Abstract:
A first communication apparatus to be coupled to a second communication apparatus and a third communication apparatus via a ring network, including: a first port to coupled to the ring network; and a second port to coupled to the ring network; wherein the first port and the second port is set to receive a control frame from the second communication apparatus and the third communication apparatus, and either the first port or the second port is set to be disabled to relay a user frame.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This is a continuation of U.S. application Ser. No. 11/833,320, filed Aug. 3, 2007 now U.S. Pat. No. 8,255,575. This application relates to and claims priority from Japanese Patent Application No. 2006-321021, filed on Nov. 29, 2006. The entirety of the contents and subject matter of all of the above is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention generally relates to an apparatus which provides redundancy of communication routes in a ring network. More specifically, the present invention is directed to such an apparatus capable of realizing high-speed switching of routes, and capable of simply coupling a plurality of ring networks, and also capable of independently performing operations in each of these ring networks so as to realize constructions of more flexible networks, and further, capable of reducing loads given to a CPU. 
     2. Description of the Related Art 
     While networks are constructed, there are some possibilities that a plurality of apparatuses are connected to each other in ring structures in order to make communication routes redundant. Networks having ring structures have a feature that necessary amounts of transfer routes and the like may be reduced, as compared with those of networks having mesh structures. On the other hand, since the networks are structured in ring shapes, when broadcast packets flow through the networks having the ring structures, transfer operations of these broadcast packets are repeatedly carried out in an infinite time, so that such a loop is produced in which the broadcast packets are continuously circulated in the same route for a long time. As a result, traffic loads on the networks caused by broadcast streams are increased, or CPU loads caused by unstable conditions of learning are increased, which may give adverse influences to communications. In order to avoid the occurrence of this loop, various methods capable of logically cutting off loop structures have been proposed in networks having ring structures. 
     In the spanning tree protocol described in IEEE Std 802.1D, 1998 Edition, such a control frame called as “BPDU” is periodically exchanged among apparatuses which construct a network in order to cut off a loop structure. While a plurality of protocol information used to determine operations of the protocol have been superimposed on the BPDU, each of these apparatuses which receive this BPDU changes logical statuses in ports which constitute a loop into blocking statuses based upon the protocol information of the received BPDU, and constructs such a network having a tree structure, so that the loop is logically cut off. 
     While the redundant protocol described in JP-A-2004-201009 is specifically used in a ring network, one structural apparatus among apparatuses which constitutes the ring network is defined as a monitoring apparatus, whereas other structural apparatuses are defined as relay apparatuses. In the monitoring apparatus, a control frame is periodically transmitted from an one-sided port thereof, and a reception of this transmitted is monitored at an opposite-sided port thereof. While the control frame is being received at the opposite-sided port, the monitoring apparatus transmits another control frame from this opposite-sided port, and sets the opposite-sided port to such a condition that a user frame cannot be relayed. As a consequence, the loop structure is logically cut off. If the control frame cannot be received at the opposite-sided port, then the monitoring apparatus stops the transmission of the control frame from this opposite-sided port, and permits the relay of the user frame. As a result, the communication routes can be made redundant. 
     Within the above-described conventional techniques, since the spanning tree protocol disclosed in IEEE Std 802.1D 1988 Edition may also be directed to complex network structures, the tree structure must be determined based upon the plurality of information superimposed on BPDU exchanged among the apparatuses, and thus, the protocol operations become complex. As a result, even when a structure of a ring network is made simple, the below-mentioned problems may occur. That is, while the structure of such a simple ring network is changed when a failure occurs and the failure is recovered within this simple ring network, and also while apparatuses are additionally conducted to the simple ring network, there are some possibilities that several tens of seconds are required until communications are stabilized, during which the communications are stopped. Also, since it is practically difficult to realize the complex protocol operations by employing hardware, these complex protocol operations are realized by employing software. As a result, if loads given to a CPU are increased, then there is another problem that an adverse influence may be given to the complex protocol operations. 
     SUMMARY OF THE INVENTION 
     Further, as to the redundant protocol described in JP-A-2004-201009 within the conventional techniques, the problems occurred in the spanning tree protocol have been solved by specifying the redundant protocol to the ring network. However, this redundant protocol has the below-mentioned problems to be solved. That is, since the control frame is transmitted from only one-sided port (namely, only one direction) of the monitoring apparatus provided in the ring network, in such a case that a failure of one direction happens to occur in a link which constitutes the ring network, the monitoring apparatus judges that a ring status is under abnormal condition, and thus, permits the opposite-sided port to relay a user frame, so that a ring status of the other direction is brought into a loop status. Furthermore, in another network structure that a plurality of ring networks are coupled to each other and coupled apparatuses become redundant, there is such a limitation that an apparatus at the coupling portion must be a monitoring apparatus. As a consequence, there is another problem to be solved that the network structure cannot have a flexible network structure. Also, as a problem specific to a ring network, in the case that the plurality of ring networks are coupled to each other and apparatuses of coupling portions are made redundant, if a failure happens to occur in a link between the apparatuses made redundant, then the following problem occurs. That is, in the coupled ring networks, there are certain possibilities that a large loop structure may be formed. 
     An object of the present invention has been made to solve the above-described problems, and therefore, is to provide an apparatus capable of realizing high-speed switching of communication routes, capable of preventing an occurrence of a loop which is caused by an one-directional failure of a link within a ring network, and capable of realizing a simple and flexible structure constructed by coupling a plurality of ring networks, and further, capable of avoiding a construction of a loop caused by a failure of a specific link. 
     To achieve the above-described object, an apparatus according to the present invention is featured as follows: That is, as apparatuses which construct a ring network, such a structure made of a monitoring apparatus and other relay apparatuses is employed. The monitoring apparatus monitors only a ring status of such a ring network to which the own monitoring apparatus belongs. The monitoring apparatus periodically transmits health check frames from ring ports thereof respectively so as to monitor whether or not the transmitted health check frame has been received by the ring port located opposite thereto. Also, in a structure of a multi-ring network having a shared link, auxiliary health check frames are periodically transmitted from two sets of shared apparatuses respectively with respect to a monitoring apparatus of a shared link non-monitored ring network. The monitoring apparatus also monitors the auxiliary health check frames in combination with the health check frames in order to avoid an occurrence of a loop which bridges a plurality of ring networks. Since the above-described apparatus is provided, the above-described object may be achieved. 
     In accordance with the present invention, the monitoring apparatus which constructs the ring network judges whether or not relaying of a user frame is permitted by checking whether or not the periodically transmitted health check frame is received. As a consequence, the high-speed switching of the routes can be realized. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1A  and  FIG. 1B  are diagrams for schematically showing a monitoring apparatus employed in a ring network constructing apparatus according to an embodiment of the present invention. 
         FIG. 2A  and  FIG. 2B  are diagrams for indicating a relay apparatus employed in the ring network constructing apparatus of the embodiment. 
         FIG. 3A  and  FIG. 3B  are diagrams for representing a monitoring apparatus in a shared link non-monitored ring network of the ring network constructing apparatus of the embodiment. 
         FIG. 4A  and  FIG. 4B  are diagrams for showing a relay apparatus (which is commonly operable as shared apparatus) in the shared link non-monitored ring network of the ring network constructing apparatus of the embodiment. 
         FIG. 5A  and  FIG. 5B  are diagrams for showing a relay apparatus in the shared link non-monitored ring network of the ring network constructing apparatus of the embodiment. 
         FIG. 6  is a structural diagram of a single ring network with employment of the ring network constructing apparatus of the embodiment. 
         FIG. 7  is another structural diagram of the single ring network with employment of the ring network constructing apparatus of the embodiment. 
         FIG. 8  is a structural diagram of a multi-ring network (having no shared link) with employment of the ring network constructing apparatus of the embodiment. 
         FIG. 9  is another structural diagram of the multi-ring network (having no shared link) with employment of the ring network constructing apparatus of the embodiment. 
         FIG. 10  is a structural diagram of a multi-ring network (having shared link) with employment of the ring network constructing apparatus of the embodiment. 
         FIG. 11  is another structural diagram of the multi-ring network (having shared link) with employment of the rink network constructing apparatus of the embodiment. 
         FIG. 12  is a structural diagram of a multi-ring network (having no shared link) with employment of the ring network constructing apparatus of the embodiment. 
         FIG. 13  is a structural diagram of a multi-ring network (having no shared link) with employment of the ring network constructing apparatus of the embodiment. 
         FIG. 14  is a structural diagram of a multi-ring network (having no shared link) with employment of the ring network constructing apparatus of the embodiment. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring now to drawings, a detailed description is made of a ring network constructing apparatus according to an embodiment of the present invention. 
       FIG. 1A  and  FIG. 1B  schematically show an apparatus  101  (will be referred to as “monitoring apparatus” hereinafter) which monitors a ring network employed in a ring network constructing apparatus according to an embodiment of the present invention. The monitoring apparatus  101  constitutes a portion of a ring network indicated, for example, in  FIG. 6 , and corresponds to an apparatus  601  which monitors this ring network. The ring network is arranged by the monitoring apparatus  601  and other relay apparatuses  602 ,  603 , and  604 . Although a detailed description as to  FIG. 6  will be made later, in this example, for instance, operations of the monitoring apparatus  601  in the ring network of  FIG. 6  are described with reference to  FIG. 1A  and  FIG. 1B . The monitoring apparatus  101  contains two pieces of ports (will be referred to as “ring ports” hereinafter)  102  and  103 , which constitute the same ring network. The monitoring apparatus  101  periodically transmits a control frame  111  from the ring port  102  so as to monitor whether or not a health check frame  113  via other apparatuses which constitute the ring network has been received at the ring port  103 . The control frame  111  (will be referred to as “health check frame” hereinafter) monitors such a status (will be referred to as “ring status” hereinafter) as to whether or not the ring network has performed a communicating operation under normal condition, and after the health check frame  113  has been received, this health check frame  113  is discarded. Similarly, the monitoring apparatus  101  periodically transmits a health check frame  112  from the ring port  103  so as to monitor whether or not this health check frame  114  has been received at the ring port  102 , and after this health check frame  114  has been received, the received health check frame  114  is discarded. While any one of the health check frame  113  and the health check frame  114  is being received, the monitoring apparatus  101  judges that the ring status is under normal condition, and brings the ring port  103  into such a condition that a user frame is not transmitted/received in order not generate a loop ( FIG. 1A ). When both the health check frames  113  and  114  cannot be received, the monitoring apparatus  101  judges that the ring status is under abnormal condition, and transfers the ring port  103  to such a condition that the user frame is transmitted/received ( FIG. 1B ). In this example, the ring port  103  controls whether or not the transmission/reception of the user frame are permitted in response to the ring status. Alternatively, the ring port  102  may control whether or not the transmission/reception of the user frame are permitted in response to the ring status. A ring port for controlling whether or not the transmission/reception of the user frame are permitted may be automatically determined based upon the numbers of the two ring ports, or may be alternatively designated based upon a configuration by a user. 
     It should also be noted that in the monitoring apparatus  101 , the health check frames  111  and  112  are periodically transmitted from the two ring ports  102  and  103 , and the reception of the health check frames  111  and  112  are monitored at the ring ports which are different from those used in the frame transmissions. Alternatively, the health check frames  111  and  112  may be periodically transmitted only from any one of the ring ports  102  and  103 , and the received health check frames  111  and  112  may be monitored only at the other ring port thereof. Also, at this time, while the health check frames  111  and  112  can be received at the other ring port, the monitoring apparatus  101  may judge that the ring status is under normal condition, and defines this ring status as such a status (which is painted in solid condition in drawings) that a user frame is not transmitted/received at any one of these two ring ports  102  and  103 . Also, another status that the user frame is transmitted/received at the other ring port thereof is indicated in a blank condition. When the health check frames  112  and  113  cannot be received at the other ring port, the monitoring apparatus  101  judges that the ring status is under abnormal condition, and changes such a ring port from which the transmissions/reception of the user frame are not permitted into a condition under which the user frame is transmitted/received. Thereafter, the monitoring apparatus  102  clears MAC address information and transmits flush request frames from the two ring ports  102  and  103 , and when this flush request frame is received at the opposite ring port, the received flush request frame may be discarded. 
     Further, in the case that in the monitoring apparatus  101 , the ring status is changed (ring status is changed from normal condition to abnormal condition, or from abnormal condition to normal condition) by permitting, or not permitting the receptions of the health frames  113  and  114 , the monitoring apparatus  101  transmits a control frame  131  (will be referred to as “flush request frame” hereinafter) from the ring port  102  with respect to other apparatus (will be referred to as “relay apparatus” hereinafter) which constitutes the ring network, and when a flush request frame  132  is received at the ring port  103 , the monitoring apparatus  101  discards the received flush request frame  132 . The above-described control frame  131  instructs to clear MAC address information which has been studded by receiving a user frame with respect to the relay apparatus. Similarly, in such a case that the monitoring apparatus  101  transmits a flush request frame  133  from the ring port  103  and receives a flush request frame  134  at the ring port  102 , the monitoring apparatus  101  discards the received flush request frame  134 . In the monitoring apparatus  101 , when the ring status is changed, this monitoring apparatus  101  also clears the MAC address information which has been learned by receiving the user frame. Generally speaking, MAC address information which has been learned by receiving a user frame implies, for example, such MAC address information that port numbers for transferring combinations between MAC address and VLAN information to a key have been held as a table. 
       FIG. 2A  and  FIG. 2B  show another relay apparatus  201  which constitutes the ring network. The relay apparatus  201  corresponds to, for example, relay apparatuses  602 ,  603 , and  604  other than a monitoring apparatus  601 , which constitute a ring network shown in  FIG. 6 . The relay apparatus  201  contains two ring ports  202  and  203 , which constitute the same ring network. When a health check frame  211  transmitted from a monitoring apparatus is received at the ring port  202 , the relay apparatus  201  relays a health check frame  212  to the ring port  203 . Similarly, when a health check frame  213  is received at the ring port  203 , the relay apparatus  201  relays a health check frame  214  to the ring port  202  provided on the opposite side ( FIG. 2A ). In the relay apparatus  201 , only health check frames are relayed, but are not monitored. 
     Also, for example, in such a case that due to a failure of the ring network, a ring port to which a received health check frame is transferred is brought into a link down condition, the health check frame cannot be transferred to a succeeding apparatus from a relay apparatus brought into the link down condition. As a result, the health check frame is not transferred up to a monitoring apparatus. In this case, the monitoring apparatus judges that the ring status is under abnormal condition. However, if the network failure is recovered and the link down condition is changed into a link up condition, then a health check frame can be again transferred. As a result, the monitoring apparatus judges that the ring status becomes a normal condition. 
     When a flush request frame  221  transmitted from a monitoring apparatus is received at the ring port  202  in the relay apparatus  201 , the relay apparatus  201  relays a flush request frame  222  to the ring port  203 , and at the same time, clears MAC address information which has been learned by receiving a user frame. Similarly, when a flush request frame  223  transmitted from a monitoring apparatus is received at the ring port  203  in the relay apparatus  201 , the relay apparatus  201  relays a flush request frame  224  to the ring port  202  provided on the opposite side, and at the same time, clears the MAC address information which has been learned by receiving the user frame ( FIG. 2B ). It should also be noted that when the relay apparatus  201  has already cleared the MAC address information by receiving the previous flush request frame  221 , the relay apparatus  201  does not clear the MAC address information by receiving the flush request frame  223 . 
     When either the ring port  202  or the ring port  203  has been brought into the link down condition, the condition of the relay apparatus  201  is brought into such a condition that a control frame and a user frame are not relayed. However, when this link down condition is recovered to a link up condition, the condition of the relay apparatus  201  is changed into such a condition that the control frame is relayed, and the user frame is not relayed. This reason is given as follows: That is, when the condition of the relay apparatus  201  is brought into such a condition that the relay apparatus  201  relays the user frame just after the link down condition is recovered to the link up condition, if the monitoring apparatus has still set to such a condition that one of the ring ports  202  and  203  is used to transmit/receive the user frame, then a loop may occur. Then, under this condition, when the relay apparatus  201  receives any one of the flush request frames  221  and  223  transmitted from the monitoring apparatus, the condition of the relay apparatus  201  is changed into such a condition that the user frame is relayed. Otherwise, even when a flush request frame has not yet been received, after a predetermined time has passed, the condition of the relay apparatus  201  is changed into the condition that the user frame is relayed. 
     Next, a description is made of a network structure in which a plurality of ring networks have been coupled to each other, which will be referred to as a multi-ring network hereinafter. The multi-ring network has been arranged in such a manner that, for instance, as represented in  FIG. 10 , two sets of ring networks  1061  and  1062  have been coupled to each other by employing shared apparatuses  1002  and  1003 . In this case, 2 sets, or more sets of ring networks to be coupled to each other may be alternatively employed, and 2 pieces, or more pieces of shared apparatuses may be alternatively employed. In the two ring networks  1061  and  1062 , monitoring apparatuses  1001  and  1005  are present respectively. While control frames which are transmitted by the monitoring apparatuses  1001  and  1005  are transferred only within the respective ring networks  1061  and  1062 , both the control frames of the monitoring apparatuses  1001  and  1005  are transferred between the shared apparatus  1002  and the shared apparatus  1003  (will be referred to as “shared link” hereinafter). In the present embodiment, while the monitoring apparatuses  1001  and  1005  monitor ring status of the respective ring networks  1061  and  1062 , as to the shared link, only the monitoring apparatus  1001  monitors, whereas the monitoring apparatus  1005  monitors ring status of the ring network other than the shared link within the ring network  1062 . As a consequence, these ring networks are divided into a ring network (will be referred to as “shared link monitored ring network” hereinafter)  1061  which monitors the ring status containing also the shared link, and another ring network (will be referred to as “shared link non-monitored ring network” hereinafter)  1062  which monitors the ring status which do not contain the shared link. At this time, the shared link monitored ring network  1061  and the shared link non-monitored ring network  1062  correspond to independent ring networks respectively. It should also be noted that a detailed description as to  FIG. 10  will be discussed later. 
     Next, a description is made of operations of the respective apparatuses which constitute the multi-ring network. Since the shared link monitored ring network  1061  is identical to such a ring network shown in  FIG. 6 , the monitoring apparatus  1001  performs the same operation as that of the monitoring apparatus  101  described in  FIG. 1A  and  FIG. 1B , whereas the shared apparatuses  1002  and  1003 , and the relay apparatus  1004  perform the same operations as those of the relay apparatus  201  explained in  FIG. 2A  and  FIG. 2B . As a result, the shared link monitored ring network  1061  is monitored. 
     Next, a description is made of operations of the respective apparatuses employed in the shared link non-monitored ring network  1062 . 
       FIG. 3A  and  FIG. 3B  indicate a monitoring apparatus  301  provided in the shared link non-monitored ring network  1062 . The monitoring apparatus  301  contains two pieces of ring ports  302  and  303 , which constitute the same ring network. The monitoring apparatus  301  periodically transmits a control frame  311  (will be referred to as “health check frame” hereinafter) for monitoring a ring status from the ring port  302  via a relay apparatus so as to monitor whether or not a health check frame  312  has been received at the ring port  302 , and after the health check frame  312  has been received, this health check frame  312  is discarded. Similarly, the monitoring apparatus  301  periodically transmits a health check frame  313  from the ring port  303  so as to monitor whether or not this health check frame  314  has been received at the ring port  302 , and after this health check frame  314  has been received, the received health check frame  314  is discarded. Also, in combination with this operation, the monitoring apparatus  301  monitors both an auxiliary health check frame (will be referred to as “auxiliary health check frame” hereinafter)  315 , and another auxiliary health check frame  316 , and after these auxiliary health check frames  315  and  316  have been received, these received auxiliary health check frames  315  and  316  are discarded. The auxiliary health check frames  315  and  316  are periodically transmitted from a shared apparatus  401  which will be discussed later in  FIG. 4A  and  FIG. 4B  with respect to the ring port  302  and to the ring port  303  respectively. While any one of the health check frame  312  and the health check frame  314  is being received, the monitoring apparatus  301  judges that the ring status is under normal condition, and sets the ring port  302  into such a condition that a user frame is not transmitted/received ( FIG. 3A ). Although both the health check frames  312  and  314  cannot be received, while both the auxiliary health check frames  315  and  316  are being received, the monitoring apparatus  301  judges that the ring status is under normal condition, and sets the ring port  303  to such a condition  321  that the user frame is not transmitted/received, since it is conceivable that the apparatuses other than the shared link which should be monitored are under normal conditions. When both the health check frames  312  and  314  cannot be received, but also, any one of the auxiliary health check frames  315  and  316  cannot be received, the monitoring apparatus  301  judges that the ring status is under abnormal condition, and thus, transfers the ring port  303  to such a condition  322  that the user frame is transmitted/received ( FIG. 3B ). In this case, although the ring port  303  controls whether or not the transmission/reception of the user frame are permitted in response to the ring status, the ring port  302  may control whether or not the transmission/reception of the user frame are permitted in response to the ring status. A ring port for controlling whether or not the transmission/reception of the user frame are permitted may be automatically determined based upon the numbers of the two ring ports, or may be alternatively designated based upon a configuration by a user. 
     Further, in the case that in the monitoring apparatus  301 , the ring status is changed (ring status is changed from normal condition to abnormal condition, or from abnormal condition to normal condition) by permitting, or not permitting the receptions of the health frames  312  and  314 , the monitoring apparatus  301  transmits a control frame (will be referred to as “flush request frame” hereinafter)  331  from the ring port  302  with respect to other apparatuses which constitute the ring network, and when a flush request frame  332  is received at the ring port  303 , the monitoring apparatus  301  discards the received flush request frame  332 . The above-described control frame  331  instructs to clear MAC address information which has been learned by receiving a user frame with respect to other apparatuses. Similarly, in such a case that the monitoring apparatus  301  transmits a flush request frame  333  from the ring port  303  and receives a flush request frame  334  at the ring port  302 , the monitoring apparatus  301  discards the received flush request frame  334 . In the monitoring apparatus  301 , this monitoring apparatus  301  also clears the MAC address information which has been learned by receiving the user frame. 
       FIG. 4A  and  FIG. 4B  show a shared apparatus  401  employed in the shared link non-monitored ring network  1062 . The shared apparatus  401  has two ring ports  402  and  403 , which constitute the same ring network. The ring port  403  corresponds to a port which is used as a shared link with another ring network. The shared apparatus  401  periodically transmits an auxiliary health check frame  415  from the ring port  402  toward the monitoring apparatus  301  of the ring network, which does not monitor the shared link. Also, since the shared apparatus  401  also constitutes the relay apparatus, when a health check frame  411  transmitted from the monitoring apparatus  301  is received at the ring port  402 , the shared apparatus  401  relays a health check frame  412  to the ring port  403 . Also, similarly, when a health check frame  413  is received at the ring port  403 , the shared apparatus  401  relays a health check frame  414  to the ring port  402  provided on the opposite side ( FIG. 4A ). 
     It should also be noted that such a fact whether or not a health check frame transmitted from the monitoring apparatus  301  of the shared link non-monitored ring network  1062  has been received may be alternatively superimposed on the auxiliary health check frame  415  which is transmitted from the shared apparatus  401 . Also, while the shared apparatus  401  does not periodically transmit the auxiliary health check frame  415  which is transmitted from the shared apparatus  401 , the shared apparatus  401  may alternatively transfer such a health check frame transmitted from the monitoring apparatus  301  to the next apparatus, and also may fold back this transferred health check frame, and then, may alternatively transmit the folded health check frame to the monitoring apparatus  301  as the auxiliary health check frame  415 . 
     Also, when the shared apparatus  401  receives a flush request frame  421  at the ring port  402 , which is transmitted from the monitoring apparatus  301 , the shared apparatus  401  relays a flush request frame  422  to the ring port  403 , and at the same time, clears MAC address information which has been learned by receiving a user frame. Similarly, when the shared apparatus  401  receives a flush request frame  423  at the ring port  403 , the shared apparatus  401  relays a flush request frame  424  to the ring port  402  provided on the opposite side, and at the same time, clears the MAC address information. It should be noted that when the shared apparatus  401  has already cleared the MAC address information by receiving the previous flush request frame  421 , the shared apparatus  401  does not clear the MAC address information by receiving the flush request frame  423 . 
     When either the ring port  402  or the ring port  403  has been brought into the link down condition, the condition of the shared apparatus  401  is brought into such a condition that a control frame and a user frame are not relayed. However, when this link down condition is recovered to a link up condition, the condition of the shared apparatus  401  is changed into such a condition that the control frame is relayed, and the user frame is not relayed. This reason is given as follows: That is, when the condition of the shared apparatus  401  is brought into such a condition that the shared apparatus  401  relays the user frame just after the link down condition is recovered to the link up condition, if the monitoring apparatus has still set to such a condition that one of the ring ports  402  and  403  is used to transmit/receive the user frame, then a loop may occur. Then, under this condition, when the shared apparatus  401  receives any one of the flush request frames  421  and  423  transmitted from the monitoring apparatus, the condition of the shared apparatus  401  is changed into such a condition that the user frame is relayed. Otherwise, even when a flush request frame has not yet been received, after a predetermined time has passed, the condition of the shared apparatus  401  is changed into the condition that the user frame is relayed. 
       FIG. 5A  and  FIG. 5B  indicate a relay apparatus  501  provided in the shared link non-monitored ring network  1062 . The relay apparatus  501  contains two ring ports  502  and  503 , which constitute the same ring network. When the relay apparatus  501  receives either a health check frame  511  transmitted from a monitoring apparatus or an auxiliary health check frame  515  transmitted from a shared apparatus, the relay apparatus  501  relays either a health check frame  512  or a health check frame  516  to the ring port  503 . Similarly, when the relay apparatus  501  receives the health check frame  513  at the ring port  503 , the relay apparatus  501  relays a health check frame  514  to the ring port  202  located opposite side thereof ( FIG. 5A ). 
     Also, when the relay apparatus  501  receives a flush request frame  521  at the ring port  502 , which is transmitted from the monitoring apparatus  501 , the relay apparatus  501  relays a flush request frame  522  to the ring port  503 , and at the same time, clears MAC address information which has been learned by receiving a user frame. Similarly, when the relay apparatus  501  receives a flush request frame  523  at the ring port  503 , the relay apparatus  501  relays a flush request frame  524  to the ring port  502  provided on the opposite side, and at the same time, clears the MAC address information. It should be noted that when the relay apparatus  501  has already cleared the MAC address information by receiving the previous flush request frame  521 , the relay apparatus  501  does not clear the MAC address information by receiving the flush request frame  523 . 
     When either the ring port  502  or the ring port  503  has been brought into the link down condition, the condition of the relay apparatus  501  is brought into such a condition that a control frame and a user frame are not relayed. However, when this link down condition is recovered to a link up condition, the condition of the relay apparatus  501  is changed into such a condition that the control frame is relayed, and the user frame is not relayed. This reason is given as follows: That is, when the relay apparatus  501  is brought into such a condition that the relay apparatus  501  relays the user frame just after the link down condition is recovered to the link up condition, if the monitoring apparatus has still set to such a condition that one of the ring ports  502  and  503  is used to transmit/receive the user frame, then a loop may occur. Then, under this condition, when the relay apparatus  501  receives any one of the flush request frames  521  and  523  transmitted from the monitoring apparatus, the condition of the relay apparatus  501  is changed into such a condition that the user frame is relayed. Otherwise, even when a flush request frame has not yet been received, after a predetermined time has passed, the condition of the relay apparatus  501  is changed into the condition that the user frame is relayed. 
     In the relay apparatus  501  and the shared apparatus  401 , when a link condition of a ring port is changed, a control frame (will be referred to as “link condition change notification frame” hereinafter) may be alternatively transmitted from a ring port under normal condition, while this control frame notifies the change in the link condition. Also, the monitoring apparatus  301  may not only monitor the ring status based upon either the health check frame or the auxiliary health check frame, but also may monitor the ring status, while including the reception of the link condition change notification frame which is transmitted from either the relay apparatus  501  or the shred apparatus  401  in addition to these health check frames. It is so assumed that the transmission time periods as to the health check frames and the auxiliary health check frames, and the time durations required for judging that the ring status are under the abnormal conditions since the health check frames are not yet received may be changed based upon a configuration by a user. These transmission time periods and time durations have been described in the monitoring apparatuses  101  and  301  shown in  FIG. 1A ,  FIG. 1B ,  FIG. 3A ,  FIG. 3B ; and also in the shared apparatus  401  shown in  FIG. 4A ,  FIG. 4B . 
     While the monitoring apparatuses  101  and  301  explained in  FIG. 1A ,  FIG. 1B ,  FIG. 3A , and  FIG. 3B  are equipped with hardware circuits, these hardware circuits transmit/receive various sorts of control frames and user frames from ring ports, and also, monitor whether or not either a health check frame or an auxiliary health check frame is received within a predetermined time at a ring port. In such a case that either the health check frame or the auxiliary health check frame has not yet been received for the predetermined time, the monitoring apparatuses  101  and  301  judge that a ring status is brought into an abnormal condition. Also, while the monitoring apparatuses  101  and  301  monitor the receptions of the health check frame and the auxiliary health check frame under such a condition that the ring status is under abnormal condition, the monitoring apparatus  101  and  301  judge that the ring status is under normal condition by receiving these health check frames. It should also be noted that the above-described functions may be alternatively realized by employing software. 
     While the relay apparatuses  201  and  501  shown in  FIG. 2A ,  FIG. 2B ,  FIG. 5A , and  FIG. 5B , and also, the shared apparatus  401  shown in  FIG. 4A  and  FIG. 4B  are equipped with hardware circuits, these hardware circuits transmit/receive various sorts of control frames and user frames, and also, judge that the ring status is changed. 
     It should also be noted that the above-described functions may be alternatively realized by employing software. 
     Based upon  FIG. 6 , a description is made of operations as to respective apparatuses employed in a single ring network (will be referred to as “single ring network” hereinafter). In  FIG. 6 , among apparatuses  601  to  604 , according to an embodiment of the present invention, which constitute the single ring network, a monitoring apparatus is defined as  601 , and relay apparatuses are defined as  602  to  604 . Although the single ring network is constituted by employing 4 sets of the apparatuses  601  to  604 , there is no limitation as to a total number of these apparatuses. 
     The monitoring apparatus  601  periodically transmits health check frames  611  and  615  from two ring ports thereof. These health check frames  611  and  615  are relayed in the relay apparatuses  602  to  604 , and then, the health check frames  611  and  615  are received at ring ports which are different from the above-described ring ports for transmitting these health check frames  611  and  615 . In the case that the health check frame  611 , or  615  is being received at any one of these two ring ports, the monitoring apparatus  601  judges that a ring status is under normal condition, and sets such a condition that a user frame is not transmitted/received at a ring port  621  thereof. 
       FIG. 7  indicates a condition that a failure  751  occurs in the single ring network of  FIG. 6 . When the failure  751  happens to occur, a health check frame  711  transmitted from a monitoring apparatus  701  cannot be relayed from a relay apparatus  702  to a succeeding apparatus. Similarly, a health check frame  715  cannot be also relayed from a relay apparatus  703  to a succeeding apparatus. As a result, the monitoring apparatus  701  cannot receive the health check frames  711  and  715  at the two ring ports thereof, so that the monitoring apparatus  701  judges that a ring status is under abnormal condition, and changes a ring port  721  thereof into such a condition that a user frame can be transmitted/received. Also, the monitoring apparatus  701  clears MAC address information which has been learned by receiving the user frame, and transmits flush request frames  731  and  735  from the two ring ports thereof with respect to other relay apparatuses  702  to  704 . The relay apparatuses  702  to  704  which receive either the flush request frame  731  or the flush request frame  735  clear the MAC address information which has been learned by receiving the user frame. 
     When the failure is recovered from the condition where the failure has occurred in  FIG. 7  and then the failure condition is returned to the normal condition of  FIG. 6 , the monitoring apparatus  601  receives any one of the health check frames  611  and  615  which are transmitted by the monitoring apparatus  601  at the ring port. As a result, the monitoring apparatus  601  judges that the ring status becomes the normal condition, and again changes such a ring port where the transmission/reception of the user frame have been permitted into another condition where the user frame is not transmitted/received. Also, the monitoring apparatus  601  clears the MAC address information which has been learned by receiving the user frame, and also, transmits a flush request frame from the two ring ports with respect to other relay apparatuses  602  to  604  in a similar manner when the failure  751  occurs in  FIG. 7 . The relay apparatuses  602  to  604  which receive this flush request frame clear the MAC address information which has been learned by receiving the user frame. 
     Also, the ring ports of the relay apparatuses  602  and  603  where a failure of a link has occurred can transmit/receive such a control frame as a health check frame and a flush request frame when the failure is recovered, and are brought into such a condition that a user frame cannot be transmitted/received. When the relay apparatuses  602  and  603  receive the flush request frame, the relay apparatuses  602  and  603  change the ring ports where the transmission/receptions of the user frames cannot be permitted into such a condition that the user frames can be transmitted/received. Otherwise, even when the flush request frame is not received, after a predetermined time has passed, the relay apparatuses  602  and  603  changes the present condition into such a condition that the user frames can be transmitted/received. 
     Referring now to  FIG. 8 , a description is made of operations as to respective apparatuses provided in such a structure that a plurality of ring networks are coupled to each other, and furthermore, a coupling apparatus has not been made redundant. This structure corresponds to such a network structure that there is no link (namely, shared link) which is shared by the plural ring networks. In  FIG. 8 , although a total number of the coupled ring networks is selected to be 2, there is no limitation with respect to a total quantity of these ring networks to be coupled to each other. 
     In  FIG. 8 , among apparatuses  801  to  803  which constitute a ring network  841 , a monitoring apparatus is defined as  801 , and relay apparatuses are defined as  802  and  803 . Also, among apparatuses  802 ,  804 , and  805 , which constitute another ring network  842 , a monitoring apparatus is defined as  804 , and relay apparatuses are defined as  802  and  805 . While the relay apparatus  802  constitutes a coupling apparatus of the ring networks  841  and  842 , the relay apparatus  802  has constructed the ring networks  841  and  842  by two different ring ports with respect to each of the ring networks  841  and  842 . In this coupling apparatus, control frames which are transmitted/received in the respective ring networks  841  and  842  are relayed by being closed in the respective ring networks  841  and  842 , but are not transferred to the other ring network. The monitoring apparatus  801  monitors a ring status of the ring network  841 , and sets a ring port  831  thereof to such a condition that a user frame is not transmitted and received. The monitoring apparatus  804  monitors a ring status of the ring network  842 , and sets a ring port  832  thereof to such a condition that a user frame is not transmitted and received. As a result, the ring networks  841  and  842  can be independently operated. 
       FIG. 9  indicates a structure made in the case that an apparatus for coupling two ring networks is used as a monitoring apparatus. In  FIG. 9 , among apparatuses  901  to  903  which constitute a ring network  941 , a monitoring apparatus is defined as  901 , and relay apparatuses are defined as  902  and  903 . Also, among apparatuses  902 ,  904 , and  905 , which constitute another ring network  942 , a monitoring apparatus is defined as  901 , and relay apparatuses are defined as  904  and  905 . While the coupled apparatus becomes the monitoring apparatus  901 , this coupled apparatus constitutes the ring networks  941  and  942  at two different ring ports with respect to each of the ring networks  941  and  942 , and a control frame is not transferred to the other ring network  941 , or  942 . The monitoring apparatus  901  monitors the ring network  941 , and sets a ring port  931  thereof to such a condition that a user frame is not transmitted and received. At the same time, the monitoring apparatus  901  monitors the ring network  942 , and sets a ring port  932  thereof to such a condition that a user frame is not transmitted and received. As a result, the ring networks  941  and  942  can be independently operated. 
     Referring now to  FIG. 10 , a description is made of operations as to respective apparatuses provided in such a structure that a plurality of ring networks are coupled to each other, and furthermore, a coupling apparatus has been made redundant. In  FIG. 10 , although a total number of the coupled ring networks is selected to be 2, there is no limitation with respect to a total quantity of these ring networks to be coupled to each other. 
     In  FIG. 10 , among apparatuses  1001  to  1004  which constitute a ring network  1061 , a monitoring apparatus is defined as  1001 , and relay apparatuses are defined as  1002  to  1004 . Also, among apparatuses  1002 ,  1003 ,  1005 , and  1006 , which constitute another ring network  1062 , a monitoring apparatus is defined as  1005 , and relay apparatuses are defined as  1002 ,  1003 , and  1006 . While the relay apparatus  1002  and  1003  constitute coupling apparatuses of the ring networks  1061  and  1062 , the relay apparatuses  1002  and  1003  have been constructed as having redundant structures (will be referred to as “shared apparatus” hereinafter). Also, a link between the shared apparatuses  1002  and  1003  constitutes such a link (will be referred to as “shared link” hereinafter) which is shared by the ring networks  1061  and  1062 . In the shared apparatuses  1062  and  1063 , control frames which are transmitted/received in the respective ring networks  1061  and  1062  are relayed by being closed in the respective ring networks  1061  and  1062 , but are not transferred to the other ring network. Within the two ring networks, the ring network  1061  is defined as such a ring network (will be referred to as “shared link monitored ring network” hereinafter) which monitors a ring status containing also the shared link, whereas the ring network  1062  is defined as such a ring network (will be referred to as “shared link non-monitored ring network” hereinafter) which monitors a ring status which does not contain the shared link. The monitoring apparatus  1001  of the shared link monitored ring network  1061  periodically transmits health check frames  1011  and  1015  from two ring ports thereof. These health check frames  1011  and  1015  are relayed via the relay apparatuses  1002  to  1004 , and the monitoring apparatus  1002  receives the health check frames  1011  and  1015  at different ring ports from the ring ports for transmitting these health check frames  1011  and  1015 . Within the two ring ports, while the health check frame is being received at any one of these ring ports, the monitoring apparatus  1002  judges that the ring status is under normal condition, and sets such a condition that a user frame is not transmitted/received at a ring port  1051 . 
     The monitoring apparatus  1005  of the shared link non-monitored ring network  1062  periodically transmits health check frames  1021  and  1025  from two ring ports thereof. Also, since the shared nodes  1002  and  1003  do not monitor the shared link, the shared nodes  1002  and  1003  periodically transmit auxiliary health check frames  1031  and  1032  from such ring ports which are located opposite to the ring ports which become the shred links toward the monitoring apparatus  1005 . The auxiliary health check frames  1031  and  1032  are not transmitted to the shared link monitored ring network  1061 . These health check frames  1021  and  1025 , and these auxiliary health check frames  1031  and  1032  are relayed in the relay apparatuses (including shared apparatus)  1002 ,  1003 , and  1006 , and then, the monitoring apparatus  1005  receives these health check frames  1021  and  1025 , and these auxiliary health check frames  1031  and  1032  at the two ring ports thereof. Within the two ring ports, while the health check frames  1021  and  1025  are being received at any one of these two ring ports, or while the auxiliary health check frames  1031  and  1032  transmitted from the shared apparatuses  1002  and  1003  are being received at the two ring ports, the monitor apparatus  1005  judges that the ring status is under normal condition, and sets such a condition that the user frame is not transmitted and received at the ring port  1052 . 
       FIG. 11  indicates a condition that a failure  1171  occurs in the shared link monitored ring network  1061  of  FIG. 10 . When the failure  1171  happens to occur, a health check frame  1111  transmitted from a monitoring apparatus  1101  cannot be relayed from a relay apparatus  1103  to a succeeding apparatus. Similarly, a health check frame  1115  cannot be also relayed from a relay apparatus  1104  to a succeeding apparatus. As a result, the monitoring apparatus  1101  cannot receive the health check frames  1111  and  1115  at the two ring ports thereof, so that the monitoring apparatus  1101  judges that a ring status is under abnormal condition, and changes a ring port  1151  thereof into such a condition that a user frame can be transmitted/received. Also, the monitoring apparatus  1101  clears MAC address information which has been learned by receiving the user frame, and transmits flush request frames  1111  and  1115  from the two ring ports thereof with respect to other relay apparatuses  1102  to  1104 . The relay apparatuses  1102  to  1104  which receive the flush request frames  111  and  1115  clear the MAC address information which has been learned by receiving the user frame. At this time, since the monitoring apparatus  1105  of the shared link non-monitored ring network  1162  has already received the health check frames  1111  and  1115  at the two ring ports thereof, this monitoring apparatus  1105  judges that there is no change in the ring status, and is continuously operated. 
     When the failure is recovered from the condition where the failure has occurred in  FIG. 11  and then the failure condition is returned to the normal condition of  FIG. 10 , the monitoring apparatus  1001  receives any one of the health check frames  1011  and  1015  which are transmitted by the monitoring apparatus  1001  at the ring port. As a result, the monitoring apparatus  1001  judges that the ring status becomes the normal condition, and again changes such a ring port where the transmission/reception of the user frame have been permitted into another condition where the user frame is not transmitted/received. Also, the monitoring apparatus  1001  clears the MAC address information which has been learned by receiving the user frame, and also, transmits a flush request frame from the two ring ports with respect to other relay apparatuses  1002  to  1004  in a similar manner when the failure  751  occurs in  FIG. 7 . The relay apparatuses  1002  to  1004  which receive this flush request frame clear the MAC address information which has been learned by receiving the user frame. At this time, since the monitoring apparatus  1005  of the shared link non-monitored ring network  1062  has continuously received the health check frames  1124  and  1125 , this monitoring apparatus  1005  is continuously operated. 
       FIG. 12  represents such a condition that a shared link failure  1271  of  FIG. 10  happens to occur. When the shared link failure  1271  occurs, a health check frame  1211  transmitted from a monitoring apparatus  1201  of the shared link monitored network  1261  cannot be relayed from a relay apparatus  1202  to a succeeding apparatus. Similarly, a health check frame  1215  cannot be also relayed from a relay apparatus  1203  to a succeeding apparatus. As a result, the monitoring apparatus  1201  cannot receive the health check frames  1211  and  1215  at the two ring ports thereof, so that the monitoring apparatus  1201  judges that a ring status is under abnormal condition, and changes a ring port  1251  thereof into such a condition that a user frame can be transmitted/received. Also, the monitoring apparatus  1201  clears MAC address information which has been learned by receiving the user frame, and transmits flush request frames  1211  and  1215  from the two ring ports thereof with respect to other relay apparatuses  1202  to  1204 . The relay apparatuses  1202  to  1204  which receive the flush request frames  1211  and  1215  clear the MAC address information which has been learned by receiving the user frame. At this time, a health check frame  1221  transmitted from a monitoring apparatus  1205  of a shared link non-monitored network  1262  cannot be relayed from the relay apparatus  1202  to a succeeding apparatus. Similarly, a health check frame  1225  cannot be also relayed from the relay apparatus  1203  to a succeeding apparatus. However, auxiliary health check frames  1231  and  1232  transmitted from the shared apparatuses  1202  and  1203  can be received at the two ring ports of the monitoring apparatus  1205 , so that the monitoring apparatus  1205  judges that a ring status except for the shred link is under normal condition, and is continuously operated. In accordance with the above-described operations, even when the failure happens to occur in the shared link, it is possible to avoid an occurrence of a large loop which bridges the plurality of ring networks. 
     When the failure is recovered from the condition where the failure has occurred in  FIG. 12  and then the failure condition is returned to the normal condition of  FIG. 10 , the monitoring apparatus  1001  of the shared link monitored ring network  1061  receives any one of the health check frames  1011  and  1015  which are transmitted by the monitoring apparatus  1001  at the ring port. As a result, the monitoring apparatus  1001  judges that the ring status becomes the normal condition, and again changes such a ring port where the transmission/reception of the user frame have been permitted into another condition that the user frame is not transmitted/received. Also, the monitoring apparatus  1001  clears the MAC address information which has been learned by receiving the user frame, and also, transmits a flush request frame from the two ring ports with respect to other relay apparatuses  1002  to  1004  in a similar manner when the failure  1271  occurs in  FIG. 12 . The relay apparatuses  1002  to  1004  which receive this flush request frame clear the MAC address information which has been learned by receiving the user frame. 
     At this time, the monitoring apparatus  1005  of the shared link non-monitored ring network  1062  also receives the health check frames  1021  and  1025  transmitted from the monitoring apparatus  1005  at the two ring ports thereof. It should also be noted that since the ring status remains under the normal condition, the operation is merely continued. 
       FIG. 13  indicates a condition that a failure  1371  occurs in the shared link non-monitored ring network  1062  of  FIG. 10 . When the failure  1371  happens to occur, a health check frame  1321  transmitted from a monitoring apparatus  1305  cannot be relayed from a relay apparatus  1306  to a succeeding apparatus. Similarly, a health check frame  1325  cannot be also relayed from a relay apparatus  1303  to a succeeding apparatus. As a result, the monitoring apparatus  1305  cannot receive the health check frames  1321  and  1325  at the two ring ports thereof. Also, an auxiliary health check frame transmitted from the shared apparatus  1303  cannot also be relayed from the relay apparatus  1306  to a succeeding apparatus. An auxiliary health check frame  1331  transmitted from the shared apparatus  1302  is received at the ring port of the monitoring apparatus  1305 . As a result, since the health check frames  1321  and  1325  transmitted from the monitoring apparatus  1305  cannot be received at the two ring ports, and further, the auxiliary health check frame transmitted from the shared apparatus  1303  cannot be received at any one of the two ring ports, the monitoring apparatus  1305  judges that a ring status is under abnormal condition, and changes a ring port  1352  thereof into such a condition that a user frame can be transmitted/received. Also, the monitoring apparatus  1305  clears MAC address information which has been learned by receiving the user frame, and transmits a flush request frame from the two ring ports thereof with respect to other relay apparatuses  1302 ,  1303 , and  1306 . The relay apparatuses  1302 ,  1303 , and  1306  which receive the flush request frame clear the MAC address information which has been learned by receiving the user frame. At this time, since the monitoring apparatus  1301  of the shared link monitored ring network  1361  has received the health check frame at the two ring ports, the monitoring apparatus  1301  judges that there is no change in the ring status, and the operation thereof is continued. 
     When the failure  1371  is recovered from the condition where the failure  1371  has occurred in  FIG. 13  and then the failure condition is returned to the normal condition of  FIG. 10 , the monitoring apparatus  1005  receives any one of the health check frames  1021  and  1025  which are transmitted by the monitoring apparatus  1005  at the ring port. Otherwise, the monitoring apparatus  1005  receives the auxiliary health check frames  1031  and  1033  which are transmitted from the shared apparatuses  1002  and  1002  at the two ring ports respectively. As a result, the monitoring apparatus  1005  judges that the ring status becomes the normal condition, and again changes such a ring port where the transmission/reception of the user frame have been permitted into another condition that the user frame is not transmitted/received. Also, the monitoring apparatus  1005  clears the MAC address information which has been learned by receiving the user frame, and also, transmits a flush request frame from the two ring ports with respect to other relay apparatuses  1002 ,  1003 , and  1006  in a similar manner when the failure  1371  occurs in  FIG. 13 . The relay apparatuses  1002 ,  1003  and  1006  which receive this flush request frame clear the MAC address information which has been learned by receiving the user frame. At this time, since the monitoring apparatus  1005  of the shared link monitored ring network  1061  continuously receives the health check frames at the two ring ports thereof, the monitoring apparatus  105  continues the operation thereof. 
       FIG. 14  shows a structure made in the case that a shared apparatus is used as a monitoring apparatus. Among apparatuses  1401  to  1404  of an embodiment of the present invention, which constitute a shared link monitored ring network  1461 , a monitoring apparatus is defined as  1401 , and relay apparatuses are defined as  1402  to  1404 . Also, among apparatuses  1401 , and  1404  to  1406  of the embodiment of the present invention, which constitute a shared link non-monitored ring network  1462 , a monitoring apparatus is defined as  1401 , and relay apparatuses are defined as  1404  to  1406 . While the apparatus  1401  corresponding to the monitoring apparatus of each of the ring networks  1461  and  1462  constitutes one shared apparatus, this monitoring apparatus  1401  independently monitors the respective ring networks  1461  and  1462 . The monitoring apparatus  1401  monitors a ring status as the monitoring apparatus for the shred link monitored network  1461 , and sets a ring port  1451  to such a condition that a user frame is not transmitted and received. At the same time, the monitoring apparatus  1401  monitors a ring status as the monitoring apparatus for the shared link non-monitored network  1462 , and sets a ring port  1452  to such a condition that a user frame is not transmitted and received. At this time, the ring ports  1451  and  1452  cannot be constituted as ring ports on the shared link side, but must be constituted as ring ports which are located opposite to the ring ports corresponding to the shared link. Also, since the monitoring apparatus  1401  of the shared link non-monitored ring network  1462  is commonly employed as the shared apparatus, the monitoring apparatus  1401  does not transmit such an auxiliary health check frame so as not to monitor the shared link, but monitors another auxiliary health check frame  1431  transmitted from another shared apparatus  1404  in combination with the health check frames  1421  and  1425  which are transmitted by the own shared apparatus  1401 . As a result, similar to  FIG. 10 , the ring networks  1461  and  1462  can be independently operated. 
     In accordance with the monitoring apparatuses in the present embodiments, the shared apparatuses, and the relay apparatuses of the present invention, the below-mentioned effects can be achieved. 
     (1) In a monitoring apparatus which constitutes a ring network, the monitoring apparatus judges whether or not a relay of a user frame is permitted by determining whether or not a health check frame is received which is periodically transmitted. As a result, routes can be switched in a high speed. 
     (2) As previously described, since the monitoring apparatus may merely monitor a reception of a control frame, the system may be easily realized in a hardware fashion. As a result, while there is no unwanted protocol operation, loads given to a CPU can be reduced, so that a stable protocol operation can be realized. 
     (3) Health check frames are transmitted from two ring ports of a monitoring apparatus which constitutes a ring network, and these health check frames are monitored at the two ring ports (namely, health check frames are monitored in bidirectional manner). As a result, even when a failure happens to occur along one direction in a portion of the link which constitutes the ring network, it is possible to prevent an occurrence of a loop status in connection with the failure. 
     (4) In the case that a multi-ring network is constructed, the following conditions may be merely set: any one of a monitoring apparatus, a shared apparatus, and a relay apparatus may be merely set with respect to this apparatus, and which port may be merely set to a ring port. As a result, these monitoring apparatus, shared apparatus, and relay apparatus may be simply coupled to the multi-ring network. Also, since operations in the respective rings are independently carried out, a ring network may be readily added to be coupled to the existing ring network without giving an adverse influence to the existing ring network. 
     (5) In such a case that a multi-ring network is constructed, a structural position of a monitoring apparatus may be freely determined irrespective of such a fact whether or not an apparatus to be coupled takes a redundant structure, so that apparatuses may be arranged in a flexible manner. Also, the apparatus to be coupled is not necessarily a monitoring apparatus. 
     (6) In the case where a multi-ring network is constructed and an apparatus to be coupled takes a redundant structure, an auxiliary health check frame is transmitted from this apparatus having the redundant structure, and a monitoring apparatus monitors a health check frame in combination with this auxiliary health check frame. As a result, even when a failure happens to occur in a link between apparatuses which are made redundant, it is possible to avoid that a loop is constructed. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.