame or detection of the occurrence of a link failure via its own port, each of the common link terminal devices  1102  and  1108  adds link failure information as shown in  FIG. 4  into the link failure information management database  122  sown in  FIG. 2  and performs multi-failure detection based on the link failure information according to a flowchart of  FIG. 5 . 
       FIG. 4  shows the contents of the link failure information management database  122  stored in the common link terminal devices  1102  and  1108  in the state of  FIG. 3 . Each piece of link failure information includes the MAC address, the monitoring ring ID, and the common ring IDs. The numeral in each pair of parentheses denotes a device having the corresponding MAC address in  FIG. 4  and subsequent similar drawings. 
       FIG. 5  is a flowchart showing a multi-failure detection routine executed by the common link terminal device  1102  or  1108  shown in  FIG. 1 . 
     In the event of the occurrence of the failure  1131  on the common link  15 , each of the common link terminal devices  1102  and  1108  receives a failure detection frame at its communication unit  130  (see  FIG. 2 ) or detects the occurrence of a link failure via its own port. In response to the reception of the failure detection frame or the detection of the link failure, the CPU  110  registers pieces of link failure information R 1  to R 4  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 4  and executes the multi-failure detection routine of  FIG. 5  according to the registered pieces of link failure information R 1  to R 4 . 
     According to the pieces of link failure information R 1  and R 3  including the monitoring ring ID ‘ 10 ’ and the common ring IDs ‘ 10 ’ and ‘ 20 ’, the CPU  110  specifies the ring  10  as the ring with the occurrence of a failure and identifies the location of the failure as the common link  15  shared by the rings  10  and  20 . According to the pieces of link failure information R 2  and R 4  including the monitoring ring ID ‘ 20 ’ and the common ring IDs ‘ 10 ’ and ‘ 20 ’, the CPU  110  specifies the ring  20  as the ring with the occurrence of a failure and identifies the location of the failure as the common link  15  shared by the rings  10  and  20 . The CPU  110  determines whether failures occur at multiple different positions with regard to an identical monitoring ring ID (step S 102 ). In the state of  FIG. 3 , the two different pieces of link failure information R 1  and R 3  give the identical common ring IDs ‘ 10 ’ and ‘ 20 ’ in correlation to the identical monitoring ring ID ‘ 10 ’. The two different pieces of link failure information R 2  and R 4  also give the identical common ring IDs ‘ 10 ’ and ‘ 20 ’ in correlation to the identical monitoring ring ID ‘ 20 ’. The CPU  110  accordingly determines no occurrence of failures at multiple different positions with regard to any identical monitoring ring ID and proceeds to step S 116  to control the communication unit  130  to continue transmission of the auxiliary monitor frame  1113  and  1114 . 
     In response to non-reception of the monitor frame  1112  sent by itself for a preset time period, the monitor device  1103  in the ring  10  recognizes the occurrence of a failure on the ring  10 , cancels the blocking of the specific port  1122 , and sends an FDB flush frame  1115 . For a high-speed route change responding to the occurrence of a failure, in response to reception of a failure detection frame, the monitor device  1103  may immediately cancel the blocking of the specific port  1122  and send the FDB flush frame  1115 . 
     The auxiliary monitor device  1101  receives the auxiliary monitor frames  1113  and  1114  from the common link terminal devices  1102  and  1108  with failure in reception of the monitor frame  1111  sent by itself and accordingly keeps the specific port  1121  blocked. 
     In response to reception of the FDB flush frame  1115  from the monitor device  1103 , the common link terminal devices  1102  and  1108  and the relay devices  1105 ,  1106 , and  1109  delete the FDBs stored therein and urge the communication network to change the communication route. Deletion of the FDB may be carried out simultaneously over the whole device or at each port. 
       FIG. 6  is a block diagram showing the occurrence of multiple failures in the Ethernet communication network of  FIG. 1 . In the state of  FIG. 6 , a failure  1132  occurs on a link between the relay device  1109  and the common link terminal device  1108 , in addition to the failure  1131  occurring on the common link  15 . In this case, the relay device  1109  sends a failure detection frame  1141  to only the ring  10  connected to the port detecting the occurrence of the failure  1132 . The common link terminal device  1108  also sends a failure detection frame  1142  to only the ring  10  connected to the port detecting the occurrence of the failure  1132 . 
     In each of the common link terminal devices  1102  and  1108 , in response to reception of a failure detection frame at the communication unit  130  or detection of the occurrence of a link failure via its own port, the CPU  110  registers pieces of link failure information R 5  and R 6  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 4  and executes the multi-failure detection routine of  FIG. 5  according to the registered pieces of link failure information. 
     According to the pieces of link failure information R 5  and R 6  including the monitoring ring ID ‘ 10 ’ and no common ring IDs, the CPU  110  specifies the ring  10  as the ring with the occurrence of a failure and identifies the location of the failure as a link other than the common link  15 . 
     The CPU  110  determines whether failures occur at multiple different positions with regard to an identical monitoring ring ID (step S 102 ). In the state of  FIG. 6 , with regard to the identical monitoring ring ID ‘ 10 ’, the pieces of link failure information R 1  and R 3  include the common ring IDs ‘ 10 ’ and ‘ 20 ’ suggesting the occurrence of a failure on the common link  15  shared by the rings  10  and  20 , while the pieces of link failure information R 5  and R 6  include no common ring IDs suggesting the occurrence of a failure on a link other than the common link  15 . Based on detection of the two different locations with regard to the identical monitoring ring ID ‘ 10 ’, it is determined that the ring  10  has failures at two different positions. The CPU  110  accordingly determines the occurrence of failures at multiple different locations with regard to an identical monitoring ring ID and proceeds to step S 104 . 
     The CPU  110  determines whether any of the monitoring ring IDs is identical with an ID represented by ID information included in an auxiliary monitor frame sent by the self device (step S 104 ). In the state of  FIG. 6 , the auxiliary monitor frames sent by the common link terminal devices  1102  and  1108  include the ID ‘ 20 ’ representing the ring  20  as the ID information, while the pieces of link failure information R 2  and R 4  include the monitoring ring ID ‘ 20 ’. The CPU  110  accordingly determines that any of the monitoring ring IDs is identical with the ID represented by the ID information included in the auxiliary monitor frame and proceeds to step S 106 . 
     The CPU  110  determines whether any of the common ring IDs has a greater number than the ID represented by the ID information included in the auxiliary monitor frame sent by the self device (step S 106 ). In the state of  FIG. 6 , the pieces of link failure information R 1  to R 4  include the common ring IDs ‘ 10 ’ and ‘ 20 ’, and there is no ID having a greater number than the ID ‘ 20 ’ represented by the ID information included in the auxiliary monitor frame. The CPU  110  accordingly determines that there is no ring in failure having an ID greater in number than ‘ 20 ’ and proceeds to step S 108 . 
     The CPU  110  determines whether a failure occurs on a link other than the common link with regard to an identical monitoring ring ID (step S 108 ). In the state of  FIG. 6 , the pieces of link failure information R 5  and R 6  include no common ring IDs. The CPU  110  accordingly detects the occurrence of a failure on a link other than the common link and proceeds to step S 110 . 
     The CPU  110  determines whether the self device sends multiple auxiliary monitor frames (step S 110 ). In the state of  FIG. 6 , the common link terminal devices  1102  and  1108  send the auxiliary monitor frame  1113  or  1114  to only one ring  20 . The CPU  110  accordingly identifies no transmission of multiple auxiliary monitor frames and proceeds to step S 114 . 
     The CPU  110  controls the communication unit  130  to stop the transmission of the auxiliary monitor frame  1113  or  1114  (step S 114 ). The common link terminal devices  1102  and  1108  accordingly stop the transmission of the auxiliary monitor frames  1113  and  1114 . 
     In response to non-reception of the auxiliary monitor frames from the common link terminal devices  1102  and  1108 , the auxiliary monitor device  1101  in the ring  20  recognizes the occurrence of a failure on the ring  20 , cancels the blocking of the specific port  1121 , and sends an FDB flush frame  1118 . The common link terminal devices  1102  and  1108  receive the FDB flush frame  1118  from the auxiliary monitor device  1101  and respectively send FDB flush frames  1119  and  1120  to the ring  10  with multiple failures. 
     In response to reception of the FDB flush frame, the common link terminal devices  1102  and  1108 , the relay devices  1104 ,  1105 ,  1106 ,  1107 , and  1109 , and the monitor  1103  delete the FDBs stored therein and urge the communication network to change the communication route. 
       FIG. 7  is a block diagram showing recovery of one failed link from the multi-failure state in the Ethernet communication network of  FIG. 1 . In the state of  FIG. 7 , in response to recovery of the link between the relay device  1109  and the common link terminal device  1108  from the failure  1132 , a specific port  1124  of the common link terminal device  1108  and a specific port  1123  of the relay device  1109  are blocked to avoid the loop. 
     The failure  1131  occurring on the common link  15  between the common link terminal device  1102  and the relay device  1105 , however, still remains. The recovery of the link between the relay device  1109  and the common link terminal device  1108  from the failure  1132  thus does not enable the monitor frame  1112  sent from the monitor device  1103  to go round the ring  10 . This means no recovery of the ring  10 , so that the monitor device  1103  does not send an FDB flush frame. In response to non-reception of the FDB flush frame, the common link terminal device  1108  and the relay device  1109  keep the respective specific ports  1124  and  1123  blocked. Communication of user frames takes a detour around the blocked specific ports  1124  and  1123  and is thus not affected. 
       FIG. 8  is a block diagram showing recovery of all links from multiple failures in the Ethernet communication network of  FIG. 1 . In the state of  FIG. 8 , in response to recovery of the common link  15  between the common link terminal device  1102  and the relay device  1105  from the failure  1131 , a specific port  1125  of the common link terminal device  1102  and a specific port  1126  of the relay device  1105  are blocked to avoid the loop. 
     The recovery of the common link  15  between the common link terminal device  1102  and the relay device  1105  enables the monitor frame  1112  sent from the monitor device  1103  and the monitor frame  1111  sent from the auxiliary monitor device  1101  to be returned respectively after circling the ring  10  and after circling the ring  20 . In response to reception of the monitor frames  1112  and  1111  sent by the respective self devices, the monitor device  1103  and the auxiliary monitor device  1101  recognize recovery of the ring  10  and recovery of the ring  20 . The monitor device  1103  and the auxiliary monitor device  1101  then fall into a recovery deterrent state not to immediately send FDB flush frames after recognition of the recovery of the rings  10  and  20 . 
     In response to the user&#39;s operation to cancel out the recovery deterrent state, the monitor device  1103  and the auxiliary monitor device  1101  respectively block the specific port  1122  connecting with the ring  10  and the specific port  1121  connecting with the ring  20  and send the FDB flush frame  1115  and the FDB flush frame  1118 . The user is required to cancel out the recovery deterrent state of the auxiliary monitor device  1101  prior to that of the monitor device  1103 , in order to avoid the loop. 
     In response to the user&#39;s operation to cancel out the recovery deterrent state of the auxiliary monitor device  1101 , the auxiliary monitor device  1101  blocks the specific port  1121  and sends the FDB flush frame  1118 . In the common link terminal devices  1102  and  1108 , when the communication unit  130  receives the FDB flush frame  1118 , the CPU  110  recognizes recovery of the ring  20  from the failure, deletes the pieces of link failure information R 2  and R 4  correlated to the ring  20  from the link failure information management database  122  set in the memory  120 , and controls the communication unit  130  to start transmission of the auxiliary monitor frames  1113  and  1114 . The common link terminal device  1102  and the relay device  1105  then cancel the blocking of the respective specific ports  1125  and  1126 . In response to reception of the FDB flush frame  1118 , the relay devices  1104 ,  1105 , and  1107  and the common link terminal devices  1102  and  1108  delete the FDBs stored therein and urge the communication network to change the communication route. At the moment of recovery, neither of the common link terminal devices  1102  and  1108  transfers the FDB flush frame  1118  to the ring  10 . 
     In response to the user&#39;s subsequent operation to cancel out the recovery deterrent state of the monitor device  1103 , the monitor device  1103  blocks the specific port  1122  and sends the FDB flush frame  1115 . In the common link terminal devices  1102  and  1108 , when the communication unit  130  receives the FDB flush frame  1115 , the CPU  110  recognizes recovery of the ring  10  from the failure and deletes the pieces of link failure information R 1 , R 3 , R 5 , and R 6  correlated to the ring  10  from the link failure information management database  122  set in the memory  120 . The common link terminal device  1108  and the relay device  1109  then cancel the blocking of the respective specific ports  1124  and  1123 . In response to reception of the FDB flush frame  1115 , the relay devices  1105 ,  1106 , and  1109  and the common link terminal devices  1102  and  1108  delete the FDBs stored therein and urge the communication network to change the communication route. 
     As described above, in the communication network of the first embodiment, in the multiple-failure state with the failure  1131  on the common link  15  and the failure  1132  on another link, the common link terminal devices  1102  and  1108  are controlled to stop transmission of the auxiliary monitor frames  1113  and  1114 . In response to non-reception of the monitor frame  1111  for a preset time period and non-reception of the auxiliary monitor frames  1113  and  1114 , the auxiliary monitor device  1101  in the ring  20  cancels the blocking of the specific port  1121 . Cancellation of the blocking of the specific port  1121  in this manner enables continuous communication without increasing the number of relay devices as passing points of a monitor frame. This arrangement desirably allows continuous communication even in the event of the occurrence of multiple failures in the communication network of the multi-ring structure. Because of no increase in number of the relay devices as the passing points of monitor frame, no countermeasure against the delay of the monitor frame is required. 
     B. Second Embodiment 
       FIG. 9  is a block diagram showing an Ethernet communication network in a multi-ring structure according to a second embodiment of the invention. A ring  10  includes a monitor device  2107 , a relay device  2106 , and common link terminal devices  2103  and  2110 . A ring  20  includes an auxiliary monitor device  2105 , relay devices  2102  and  2109 , the relay device  2106 , and the common link terminal devices  2103  and  2110 . A ring  30  includes an auxiliary monitor device  2104 , relay devices  2101  and  2108 , the relay device  2106 , and the common link terminal devices  2103  and  2110 . IDs ‘ 10 ’, ‘ 20 ’, and ‘ 30 ’ are allocated respectively to the ring  10 , to the ring  20 , and to the ring  30 . 
     A link connecting the common link terminal device  2103  with the relay device  2106  and the common link terminal device  2110  is a common link shared by the three rings  10 ,  20 , and  30 . 
     In the communication network of the second embodiment, each of the common link terminal devices  2103  and  2110  sends multiple auxiliary monitor frames. More specifically the common link terminal devices  2103  and  2110  respectively send auxiliary monitor frames  2113  and  2114  to the auxiliary monitor device  2105  in the ring  20 , while sending auxiliary monitor frames  2115  and  2116  to the auxiliary monitor device  2104  in the ring  30 . This assigns the rings  20  and  30  to non-working rings of monitoring the occurrence of a failure on the common link. The auxiliary monitor frames  2113  and  2114  include the ID ‘ 20 ’ representing the ring  20  as ID information. The auxiliary monitor frames  2115  and  2116  include the ID ‘ 30 ’ representing the ring  30  as ID information. 
     The monitor device  2107  sends a monitor frame  2117  and receives the monitor frame  2117  returned after circling the ring  10  and sequentially transferred through the constituents of the ring  10 , with a view to confirming the normal state of the ring  10 . The auxiliary monitor device  2105  sends a monitor frame  2112  and receives the monitor frame  2112  returned after circling the ring  20  and sequentially transferred through the constituents of the ring  20 , as well as the auxiliary monitor frames  2113  and  2114  sent by the common link terminal devices  2103  and  2110 , with a view to confirming the normal state of the ring  20 . The auxiliary monitor device  2104  sends a monitor frame  2111  and receives the monitor frame  2111  returned after circling the ring  30  and sequentially transferred through the constituents of the ring  30 , as well as the auxiliary monitor frames  2115  and  2116  sent by the common link terminal devices  2103  and  2110 , with a view to confirming the normal state of the ring  30 . The monitor frame may be sent individually from both ports connected to the ring as in this embodiment or may be sent from only one port connected to the ring as in the first embodiment. 
     The relay devices  2101 ,  2102 ,  2106 ,  2108 , and  2109  simply work to transfer the frames and have no other operations in the state of no failure. 
     The structure of the common link terminal devices  2103  and  2110  is identical with the structure of the common link terminal devices  1102  and  1108  shown in  FIG. 2  and is not specifically explained here. 
       FIG. 10  is a block diagram showing the occurrence of multiple failures in the Ethernet communication network of  FIG. 9 . In the state of  FIG. 10 , in the event of the occurrence of a failure  2131  on the common link between the common link terminal device  2103  and the relay device  2106 , the relay device  2106  sends a failure detection frame  2141  to all the rings  10 ,  20 , and  30  sharing the common link. The common link terminal device  2103  also sends a failure detection frame  2142  to all the rings  10 ,  20 , and  30  sharing the common link. In the event of the subsequent occurrence of another failure  2132  on a link between the monitor device  2107  and the common link terminal device  2110 , the monitor device  2107  sends a failure detection frame  2118  to only the ring  10  including the link. The common link terminal device  2110  also sends a failure detection frame  2119  to only the ring  10  including the link. The monitor device  2107  does not receive the monitor frame  2117  sent by itself and thereby cancels the blocking of a specific port  2123 . 
       FIG. 11  shows the contents of a link failure information management database stored in the common link terminal devices  2103  and  2110  in the state of  FIG. 10 . 
     In each of the common link terminal devices  2103  and  2110 , in response to reception of a failure detection frame by the communication unit  130  (see  FIG. 2 ) or detection of the occurrence of a link failure via its own port, the CPU  110  registers pieces of link failure information R 11  to R 18  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 11  and executes the multi-failure detection routine of  FIG. 5  according to the registered pieces of link failure information R 11  to R 18 . 
     According to the pieces of link failure information R 11  and R 14  including the monitoring ring ID ‘ 10 ’ and the common ring IDs ‘ 10 ’, ‘ 20 ’, and ‘ 30 ’, the CPU  110  specifies the ring  10  as the ring with the occurrence of a failure and identifies the location of the failure as the common link shared by the rings  10 ,  20 , and  30 . According to the pieces of link failure information R 12  and R 15  including the monitoring ring ID ‘ 20 ’ and the common ring IDs ‘ 10 ’, ‘ 20 ’, and ‘ 30 ’, the CPU  110  specifies the ring  20  as the ring with the occurrence of a failure and identifies the location of the failure as the common link shared by the rings  10 ,  20 , and  30 . According to the pieces of link failure information R 13  and R 16  including the monitoring ring ID ‘ 30 ’ and the common ring IDs ‘ 10 ’, ‘ 20 ’, and ‘ 30 ’, the CPU  110  specifies the ring  30  as the ring with the occurrence of a failure and identifies the location of the failure as the common link shared by the rings  10 ,  20 , and  30 . According to the pieces of link failure information R 17  and R 18  including the monitoring ring ID ‘ 10 ’ and no common ring IDs, the CPU  110  specifies the ring  10  as the ring with the occurrence of a failure and identifies the location of the failure as a link other than the common link. 
     The CPU  110  first executes the processing of step S 102  in the flowchart of  FIG. 5 . In the state of  FIG. 10 , with regard to the identical monitoring ring ID ‘ 10 ’, the pieces of link failure information R 11  and R 14  include the common ring IDs ‘ 10 ’, ‘ 20 ’, and ‘ 30 ’ suggesting the occurrence of a failure on the common link shared by the rings  10 ,  20 , and  30 , while the pieces of link failure information R 17  and R 18  include no common ring IDs suggesting the occurrence of a failure on a link other than the common link. Based on detection of the two different locations with regard to the identical monitoring ring ID ‘ 10 ’, it is determined that the ring  10  has failures at two different positions. The CPU  110  accordingly determines the occurrence of failures at multiple different locations with regard to an identical monitoring ring ID at step S 102  and proceeds to step S 104 . 
     In the state of  FIG. 10 , among the auxiliary monitor frames sent by the common link terminal devices  2103  and  2110 , the auxiliary monitor frames  2113  and  2114  include the ID ‘ 20 ’ representing the ring  20  as the ID information, whereas the auxiliary monitor frame  2115  and  2116  include the ID ‘ 30 ’ representing the ring  30  as the ID information. The pieces of link failure information R 12  and R 15  include the monitoring ring ID ‘ 20 ’, whereas the pieces of link failure information R 13  and R 16  include the monitoring ring ID ‘ 30 ’. The CPU  110  accordingly determines that any of the monitoring ring IDs is identical with the ID represented by the ID information included in the auxiliary monitor frames at step S 104  and proceeds to step S 106 . 
     In the state of  FIG. 10 , the pieces of link failure information R 11  to R 16  include the common ring IDs ‘ 10 ’, ‘ 20 ’, and ‘ 30 ’, and there is no ID having a greater number than the IDs ‘ 20 ’ and ‘ 30 ’ represented by the ID information included in the auxiliary monitor frames. The CPU  110  accordingly determines that there is no ring in failure having an ID greater in number than the IDs represented by the ID information in the auxiliary monitor frames at step S 106  and proceeds to step S 108 . 
     In the state of  FIG. 10 , the pieces of link failure information R 17  and R 18  include no common ring IDs. The CPU  110  accordingly detects the occurrence of a failure on a link other than the common link at step S 108  and proceeds to step S 110 . 
     In the state of  FIG. 10 , the common link terminal devices  2103  and  2110  send auxiliary monitor frames to the two rings  20  and  30 . The CPU  110  accordingly identifies transmission of multiple auxiliary monitor frames at step S 110  and proceeds to step S 112 . 
     The CPU  110  stops transmission of the auxiliary monitor frames to a ring having a smaller ID number among the common ring IDs at step S 112 . In the state of  FIG. 10 , the pieces of link failure information R 11  to R 16  include the common ring IDs ‘ 10 ’, ‘ 20 ’, and ‘ 30 ’. Namely a failure occurs on the common link shared by the three rings  10 ,  20 , and  30 . Among these three rings  10 ,  20 , and  30 , the rings  20  and  30  are the receivers of the auxiliary monitor frames and the ring  20  has the smaller ID number. The common link terminal devices  2103  and  2110  accordingly stop the transmission of the auxiliary monitor frames  2113  and  2114  to the ring  20  of the smaller ID number, while keeps the transmission of the auxiliary monitor frames  2114  and  2116  to the ring  30  of the greater ID number. 
     In response to non-reception of the monitor frame  2117  sent by itself for a preset time period, the monitor device  2107  in the ring  10  recognizes the occurrence of a failure on the ring  10 , cancels the blocking of the specific port  2123 , and sends an FDB flush frame  2146 . 
     In response to non-reception of the auxiliary monitor frames from the common link terminal devices  2103  and  2110 , the auxiliary monitor device  2105  in the ring  20  recognizes the occurrence of a failure on the ring  20 , cancels the blocking of a specific port  2122 , and sends an FDB flush frame  2143 . 
     In response to reception of the FDB flush frame, the relay devices  2102  and  2109  and the common link terminal devices  2103  and  2110  delete the FDBs stored therein and urge the communication network to change the communication route. 
     In response to reception of the FDB flush frame  2143 , the common link terminal devices  2103  and  2110  send FDB flush frames  2144  and  2145  to the ring  10 . The relay devices  2107  and  2106  receive the FDB flush frames  2144  and  2145 , delete the FDBs stored therein, and urge the communication network to change the communication route. 
     Even in the occurrence of the failures  2131  and  2132 , the relay devices  2101  and  2108  and the auxiliary monitor device  2104  in the ring  30  enable continuous communication without any specified operations. 
     As described above, in the communication network of the second embodiment where the common link terminal devices  2103  and  2110  individually send multiple auxiliary monitor frames, in the multiple-failure state with the failure  2131  on the common link and the failure  2132  on another link, the common link terminal devices  2103  and  2110  are controlled to stop transmission of the auxiliary monitor frames  2113  and  2114  to the ring  20  of the smaller ID number. In response to non-reception of the monitor frame  2112  for a preset time period and non-reception of the auxiliary monitor frames  2113  and  2114 , the auxiliary monitor device  2105  in the ring  20  cancels the blocking of the specific port  2122 . Cancellation of the blocking of the specific port  2121  in this manner enables continuous communication without increasing the number of relay devices as passing points of a monitor frame. This arrangement exerts the similar effects to those of the first embodiment described above. 
     C. Third Embodiment 
       FIG. 12  is a block diagram showing an Ethernet communication network in a multi-ring structure according to a third embodiment of the invention. The communication network of the third embodiment has multiple common links. A ring  10  includes a monitor device  3103 , a relay device  3106 , and common link terminal devices  3102 ,  3105 ,  3108 ,  3109 ,  3142 , and  3143 . A ring  20  includes an auxiliary monitor device  3101 , a relay device  3104 , and the common link terminal devices  3102  and  3105 . A ring  30  includes an auxiliary monitor device  3107 , a relay device  3141 , and the common link terminal devices  3108  and  3142 . A ring  40  includes an auxiliary monitor device  3110 , a relay device  3144 , and the common link terminal devices  3109  and  3143 . IDs ‘ 10 ’, ‘ 20 ’, ‘ 30 ’, and ‘ 40 ’ are allocated respectively to the ring  10 , to the ring  20 , to the ring  30 , and to the ring  40 . 
     A link between the common link terminal devices  3102  and  3105  is a common link shared by the rings  10  and  20 . A link between the common link terminal devices  3108  and  3142  is a common link shared by the rings  10  and  30 . A link between the common link terminal devices  3019  and  3143  is a common link shared by the rings  10  and  40 . 
     The common link terminal devices  3102  and  3105  respectively send auxiliary monitor frames  3112  and  3113  to the auxiliary monitor device  3101  in the ring  20 . The common link terminal devices  3108  and  3142  respectively send auxiliary monitor frames  3116  and  3117  to the auxiliary monitor device  3107  in the ring  30 . The common link terminal devices  3109  and  3143  respectively send auxiliary monitor frames  3118  and  3119  to the auxiliary monitor device  3110  in the ring  40 . This assigns the rings  20 ,  30 , and  40  to non-working rings of monitoring the occurrence of a failure on the respective common links. The auxiliary monitor frames  3112  and  3113  include the ID ‘ 20 ’ representing the ring  20  as ID information. The auxiliary monitor frames  3116  and  3117  include the ID ‘ 30 ’ representing the ring  30  as ID information. The auxiliary monitor frames  3118  and  3119  include the ID ‘ 40 ’ representing the ring  40  as ID information. 
     The monitor device  3103  sends a monitor frame  3114  and receives the monitor frame  3114  returned after circling the ring  10  and sequentially transferred through the constituents of the ring  10 , with a view to confirming the normal state of the ring  10 . The auxiliary monitor device  3101  sends a monitor frame  3111  and receives the monitor frame  3111  returned after circling the ring  20  and sequentially transferred through the constituents of the ring  20 , as well as the auxiliary monitor frames  3112  and  3113  sent by the common link terminal devices  3102  and  3105 , with a view to confirming the normal state of the ring  20 . The auxiliary monitor device  3107  sends a monitor frame  3115  and receives the monitor frame  3115  returned after circling the ring  30  and sequentially transferred through the constituents of the ring  30 , as well as the auxiliary monitor frames  3116  and  3117  sent by the common link terminal devices  3108  and  3142 , with a view to confirming the normal state of the ring  30 . The auxiliary monitor device  3110  sends a monitor frame  3120  and receives the monitor frame  3120  returned after circling the ring  40  and sequentially transferred through the constituents of the ring  40 , as well as the auxiliary monitor frames  3118  and  3119  sent by the common link terminal devices  3109  and  3143 , with a view to confirming the normal state of the ring  40 . The monitor frame may be sent individually from both ports connected to the ring as in this embodiment or may be sent from only one port connected to the ring as in the first embodiment. 
     The relay devices  3104 ,  3106 ,  3141 , and  3144  simply work to transfer the frames and have no other operations in the state of no failure. 
     The structure of the common link terminal devices  3102 ,  3105 ,  3108 ,  3109 ,  3142 , and  3143  is identical with the structure of the common link terminal devices  1102  and  1108  shown in  FIG. 2  and is not specifically explained here. 
       FIG. 13  is a block diagram showing the occurrence of multiple failures in the Ethernet communication network of  FIG. 12 . In the state of  FIG. 13 , in the event of the occurrence of a failure  3131  on the common link between the common link terminal devices  3102  and  3105 , the common link terminal device  3102  sends a failure detection frame  3151  to all the rings  10  and  20  sharing the common link. The common link terminal device  3105  also sends a failure detection frame  3154  to all the rings  10  and  20  sharing the common link. In the event of the subsequent occurrence of another failure  3132  on the common link between the common link terminal devices  3108  and  3142 , the common link terminal device  3108  sends a failure detection frame  3155  to all the rings  10  and  30  sharing the common link. The common link terminal  3142  also sends a failure detection frame  3158  to all the rings  10  and  30  sharing the common link. 
       FIG. 14  shows the contents of a link failure information management database stored in the common link terminal device  3102  in the state of  FIG. 13 .  FIG. 15  shows the contents of a link failure information management database stored in the common link terminal device  3105  in the state of  FIG. 13 . 
     In the common link terminal device  3102 , in response to reception of the failure detection frames  3154  and  3158  by the communication unit  130  or detection of the occurrence of a link failure via its own port, the CPU  110  registers pieces of link failure information R 21  to R 25  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 14 . In the common link terminal device  3105 , in response to reception of the failure detection frames  3151  and  3155  by the communication unit  130  or detection of the occurrence of a link failure via its own port, the CPU  110  registers pieces of link failure information R 21  to R 24  and R 26  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 15 . The CPUs  110  in the common link terminal devices  3102  and  3105  then execute the multi-failure detection routine of  FIG. 5  according to the registered pieces of link failure information. 
     The CPU  110  first executes the processing of step S 102 . In the state of  FIG. 13 , with regard to the identical monitoring ring ID ‘ 10 ’, the pieces of link failure information R 21  and R 23  include the common ring IDs ‘ 10 ’ and ‘ 20 ’ suggesting the occurrence of a failure on the common link shared by the rings  10  and  20 , while the pieces of link failure information R 25  and R 26  include the common ring IDs ‘ 10 ’ and ‘ 30 ’ suggesting the occurrence of a failure on the common link shared by the rings  10  and  30 . Based on detection of the two different locations with regard to the identical monitoring ring ID ‘ 10 ’, it is determined that the ring  10  has failures at two different positions. The CPU  110  accordingly determines the occurrence of failures at multiple different locations with regard to an identical monitoring ring ID at step S 102  and proceeds to step S 104 . 
     In the state of  FIG. 13 , the auxiliary monitor frames  3112  and  3113  sent by the common link terminal devices  3102  and  3105  include the ID ‘ 20 ’ representing the ring  20  as the ID information, while the pieces of link failure information R 22  and R 24  include the monitoring ring ID ‘ 20 ’. The CPU  110  accordingly determines that any of the monitoring ring IDs is identical with the ID represented by the ID information included in the auxiliary monitor frames at step S 104  and proceeds to step S 106 . 
     In the state of  FIG. 13 , the pieces of link failure information R 21  to R 24  include the common ring IDs ‘ 10 ’ and ‘ 20 ’ and the pieces of link failure information R 25  and R 26  include the common ring IDs ‘ 10 ’ and ‘ 30 ’. There is a number greater than the ID ‘ 20 ’ represented by the ID information included in the auxiliary monitor frames  3112  and  3113 . The CPU  110  accordingly determines that there is a ring in failure having an ID greater in number than the ID represented by the ID information in the auxiliary monitor frames  3112  and  3113  at step S 106  and proceeds to step S 110 . 
     In the state of  FIG. 13 , the common link terminal devices  3102  and  3105  send the auxiliary monitor frame  3112  or  3113  to only one ring  20 . The CPU  110  accordingly identifies no transmission of multiple auxiliary monitor frames at step S 110  and proceeds to step S  14 . 
     The CPU  110  controls the communication unit  130  to stop the transmission of the auxiliary monitor frames  3112  and  3113  at step S 114 . The common link terminal devices  3102  and  3105  accordingly stop the transmission of the auxiliary monitor frames  3112  and  3113 . 
     In response to non-reception of the monitor frame  3114  sent by itself for a preset time period, the monitor device  3103  in the ring  10  recognizes the occurrence of a failure on the ring  10 , cancels the blocking of a specific port  3122 , and sends an FDB flush frame  3159 . 
     In response to non-reception of the auxiliary monitor frames from the common link terminal devices  3102  and  3105 , the auxiliary monitor device  3101  in the ring  20  recognizes the occurrence of a failure on the ring  20 , cancels the blocking of a specific port  3121 , and sends an FDB flush frame  3152 . 
     In response to reception of the FDB flush frame  3152 , the relay device  3104  deletes the FDB stored therein and urges the communication network to change the communication route. 
     In response to reception of the FDB flush frame  3152 , the common link terminal devices  3102  and  3105  delete the FDBs stored therein and send FDB flush frames  3153  and  3156  to the ring  10 . The relay device  3106  and the common link terminal devices  3108 ,  3142 ,  3143 , and  3109 , and the monitor device  3103  receive these FDB flush frames  3153  and  3156 , delete the FDBs stored therein, and urge the communication network to change the communication route. 
       FIG. 16  shows the contents of a link failure information management database stored in the common link terminal device  3108  in the state of  FIG. 13 .  FIG. 17  shows the contents of a link failure information management database stored in the common link terminal device  3142  in the state of  FIG. 13 . 
     In the common link terminal device  3108 , in response to reception of the failure detection frames  3154  and  3158  by the communication unit  130  or detection of the occurrence of a link failure via its own port, the CPU  110  registers pieces of link failure information R 31  to R 35  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 16 . In the common link terminal device  3142 , in response to reception of the failure detection frames  3151  and  3155  by the communication unit  130  or detection of the occurrence of a link failure via its own port, the CPU  110  registers pieces of link failure information R 31  to R 34  and R 36  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 17 . The CPUs  110  in the common link terminal devices  3108  and  3142  then execute the multi-failure detection routine of  FIG. 5  according to the registered pieces of link failure information. 
     The CPU  110  first executes the processing of step S 102 . In the state of  FIG. 13 , with regard to the identical monitoring ring ID ‘ 10 ’, the pieces of link failure information R 31  and R 33  include the common ring IDs ‘ 10 ’ and ‘ 30 ’ suggesting the occurrence of a failure on the common link shared by the rings  10  and  30 , while the pieces of link failure information R 35  and R 36  include the common ring IDs ‘ 10 ’ and ‘ 20 ’ suggesting the occurrence of a failure on the common link shared by the rings  10  and  20 . Based on detection of the two different locations with regard to the identical monitoring ring ID ‘ 10 ’, it is determined that the ring  10  has failures at two different positions. The CPU  110  accordingly determines the occurrence of failures at multiple different locations with regard to an identical monitoring ring ID at step S 102  and proceeds to step S 104 . 
     In the state of  FIG. 13 , the auxiliary monitor frames  3116  and  3117  sent by the common link terminal devices  3108  and  3142  include the ID ‘ 30 ’ representing the ring  30  as the ID information, while the pieces of link failure information R 32  and R 34  include the monitoring ring ID ‘ 30 ’. The CPU  110  accordingly determines that any of the monitoring ring IDs is identical with the ID represented by the ID information included in the auxiliary monitor frames at step S 104  and proceeds to step S 106 . 
     In the state of  FIG. 13 , the pieces of link failure information R 31  to R 34  include the common ring IDs ‘ 10 ’ and ‘ 30 ’ and the pieces of link failure information R 35  and R 36  include the common ring IDs‘ 10 ’ and ‘ 20 ’. There is no ID having a greater number than the ID ‘ 30 ’ represented by the ID information included in the auxiliary monitor frames  3116  and  3117 . The CPU  110  accordingly determines that there is no ring in failure having an ID greater in number than the ID represented by the ID information in the auxiliary monitor frames  3116  and  3117  at step S 106  and proceeds to step S 108 . 
     In the state of  FIG. 13 , no null data is given as the common ring IDs in any of the pieces of link failure information R 31  to R 36 . The CPU  110  accordingly detects no occurrence of a failure on any link other than the common link at step S 108  and proceeds to step S 116 . 
     The CPU  110  controls the communication unit  130  to continue transmission of the auxiliary monitor frames  3116  and  3117  at step S 116 . 
       FIG. 18  shows the contents of a link failure information management database stored in the common link terminal devices  3109  and  3143  in the state of  FIG. 13 . 
     In the common link terminal devices  3109  and  3143 , in response to reception of the failure detection frames  3151  and  3158  by the communication unit  130 , the CPU  110  registers pieces of link failure information R 41  and R 42  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 18 . The CPUs  110  in the common link terminal devices  3109  and  3143  then execute the multi-failure detection routine of  FIG. 5  according to the registered pieces of link failure information. 
     The CPU  110  first executes the processing of step S 102 . In the state of  FIG. 13 , with regard to the identical monitoring ring ID ‘ 10 ’, the piece of link failure information R 41  includes the common ring IDs ‘ 10 ’ and ‘ 20 ’ suggesting the occurrence of a failure on the common link shared by the rings  10  and  20 , while the piece of link failure information R 42  includes the common ring IDs ‘ 10 ’ and ‘ 30 ’ suggesting the occurrence of a failure on the common link shared by the rings  10  and  30 . Based on detection of the two different locations with regard to the identical monitoring ring ID ‘ 10 ’, it is determined that the ring  10  has failures at two different positions. The CPU  110  accordingly determines the occurrence of failures at multiple different locations with regard to an identical monitoring ring ID at step S 102  and proceeds to step S 104 . 
     In the state of  FIG. 13 , the auxiliary monitor frames  3118  and  3119  sent by the common link terminal devices  3109  and  3143  include the ID ‘ 40 ’ representing the ring  40  as the ID information, while the pieces of link failure information R 41  and R 42  include the monitoring ring ID ‘ 10 ’. The CPU  110  accordingly determines that none of the monitoring ring IDs is identical with the ID represented by the ID information included in the auxiliary monitor frames at step S 104  and proceeds to step S 116 . 
     The CPU  110  controls the communication unit  130  to continue transmission of the auxiliary monitor frames  3118  and  3119  at step S 116 . 
     Even in the occurrence of the failures  3131  and  3132 , the respective devices included in the ring  30  or in the ring  40  enable continuous communication without any specified operations. 
     As described above, in the communication network of the third embodiment having multiple common links, in the multiple-failure state with the failures  3131  and  3132  on plural common links, the common link terminal devices  3102  and  3105  are controlled to stop transmission of the auxiliary monitor frames  3112  and  3113  to the ring  20  of the smaller ID number. In response to non-reception of the monitor frame  3111  for a preset time period and non-reception of the auxiliary monitor frames  3112  and  3113 , the auxiliary monitor device  3101  in the ring  20  cancels the blocking of the specific port  3121 . Cancellation of the blocking of the specific port  3121  in this manner enables continuous communication even in the event of multiple failures on plural common links without increasing the number of relay devices as passing points of a monitor frame. This arrangement exerts the similar effects to those of the first and the second embodiments described above. 
     D. Fourth Embodiment 
       FIG. 19  is a block diagram showing an Ethernet communication network in a multi-ring structure according to a fourth embodiment of the invention. The communication network of the fourth embodiment has a large ring covering over multiple rings. A ring  10  includes a monitor device  4101 , relay devices  4102  and  4103 , and common link terminal devices  4104 ,  4105 , and  4106 . A ring  20  includes an auxiliary monitor device  4107 , the common link terminal devices  4104  and  4105 , and another common link terminal device  4108 . A ring  30  includes an auxiliary monitor device  4109  and the common link terminal devices  4105 ,  4106 , and  4108 . IDs ‘ 10 ’, ‘ 20 ’, and ‘ 30 ’ are allocated respectively to the ring  10 , to the ring  20 , and to the ring  30 . 
     A link between the common link terminal devices  4104  and  4105  is a common link shared by the rings  10  and  20 . A link between the common link terminal devices  4105  and  4106  is a common link shared by the rings  10  and  30 . A link between the common link terminal devices  4105  and  4108  is a common link shared by the rings  20  and  30 . 
     The common link terminal devices  4104  and  4105  respectively send auxiliary monitor frames  4113  and  4114  to the auxiliary monitor device  4107  in the ring  20 . The common link terminal devices  4106  and  4108  respectively send auxiliary monitor frames  4116  and  4117  to the auxiliary monitor device  4109  in the ring  30 . This assigns the rings  20  and  30  to non-working rings of monitoring the occurrence of a failure on the respective common links. The auxiliary monitor frames  4113  and  4114  include the ID ‘ 20 ’ representing the ring  20  as ID information. The auxiliary monitor frames  4116  and  4117  include the ID ‘ 30 ’ representing the ring  30  as ID information. 
     The monitor device  4101  sends a monitor frame  4111  and receives the monitor frame  4111  returned after circling the ring  10  and sequentially transferred through the constituents of the ring  10 , with a view to confirming the normal state of the ring  10 . The auxiliary monitor device  4107  sends a monitor frame  4112  and receives the monitor frame  4112  returned after circling the ring  20  and sequentially transferred through the constituents of the ring  20 , as well as the auxiliary monitor frames  4113  and  4114  sent by the common link terminal devices  4104  and  4105 , with a view to confirming the normal state of the ring  20 . The auxiliary monitor device  4109  sends a monitor frame  4115  and receives the monitor frame  4115  returned after circling the ring  30  and sequentially transferred through the constituents of the ring  30 , as well as the auxiliary monitor frames  4116  and  4117  sent by the common link terminal devices  4106  and  4108 , with a view to confirming the normal state of the ring  30 . The monitor frame may be sent individually from both ports connected to the ring as in this embodiment or may be sent from only one port connected to the ring as in the first embodiment. 
     The relay devices  4102  and  4103  simply work to transfer the frames and have no other operations in the state of no failure. 
     The structure of the common link terminal devices  4104 ,  4105 ,  4106 , and  4108  is identical with the structure of the common link terminal devices  1102  and  1108  shown in  FIG. 2  and is not specifically explained here. 
       FIG. 20  is a block diagram showing the occurrence of multiple failures in the Ethernet communication network of  FIG. 19 . In the state of  FIG. 20 , in the event of the occurrence of a failure  4131  on the common link between the common link terminal devices  4104  and  4105 , the common link terminal device  4104  sends a failure detection frame  4118  to all the rings  10  and  20  sharing the common link. The common link terminal device  4105  also sends a failure detection frame  4119  to all the rings  10  and  20  sharing the common link. In the event of the subsequent occurrence of another failure  4132  on the common link between the common link terminal devices  4105  and  4108 , the common link terminal device  4105  sends a failure detection frame  4120  to all the rings  20  and  30  sharing the common link. All the links from the common link terminal device  4105  to the ring  20 , however, have failures, so that the failure detection frame  4120  is not actually sent to the ring  20 . The common link terminal device  4108  sends a failure detection frame  4141  to all the rings  20  and  30  sharing the common link. 
       FIG. 21  shows the contents of a link failure information management database stored in the common link terminal device  4104  in the state of  FIG. 20 .  FIG. 22  shows the contents of a link failure information management database stored in the common link terminal device  4105  in the state of  FIG. 20 .  FIG. 23  shows the contents of a link failure information management database stored in the common link terminal device  4106  in the state of  FIG. 20 .  FIG. 24  shows the contents of a link failure information management database stored in the common link terminal device  4108  in the state of  FIG. 20 . 
     In the common link terminal device  4104 , in response to reception of the failure detection frames  4119  and  4141  by the communication unit  130  or detection of the occurrence of a link failure via its own port, the CPU  110  registers pieces of link failure information R 51  to R 55  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 21 . In the common link terminal device  4105 , in response to reception of the failure detection frames  4118  and  4141  by the communication unit  130  or detection of the occurrence of a link failure via its own port, the CPU  110  registers pieces of link failure information R 51  to R 54  and R 56  to R 58  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 22 . In the common link terminal device  4106 , in response to reception of the failure detection frames  4118 ,  4119 ,  4120 , and  4141  by the communication unit  130  or detection of the occurrence of a link failure via its own port, the CPU  110  registers pieces of link failure information R 51 , R 53 , and R 57  to R 59  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 23 . In the common link terminal device  4108 , in response to reception of the failure detection frames  4118  and  4120  by the communication unit  130  or detection of the occurrence of a link failure via its own port, the CPU  110  registers pieces of link failure information R 52 , R 54 , R 56  to R 58 , and R 60  in the link failure information management database  122  set in the memory  120  as shown in  FIG. 24 . The CPUs  110  in the common link terminal devices  4104 ,  4105 ,  4106 , and  4108  then execute the multi-failure detection routine of  FIG. 5  according to the registered pieces of link failure information. 
     In the common link terminal device  4106 , the CPU  110  executes the processing of step S 102 . As shown in  FIG. 23 , in the state of  FIG. 20 , with regard to the identical monitoring ring ID ‘ 10 ’, both the pieces of link failure information R 51  and R 53  include the common ring IDs ‘ 10 ’ and ‘ 20 ’ suggesting the occurrence of a failure on the common link shared by the rings  10  and  20 . With regard to the identical monitoring ring ID ‘ 30 ’, all the pieces of link failure information R 57  to R 59  include the common ring IDs ‘ 20 ’ and ‘ 30 ’ suggesting the occurrence of a failure on the common link shared by the rings  20  and  30 . Namely there are no failures occurring at multiple different positions with regard to either the ring  10  or the ring  30 . The CPU  110  in the common link terminal device  4106  accordingly determines no occurrence of failures at multiple different locations with regard to an identical monitoring ring ID at step S 1102  and proceeds to step S 116  to control the communication unit  130  to continue transmission of the auxiliary monitor frame  4116 . 
     In the common link terminal devices  4104 ,  4105 , and  4108 , the CPUs  110  execute the processing of step S 102 . As shown in  FIGS. 21 ,  22 , and  24 , in the state of  FIG. 20 , with regard to the identical monitoring ring ID ‘ 20 ’, the pieces of link failure information R 52  and R 54  include the common ring IDs ‘ 10 ’ and ‘ 20 ’ suggesting the occurrence of a failure on the common link shared by the rings  10  and  20 , while the pieces of link failure information R 55 , R 56 , and R 60  include the common ring IDs ‘ 20 ’ and ‘ 30 ’ suggesting the occurrence of a failure on the common link shared by the rings  20  and  30 . Based on detection of the two different locations with regard to the identical monitoring ring ID ‘ 20 ’, it is determined that the ring  20  is in a multi-failure state having failures at two different positions. The CPUs  110  in the common link terminal devices  4104 ,  4105 , and  4108  accordingly determine the occurrence of failures at multiple different locations with regard to an identical monitoring ring ID at step S 102  and proceed to step S 104 . 
     In the state of  FIG. 20 , the auxiliary monitor frames  4113  and  4114  sent by the common link terminal devices  4104  and  4105  include the ID ‘ 20 ’ representing the ring  20  as the ID information, while the pieces of link failure information R 52 , R 54 , and R 56  shown in  FIGS. 21 and 22  include the monitoring ring ID ‘ 20 ’. The auxiliary monitor frame  4117  sent by the common link terminal device  4108  includes the ID ‘ 30 ’ representing the ring  30  as the ID information, while the pieces of link failure information R 57  and R 58  shown in  FIG. 24  include the monitoring ring ID ‘ 30 ’. The CPUs  110  in the common link terminals  4104 ,  4105 , and  4108  accordingly determine that any of the monitoring ring IDs is identical with the ID represented by the ID information included in the auxiliary monitor frames at step S 104  and proceed to step S 106 . 
     In the state of  FIG. 20 , the pieces of link failure information R 51  to R 54  include the common ring IDs ‘ 10 ’ and ‘ 20 ’ and the pieces of link failure information R 55  to R 58  include the common ring IDs ‘ 20 ’ and ‘ 30 ’ as shown in  FIGS. 21 and 22 . There is a number greater than the ID ‘ 20 ’ represented by the ID information included in the auxiliary monitor frames  4113  and  4114 . The CPUs  110  in the common link terminals  4104  and  4105  accordingly determine that there is a ring in failure having an ID greater in number than the ID represented by the ID information in the auxiliary monitor frames  4113  and  4114  at step S 106  and proceed to step S 110 . 
     In the state of  FIG. 20 , the common link terminal devices  4104  and  4105  send the auxiliary monitor frame  4113  or  4114  to only one ring  20 . The CPUs  110  in the common link terminal devices  4104  and  4105  accordingly identify no transmission of multiple auxiliary monitor frames at step S 110  and proceed to step S 114 . The CPUs  110  in the common link terminal devices  4104  and  4105  control the communication unit  130  to stop the transmission of the auxiliary monitor frames  4113  and  4114  at step S 114 . The common link terminal devices  4104  and  4105  accordingly stop the transmission of the auxiliary monitor frames  4113  and  4114 . 
     In the state of  FIG. 20 , the pieces of link failure information R 52  and R 54  include the common ring IDs ‘ 10 ’ and ‘ 20 ’ and the pieces of link failure information R 56  to  58  and R 60  include the common ring IDs ‘ 20 ’ and ‘ 30 ’ as shown in  FIG. 24 . There is no ID having a greater number than the ID ‘ 30 ’ represented by the ID information included in the auxiliary monitor frame  4117 . The CPU  110  in the common link terminal device  4108  accordingly determines that there is no ring in failure having an ID greater in number than the ID represented by the ID information in the auxiliary monitor frame  4117  at step S 106  and proceeds to step S 108 . 
     In the state of  FIG. 20 , no null data is given as the common ring IDs in any of the pieces of link failure information R 52 , R 54 , R 56  to R 58 , and R 60  as shown in  FIG. 24 . The CPU  110  in the common link terminal device  4108  accordingly detects no occurrence of a failure on any link other than the common link at step S 108  and proceeds to step S 116 . The CPU  110  in the common link terminal device  4108  controls the communication unit  130  to continue transmission of the auxiliary monitor frame  4117  at step S 116 . 
     In response to non-reception of the monitor frame  4111  sent by itself for a preset time period, the monitor device  4101  in the ring  10  recognizes the occurrence of a failure on the ring  10 , cancels the blocking of a specific port  4121 , and sends an FDB flush frame  4142 . 
     In response to non-reception of the auxiliary monitor frames from the common link terminal devices  4104  and  4105 , the auxiliary monitor device  4107  in the ring  20  recognizes the occurrence of a failure on the ring  20 , cancels the blocking of a specific port  4122 , and sends an FDB flush frame  4143 . 
     The respective devices included in the ring  10  receive the FDB flush frame  4142 , delete the FDBs stored therein, and urge the communication network to change the communication route. The respective devices included in the ring  20  receive the FDB flush frame  4143 , delete the FDBs stored therein, and urge the communication network to change the communication route. 
     Even in the occurrence of the failures  4131  and  4132 , the respective devices included in the ring  30  enable continuous communication without any specified operations. 
     As described above, in the communication network of the fourth embodiment having a large ring covering over multiple rings, in the multiple-failure state with the failures  4131  and  4132  on plural common links, the common link terminal devices  4104  and  4105  are controlled to stop transmission of the auxiliary monitor frames  4113  and  4114  to the ring  20  of the smaller ID number. In response to non-reception of the monitor frame  4112  for a preset time period and non-reception of the auxiliary monitor frames  4113  and  4114 , the auxiliary monitor device  4107  in the ring  20  cancels the blocking of the specific port  4122 . Cancellation of the blocking of the specific port  4122  in this manner enables continuous communication without increasing the number of relay devices as passing points of a monitor frame. This arrangement exerts the similar effects to those of the first through the third embodiments described above. The presence of the large ring covering over the multiple rings desirably forms detours in the event of multiple failures. 
     E. General Flow of Embodiments 
     As described above, in response to detection of a link failure in a ring, each of the relay devices, the monitor devices, and the common link terminal devices (hereafter referred to as ‘each device’) sends a failure detection frame to the ring. This transmits link failure information to the common link terminal device. The common link terminal device generates a link failure information management database, performs the multi-failure detection process, and stops transmission of an auxiliary monitor frame in the event of detection of multiple failures. 
     In the event of a link failure, the failure detection frame is sent by each device and is thus to be sent to and received from a blocked port in the monitor device. In order to avoid the loop, the failure detection frame is to be discarded in each device. When a common port shared by multiple rings has a failure, failure detection frames are sent to all the rings as the monitoring objects. 
     The failure detection frame includes information on the ring monitoring the port with a failure and information on the rings sharing the common link. 
     In response to reception of the failure detection frame, the common link terminal device refers to the own link failure information management database, performs the multi-failure detection process, and stops transmission of the auxiliary monitor frame in the event of detection of multiple failures. The common link terminal device also performs this series of operations in the event of a link failure in the common link terminal device. 
     In response to non-reception of a monitor frame sent by itself for a preset time period and non-reception of the auxiliary monitor frame from the common link terminal device, the auxiliary monitor device cancels the blocking of its own specific port. The blocking of the port is not cancelled as long as the auxiliary monitor device receives the monitor frame sent by itself. After cancellation of the blocking, the auxiliary monitor frame sends a control frame (FDB flush frame) for clearing the information table (FDB) stored in each device of the ring for controlling the forwarding destinations. In response to reception of the FDB flush frame, the common link terminal device sends an FDB flush frame to the ring with the multiple failures. 
     In the case of recovery of a link from a failure, each device blocks the recovered link and prohibits transmission of user frames until reception of an FDB flush frame or the user&#39;s blocking cancellation operation. 
     In response to reception of the FDB flush frame, the common link terminal device refers to the own link failure information management database and deletes information on the failed link. The common link terminal device then performs the multi-failure detection process and resumes transmission of the auxiliary monitor frame upon determination of recovery from multiple failures. 
     In response to reception of the auxiliary monitor frame from the common link terminal device or reception of the monitor frame sent by itself, the auxiliary monitor device blocks one of ring connection ports. After the blocking, the auxiliary monitor device sends a control frame (FDB flush frame) for clearing the FDB. 
     In the embodiments described above, the system detects the occurrence of multiple failures based on accumulation and analyses of information reported by failure detection frames and stop transmission of auxiliary monitor frames in the event of detection of multiple failures. This arrangement releases the blocked port and enables continuous communication without increasing the number of relay devices as passing points of a monitor frame. The system effectively allows continuous communication even in the event of multiple failures in a communication network of a multi-ring structure. 
     The embodiments discussed above are to be considered in all aspects as illustrative and not restrictive. There may be many modifications, changes, and alterations without departing from the scope or spirit of the main characteristics of the present invention. The scope and spirit of the present invention are indicated by the appended claims, rather than by the foregoing description.