Patent Publication Number: US-8972771-B2

Title: Connection control apparatus, storage system, and control method of connection control apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority of the prior Japanese Patent Application No. 2011-241578, filed on Nov. 2, 2011, the entire contents of which are incorporated herein by reference. 
     FIELD 
     The embodiments discussed herein are related to a connection control apparatus, a storage system, and a control method of the connection control apparatus. 
     BACKGROUND 
     In recent years, storage systems configured with a storage area network (SAN) architecture in which fiber channel switches are used are becoming mainstream. On a SAN, a plurality of severs, a plurality of storage units, and a plurality of other peripheral units are mutually connected through fiber channels. Storage integration, in which data in a plurality of severs is stored together, is carried out through a SAN. 
     With the SAN, a fiber channel switch called a center witch or director is used. The center switch has a function that is a kernel in data routing. Specifically, the center switch assigns port IDs (PIDs) to devices connected to it. The assigned PIDs are stored in a name server in the center switch. The center switch uses a PID stored in the name server to transfer data received from a device (a server, for example) to a specified device (a storage unit, for example). In addition, the center switch acquires data from a specified device and transfers the acquired data to a device that has requested the data. Since many devices are mutually connected in recent years, a SAN to which a plurality of center switches are connected is often used. From the viewpoint of the fiber switch, a switch that simply performs routing may be connected to a center switch of this type. 
     Some SANs use a redundant configuration to protect against communication failures. In a conventional technology, for example, a plurality of fiber channel switches are connected to a single device and data transmission routes are duplicated; in this configuration, after a switchover has been made, a state is monitored through broadcasting and a routing table is updated. 
     When a redundant configuration is used, a center switch with a redundant configuration may be used, that is, elements that implement the functions of the center switch described above are duplicated. In the description that follows, elements that perform routing in a normal state will be referred to as the active side and duplicated elements to which the center switch switches in case of a failure on the active side will be referred to as the standby side. 
     In the center switch with a redundant configuration, the active side and standby side each have a name server. Even if the name server in the active side goes down, the center switch switches to the standby side and may uses the name server on the standby side to enable routing as on the active side. 
     Japanese Laid-open Patent Publication No. 2008-242872 is an example of related art. 
     If, for example, failures occur on both the active side and standby side, device information stored in the name servers are deleted and address resolution is disabled. This suppresses the center switch, in which both the active side and standby side caused a failure, from carrying out routing. Accordingly, for example, the center switch may no longer route data to a device connected to the center switch. 
     Even if the conventional technology in which monitoring is performed through broadcasting after a switchover has been made is used, when a switch with a redundant configuration causes failures in the active side and standby side, it is suppressed to route data to devices connected to only the switch. 
     SUMMARY 
     According to an aspect of the embodiments, a connection control apparatus includes an active control unit that performs routing of data to a first device connected to a port of the connection control apparatus by using an address of the first device, and a standby control unit that, when the active control unit causes a failure, performs routing of data to the first device by using the address of the first device and by using route information indicating a connection route to a second device connected to a port of an adjacent connection control apparatus connected to the connection control apparatus, the standby control unit performing routing of data to the second device by using the route information upon the adjacent connection control apparatus being unable to perform routing. 
     The object and advantages of the invention will be realized and attained by means of the elements and combinations particularly pointed out in the claims. 
     It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory and are not restrictive of the invention, as claimed. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         FIG. 1  is an exemplary outside view illustrating the structure of a storage system in a first embodiment; 
         FIG. 2  illustrates connections of control processor (CP) blades, core blades, and port blades; 
         FIG. 3  is a block diagram illustrating the structure of the storage system in the first embodiment; 
         FIG. 4  illustrates the states of name servers and extended name servers in the first embodiment at normal times; 
         FIG. 5  illustrates the data structure of a fiber channel (FC) frame; 
         FIG. 6  illustrates a state in which one of center switches in the first embodiment goes down, and particularly the state of the expanded name server in another center switch; 
         FIG. 7  is a sequence diagram of processing executed by the storage system in the first embodiment at normal times; 
         FIG. 8  is a sequence diagram of processing executed by the storage system in the first embodiment at a time when one center switch goes down; 
         FIG. 9  is a sequence diagram of transfer processing executed by the storage system in the first embodiment while the one center switch is in a down state; 
         FIG. 10  is a flowchart of processing executed by an extended name server at a time when one center switch goes down; 
         FIG. 11  illustrates the states of name servers and extended name servers in a second embodiment at normal times; 
         FIG. 12  illustrates a state in which one of center switches in the second embodiment goes down, and particularly illustrates the state of the expanded name server in another center switch; and 
         FIG. 13  is a sequence diagram of processing executed at a time when one center switch in the storage system in the second embodiment goes down. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Embodiments of a connection control apparatus, a storage system, and a control method of the connection control apparatus disclosed in this application will be described in detail with reference to the drawings. The connection control apparatus, storage system, and the method of the connection control apparatus are not limited by embodiments described below. Particularly, although a fiber channel will be used in the description below as an example of the connection control apparatus, this is not a limitation. The fiber channel will be referred to below as the FC switch. 
     First Embodiment 
       FIG. 1  is an exemplary outside view illustrating the structure of a storage system in a first embodiment. As illustrated in  FIG. 1 , the storage system in the first embodiment has a center switch  1 , a center switch  2 , inter-chassis links (ICL)  3 , a storage unit  4 , and a server  5 . The center switch  1  is an example of a first connection control apparatus, and the center switch  2  is an example of a second connection control apparatus. 
     In this embodiment, the storage unit  4  is connected to the center switch  1  and the server  5  is connected to the center switch  2 . For convenience, only one device is connected to each of the center switches  1  and  2  in  FIG. 1 . In practice, however, a plurality of devices may be connected to each of the center switches  1  and  2 . Specifically, a maximum number of devices equal to the number of ports, described later, with which the center switch  1  is provided may be connected to the center switch  1  and a maximum number of devices equal to the number of ports with which the center switch  2  is provided may be likewise connected to the center switch  2 . In  FIG. 1 , various types of devices such as servers and storage units may be connected to both the center switches  1  and  2 . When many devices are connected, an FC switch that simply performs routing may also be connected to the center switches  1  and  2 . 
     The center switches  1  and  2  are mutually connected through ICLs  3 . The ICL  3  will be described later in detail. 
     The center switch  1  has a control processor (CP) blades  11  and  12 , each of which is a blade that includes a processor having control functions, core blades  13  and  14 , and port blades  15  to  18 . The center switch  2  has CP blades  21  and  22 , core blades  23  and  24 , and port blades  25  to  28 . In this embodiment, the center switches  1  and  2  have the same structure. 
     The CP blades  11  and  12 , core blades  13  and  14 , and port blades  15  to  18  each have a thin (blade-type) case. When the case of each blade is inserted into the case of the center switch  1  and connected to a board provided on the rear surface of the case of the center switch  1 , the blade and the center switch  1  are mutually connected. 
     Although, in  FIG. 1 , the center switches  1  and  2  each have four port blades, they many have any number of port blades within the range of 1 to 4. The center switches  1  and  2  in  FIG. 1  each may have only up to four port blades. If the cases of the center switches  1  and  2  are enlarged, however, they may have more port blades. 
       FIG. 2  illustrates connections of the CP blades  11  and  12 , core blades  13  and  14 , and port blades  15  to  18 . As illustrated in  FIG. 2 , the CP blade  11  is connected to the core blades  13  and  14 , and the CP blade  12  is also connected to the core blades  13  and  14 . The core blade  13  is connected to the port blades  15  to  18 , and the core blade  14  is also connected to the port blades  15  to  18 . That is, both the CP blades  11  and  12  may communicate with both the core blades  13  and  14 . Both the core blades  13  and  14  may communicate with all of the port blades  15  to  18 . The center switch  2 , the structure of which is similar to the structure of the center switch  1 , is also connected as in  FIG. 2 . 
     The CP blade  11  (control processor) has a central processing unit (CPU), a hard disk, and a memory. In this embodiment, the CP blades  12 ,  21 , and  22  each have a structure similar to the structure of the CP blade  11 . One of the CP blades  11  and  12  is active and the other is standby. That is, the active one of the CP blades  11  and  12  is operating at normal times; if a failure occurs on the active side, the other CP blade, which is on the standby side, operates instead of the active CP blade. This is also true for the CP blades  21  and  22 . 
     The core blade  13  has a CPU and a memory. The core blade  13  also has a plurality of ICL ports. In this embodiment, the core blades  14 ,  23 , and  24  each have a structure similar to the structure of the core blade  13 . Both the core blades  13  and  14  each carry out routing processing. If one of the core blades  13  and  14  goes down, the other takes over the processing. This is also true for the core blades  23  and  24 . When an ICL port of the core blade  13  is connected to an ICL port of a core blade inserted into another center switch through one ICL  3 , the core blade  13  and the core blade in the other center switch may mutually communicate at high speed. In this embodiment, the core blade  13  is connected to the core blade  23  though one ICL  3 , and the core blade  14  is connected to the core blade  24  through one ICL  3 . 
     The port blades  15  to  18  have ports used to connect servers, storage units, switches, and other devices. For example, the port blades  15  to  18  each have 64 ports. The storage unit  4  is connected to a port of the port blade  15 , for example. 
       FIG. 3  is a block diagram illustrating the structure of the storage system in the first embodiment. An overall control unit  101  is implemented by, for example, the CP blade  11  in  FIG. 1 . An overall control unit  102  is implemented by, for example, the CP blade  12  in  FIG. 1 . A routing control unit  103  is implemented by, for example, the core blade  13  in  FIG. 1 . A routing control unit  104  is implemented by, for example, the core blade  14  in  FIG. 1 . A port unit  105  is implemented by, for example, the port blades  15  and  16  in  FIG. 1 . A port unit  106  is implemented by, for example, the port blades  17  and  18  in  FIG. 1 . An overall control unit  201  is implemented by, for example, the CP blade  21  in  FIG. 1 . An overall control unit  202  is implemented by, for example, the CP blade  22  in  FIG. 1 . A routing control unit  203  is implemented by, for example, the core blade  23  in  FIG. 1 . A routing control unit  204  is implemented by, for example, the core blade  24  in  FIG. 1 . A port unit  205  is implemented by, for example, the core blades  25  and  26  in  FIG. 1 . A port unit  206  is implemented by, for example, the port blades  27  and  28  in  FIG. 1 . 
     In  FIG. 1 , the storage unit  4  and server  5  have been respectively connected to the center switches  1  and  2  as typical examples. In  FIG. 3 , however, devices  41  to  44  are connected to the center switch  1  and devices  51  to  54  are connected to the center switch  2 . Since the center switches  1  and  2  have the same structure as described above and symmetrically operate, the structure of the center switch  1  will be mainly described below. The described below is also applied to the center switch  2 . 
     One of the overall control unit  101  and overall control unit  102  is active and performs processing at normal times, and the other is standby, which takes over the processing when the active one goes down. In this embodiment, the overall control unit  101  is active and the overall control unit  102  is standby. The routing control units  103  and  104  have a duplicate configuration; both are active and perform routing processing. That is, both the routing control units  103  and  104  perform routing processing at normal times. If one of them goes down, however, the other performs the processing that the one has been performing instead. Since the routing control units  103  and  104  operate similarly as described above, the routing control unit  103  will be taken as an example in the description below. 
     The overall control unit  101  has a name server  110 . The overall control unit  101  acquires information about the devices  41  and  42  connected to the port unit  105  from the routing control unit  103  and also acquires information about devices  43  and  44  connected to the port unit  106  from the routing control unit  104 . The overall control unit  101  then assigns port IDs (PIDs) to the devices  41  and  42  connected to the port unit  105  and the devices  43  and  44  connected to the port unit  106 . The PID created by the overall control unit  101  will be described next. The PID is a six-digit number, for example. The first two digits of the PID indicate the DID of the center switch, the subsequent two digits indicate the number of a port of the center switch through which a device is connected to an adjacent switch, and the remaining two digits are reserved. When the DID of the center switch  1  is 1 and a device is connected to the center switch  2  through a port having port number  81 , for example, the overall control unit  101  assigns PID 018100 to the device. 
     The overall control unit  101  then records the assigned PID in an address table stored in the name server  110  in the overall control unit  101 . In the address table, devices and PIDs assigned to them are recorded in correspondence with each other. The overall control unit  101  outputs information about the assigned PID to the overall control unit  102 . 
     The overall control unit  101  acquires, from the routing control unit  103 , the PID assigned to the device connected to the center switch  2 . The overall control unit  101  records the PID assigned to the device connected to the center switch  2  in the address table stored in the name server  110  in the overall control unit  101 . That is, the name server  110  in the overall control unit  101 , which is on the active side, has an address table in which the PID assigned to the device connected to the center switches  1  and  2  is recorded. The overall control unit  101  outputs information about the PID assigned to the device connected to the center switch  2  to the overall control unit  102 . 
       FIG. 4  illustrates the states of the name servers and extended name servers in the first embodiment at normal times. In  FIG. 4 , the storage unit  4  is connected to the center switch  1  and the server  5  is connected to the center switch  2 . In this example, the world wide name (WWN) of the storage unit  4  is 50:00:00:0b:5d:42:01:6b and the WWN of the server  5  is 20:01:00:0b:5c:61:03:3a. As illustrated in  FIG. 4 , the name server  110  has an address table  111  in which the PIDs of the storage unit  4  and server  5  have been recorded. The address table  111  stores a type, a PID, a WWN and Info for each device. In this embodiment, the type indicates the type of a recorded PID. When the type is N, it indicates that the device is a node and the PID has been assigned to the device. When the type is S, it indicates that the device is a switch and the PID is a switching PID, which is used as an alternative as described later if another center switch goes down. Info indicates the type of the device. Similarly, as illustrated in  FIG. 4 , a name server  210  disposed in the overall control unit  201  in the center switch  2  also has an address table  211 . The address tables  111  and  211  have the same contents, as illustrated in  FIG. 4 . The name server  110  and address table  111  are an example of a first table and the name server  210  and address table  211  are an example of a third table. 
     The overall control unit  101  receives, from the routing control unit  103 , a request to send information about the PID assigned to a device connected to the center switch  1 . The overall control unit  101  then outputs, to the routing control unit  103 , the information about the PID assigned to the device connected to the center switch  1 . 
     Either of the routing control units  103  and  104  may be used to send the PID of the device connected to the center switch  1  and receive the PID of the device connected to the center switch  2 . In this embodiment, the routing control unit  103  is used to send and receive PIDs as an example. 
     The overall control unit  101  acquires, from the routing control unit  103 , the destination ID (DID) of a destination to which an FC frame is to be transferred, the DID being included in the FC frame. The FC frame will be described below.  FIG. 5  illustrates the data structure of the FC frame. As illustrated in  FIG. 5 , the FC frame  300  has a start-of-frame (SOF) field  301 , a header field  302 , a payload field  303 , a cyclic redundancy check (CRC) field  304 , and an end-of-frame (EOF) field  305 . The SOF field  301  is used as a delimiter that indicates the start of the frame. The size  310  of the SOF field  301  is four bytes, for example. The header field  302  is an area in which transferred information and the like are stored; a DID, a source ID (SID), and other information are stored therein. The size  320  of the header field  302  is 24 bytes, for example. The payload field  303  includes user data. The size  330  of the payload field  303  is a maximum of 2112 bytes. The CRC field  304  is an area to store information about CRC. The size  340  of the CRC field  304  is four bytes, for example. The EOF field  305  is used as a delimiter that indicates the end of the frame. The size  350  of the EOF field  305  is four bytes, for example. The total size  360  of the FC frame  300  is a maximum of 2,148 bytes, for example. The DID of the FC frame  300  may be acquired from the header field  302 . 
     The overall control unit  101  decides whether the acquired DID is included in the address table in the name server  110 . If the PID is included, the overall control unit  101  outputs the PID of the transfer destination to the routing control unit  103  to command the routing control unit  103  to transfer the FC frame. If the PID is not included, the overall control unit  101  commands the routing control unit  103  to discard the FC frame. 
     If the overall control unit  101  is notified by the routing control unit  103  that the center switch  2  has gone down, the overall control unit  101  commands the overall control unit  102  to store, in an extended name server  120 , the fact that the center switch  2  has gone down. 
     The overall control unit  102  may cause a failure after the center switch  2  has gone down. In this case, connection is preserved or performance is preserved in this embodiment. When connection is preserved, routing using the switching port is preserved without alteration. When performance is preserved, routing using the switching port is abandoned and only routing that is being carried out by the overall control unit  101  for the device connected to the center switch  1  is continued. In connection preservation, all routings that are being carried out at that time are assured, but the load to the overall control unit  101  is increased due to an increase in the amount of processing. In performance preservation, performance is preserved because the overall control unit  101  performs only processing similar to processing so far, but routing through the down center switch is disabled. 
     In connection preservation, if the overall control unit  101  causes a failure, the overall control unit  101  receives a notification of synchronization with the extended name server  120  from the overall control unit  102  so that the extended name server  120  is used. In this case, the overall control unit  101  acquires an extended address table  121  from the overall control unit  102 . If the overall control unit  102  causes a failure, the overall control unit  101  uses the extended address table  121  to perform routing to the connected device through the center switch  2 . The overall control unit  101  is an example of an active control unit. 
     The overall control unit  102  includes the extended name server  120 . The extended address table  121  stored in the extended name server  120  includes a type field, a PID field, a WWN field, a native PID field, an access flag field, and an Info field. That is, the extended address table  121  is identical to the address table stored in the name server  110 , except that the native PID field and access flag field are added. A native PID is an original PID assigned to a device connected to a down center switch. The native PID is stored so as to correspond to a switching PID, which is created to connect to the device connected to the down center switch. The switching PID will be described later in detail. The access flag field indicates whether a connection may be made by using the PID described on the row on which the access flag field is indicated. Specifically, a circle (◯) is indicated in the access flag field if an access to the device is possible by using the PID described on the row on which the access flag is indicated, and a cross (x) is indicated if not. The overall control unit  102  acquires, from the overall control unit  101 , information about the PID assigned to the center switch  1  and the PID assigned to the device connected to the center switch  2 . 
     The overall control unit  102  stores the acquired PIDs in the address table in the extended name server  120 . Furthermore, the overall control unit  102  creates information about the switching PID for a device other than the device directly connected to the center switch  1  and stores the created information in the address table. 
     Now, an example of creating a switching PID and storing it in the address table will be described. The switching PID is a six-digit number, as with the normal PID. The first two digits of the switching PID indicate the DID of the center switch. The subsequent one digit of the switching PID indicates a value obtained by adding the port number of the port in use by the ICL to the position at which the core blade of the center switch is inserted. The subsequent one digit in the switching PID indicates a position at which a port blade of the adjacent center switch is inserted. The remaining two digits indicate the port number of a port to which the device is connected. For example, it is assumed that the DID of the center switch  1  is 1, the core blade of the center switch  1  is inserted into a third slot, and the port number of the ICL port connected to the adjacent center switch is 1. It is also assumed that the port blade of the adjacent center switch is inserted into a first slot and the port number of a port, of the adjacent center switch, to which the device is connected is 1. Then, the overall control unit  102  creates 014101 as the switching PID. The switching PID indicates a route from the center switch to the port to which the device is connected, without using the original PID assigned to the device. That is, when the switching PID is used, the routing control unit  103  may transfer a FC frame to a port to which a device to which to transfer the FC frame is connected, for example. A port subject to routing in which a switching PID is used will be referred to below as a switching port. The switching PID is an example of route information indicating a connection route from the connection control apparatus to the port, of the adjacent connection control apparatus, to which the device having the address is connected. 
     The overall control unit  102  stores the created switching PID in the PID field of the extended address table  121  in the extended name server  120 . The overall control unit  102  indicates S in the type field corresponding to the switching PID and stores the value of the WWN of the device in the WWN field. The overall control unit  102  stores the value of the original PID of the device in the native PID field corresponding to the switching PID. The overall control unit  102  indicates the state of access in the access flag field. The overall control unit  102  stores the type of the device in the Info field. 
     For example, the address table in the extended name server  120  becomes like the extended address table  121  in  FIG. 4 . As illustrated in  FIG. 4 , the original PIDs of the storage unit  4  and server  5  have been stored in the extended address table  121  in the extended name server  120 . The switching PID of the server  5 , which is a device connected through the center switch  2 , has been stored in the extended address table  121 . PIDs  122  and  123  are the switching PID of the server  5 . The PID  122  is a switching PID used to connect to the server  5  through a route that passes through the ICL  3  connected to a port with port number  1  on the core blade inserted into the third slot. The PID  123  is a switching PID used to connect to the server  5  through a route that passes through the ICL  3  connected to a port with port number  1  on the core blade inserted into a sixth slot. Since, in this embodiment, two core blades are inserted and each core blade is connected to the center switch  2  through the relevant ICL  3 , two routes are provided. Accordingly, two switching PIDs, each of which uses one route, are created. If one core blade is used, for example, only one switching PID is created for one device. 
     The overall control unit  102  stores the extended address table  121  in its extended name server  120  as described above. Similarly, the overall control unit  202  in the center switch  2  stores an extended address table  221  in its extended name server  220  as illustrated in  FIG. 4 . Since the switching PID is created for a device connected to other than the center switch, the extended address table  121  and extended address table  221  have different contents as illustrated in  FIG. 4 . 
     If the overall control unit  102  detects a failure caused in the overall control unit  101 , which is on the active side, the overall control unit  101  becomes active and takes over routing that the overall control unit  101  has been carrying out. 
     If both the overall control units  201  and  202  in the center switch  2  cause a failure and the center switch  2  goes down, the overall control unit  102  takes over routing that the overall control unit  201  or overall control unit  202  has been carrying out. In this case, the overall control unit  102  uses the extended name server  120  to carry out routing. Operation carried out by the overall control unit  102  when the center switch  2  goes down will be described with reference to  FIG. 6 .  FIG. 6  illustrates a state in which one of the center switches in the first embodiment goes down, and particularly the state of the expanded name server in the other center switch. In  FIG. 6 , the center switch  2  goes down from the state in  FIG. 4 . 
     The overall control unit  102  receives, from the overall control unit  101 , a command to reflect the down state of the center switch  2  into the extended name server  120 . In the extended address table  121  stored in the extended name server  120 , the overall control unit  102  indicates x in the access flag field corresponding to the device connected through the center switch  2 . In  FIG. 6 , the device connected through the center switch  2  is the server  5 , so the overall control unit  102  indicates x in the access flag field on the row on which the WWN of the server  5  is indicated and N is indicated as the type. 
     The overall control unit  102  then decides whether a switching PID that uses the PID on the row on which x is indicated in the access flag field as the native PID has been stored in the extended address table  121 . Specifically, the overall control unit  102  searches for the PID for which x is indicated in the access flag field from native PIDs to decide whether there is a switching PID corresponding to the PID. If there is a switching PID corresponding to the PID for which x is indicated in the access flag field, the overall control unit  102  commands the routing control unit  103  to perform prioritized port setting. Prioritized port setting is to specify another center switch as the routing destination of an FC frame received from a connected device. An example of prioritized setting is that when the center switch  2  receives an FC frame from the device connected to the center switch  2 , a setting is made so that the received FC frame is transferred to the routing control unit  103  in the center switch  1 . Prioritized port setting will be described later in detail. If there is no switching PID corresponding to the PID for which x is indicated in the access flag field, the overall control unit  102  abandons routing to the device to which the PID is assigned. 
     When the overall control unit  102  receives a notification, from the routing control unit  103 , that routing using a switching port has been set, the overall control unit  102  confirms operation performed when a failure occurs in the overall control unit  102 . The overall control unit  102  may cause a failure while carrying out routing using a switching port. In this case, connection is preserved or performance is preserved in this embodiment. When connection is preserved, routing using the switching port is preserved without alteration. When performance is preserved, routing using the switching port is abandoned and only routing that is being carried out by the overall control unit  101  for the device connected to the center switch  1  is continued. In connection preservation, all routings that are being carried out at that time are assured, but the load to the overall control unit  101  is increased due to an increase in the amount of processing. In performance preservation, performance is preserved because the overall control unit  101  performs only processing similar to processing so far, but routing through the down center switch is disabled. In this embodiment, the overall control unit  102  receives a command to select connection preservation or performance preservation in advance. 
     In connection preservation, if the overall control unit  102  causes a failure, the overall control unit  102  notifies the overall control unit  101  of synchronization with the extended name server  120  so that the extended name server  120  is used. In performance preservation, however, the overall control unit  102  does not notify the overall control unit  101 . This is because when synchronization with the extended name server  120  is not commanded, even if the overall control unit  102  causes a failure, the overall control unit  101  continues the current processing. 
     In routing using a switching port, the overall control unit  102  receives, from the routing control unit  103 , an inquiry about the transfer destination of the received FC frame. Specifically, the overall control unit  102  receives the DID of the transfer destination from the routing control unit  103 . The overall control unit  102  searches the extended address table  121  stored in the extended name server  120  for a switching PID for which the DID of the transfer destination is written as a native PID. The overall control unit  102  extracts the switching PID that is searched for and notifies the routing control unit  103  of the switching PID. If there is no switching PID for which the received DID is written as a native PID, the overall control unit  102  commands the routing control unit  103  to discard the FC frame. 
     In  FIG. 6 , for example, the overall control unit  102  receives, from the routing control unit  103 , an inquiry about the transfer destination of an FC frame to be transferred to the server  5 . Specifically, the overall control unit  102  receives, from the routing control unit  103 , 028100 as the DID specified by the FC frame. The overall control unit  102  searches the extended address table  121  for the native ID the value of which is 028100. In  FIG. 6 , there are two routes to devices connected to the center switch  2 , so the overall control unit  102  extracts two rows. The overall control unit  102  then selects one of the extracted rows. For example, the overall control unit  102  selects a row on which a switching PID that indicates a low load at that time is indicated. The overall control unit  102  notifies the routing control unit  103  of the switching PID indicated on the selected row. The overall control unit  102  is an example of a standby control unit. The extended name server  120  and extended address table  121  are an example of a second table. 
     The routing control unit  103  acquires an FC frame supplied to the port unit  105  or  106 . The routing control unit  103  then acquires a DID from the acquired FC frame. If a prioritized port is not used to transfer the FC frame, the routing control unit  103  uses the acquired DID to inquire the transfer destination of the overall control unit  101 . The routing control unit  103  then receives the PID of the transfer destination of the FC frame from the overall control unit  101 . The routing control unit  103  transfers the FC frame to the device to which the received PID is assigned through the port unit  105  or  106 . 
     If a prioritized port is used to transfer the FC frame, the routing control unit  103  uses the acquired DID to inquire the transfer destination of the overall control unit  102 . The routing control unit  103  then receives the switching PID of the transfer destination of the FC frame from the overall control unit  102 . The routing control unit  103  references the received switching PID and acquires a switching port to which the device to which to transfer the FC frame is connected and also acquires a route to the switching port. The routing control unit  103  transfers the FC frame through the port unit  105  or  106  so that the FC frame arrives at the switching port through the acquired route. 
     If, in  FIG. 6 , the routing control unit  103  inquires the transfer destination of an FC frame to be transferred to the server  5  of the overall control unit  102 , for example, the routing control unit  103  receives 014810 or 017810 as the switching PID from the overall control unit  102 . In this example, it is assumed that the routing control unit  103  has received 014810. The routing control unit  103  then sends the FC frame so that it is output from a port with port number  10  on the port blade inserted into the eighth slot of the center switch  2  through the port with port number  1  on the port blade inserted into the third slot. 
     If both the overall control units  101  and  102  cause a failure, the routing control unit  103  receives a command to perform routing using a switching PID from, for example, the routing control unit  203  or  204  in the center switch  2 . The routing control unit  103  then sets a prioritized port so that when the routing control unit  103  receives a FC frame from a device (the server  5 , for example) connected to the center switch  1 , the received FC frame is transferred to the routing control unit  203  in the center switch  2  through the prioritized port. The routing control unit  103  further sets a switching port that is subject to control of transfer using a switching PID, control being carried out by the overall control unit  202  on the standby side in the center switch  2  when an FC frame is sent to the device connected to the center switch  1 . That is, with a switching port having been set, when the routing control unit  103  receives an FC frame to be transferred to the device connected to the center switch  1 , the routing control unit  103  transfers the FC frame to the routing control unit  203 . 
     The port units  105  and  106  receive FC frames from devices connected to ports. The port units  105  and  106  output the received FC frames to the routing control unit  103  or  104 . 
     The port units  105  and  106  respectively receive, from the routing control units  103  and  104 , FC frames to be transferred. The port units  105  and  106  output the received FC frames to specified devices or ports. 
     Next, three types of processing executed by the storage system in the first embodiment will be described with reference to  FIGS. 7 to 10 : processing at normal times, processing executed at a time when one center switch goes down, and processing executed while the one center switch is in a down state. In this embodiment, the two routing control units mounted in each center switch operate similarly and two port units mounted therein operate similarly. In the description below, therefore, the routing control unit  103  and port unit  105  in the center switch  1  and the routing control unit  203  and port unit  205  in the center switch  2  are taken as examples. 
       FIG. 7  is a sequence diagram of processing executed by the storage system in the first embodiment at normal times. Processing executed by the storage system in the first embodiment at normal times will be described with reference to  FIG. 7 . 
     Steps S 101  to S 111  indicate creation processing executed by the name servers  110  and  210  and extended name servers  120  and  220 . At normal times, the routing control units  103  and  203  exchange information stored in their corresponding name servers at fixed intervals through the ICL  3  (operation S 101 ). 
     The routing control unit  203  sends information acquired from the address table  111  stored in the name server  110  in the center switch  1  to the overall control unit  201  (operation S 102 ). The overall control unit  201  acquires the information in the address table  111  stored in the name server  110  from the routing control unit  203  (operation S 103 ). The overall control unit  201  then reflects the information in the address table  111  stored in the name server  110  into the address table  211  stored in the name server  210  in the overall control unit  201  (operation S 104 ). Furthermore, the overall control unit  201  sends the information in the address table  211  stored in the name server  210  to the overall control unit  202  (operation S 105 ). 
     The overall control unit  202  stores information about the device connected to the center switch  2  and information about the device connected to the center switch  1  in the extended address table  221 . Furthermore, the overall control unit  202  creates a switching PID from the PID assigned to the device connected to the center switch  1 , stores the created switching PID in the extended address table  221  together with a native PID and the like, creating information about the extended name server  220  (operation S 106 ). 
     Similarly, the routing control unit  103  sends information acquired from the address table  211  stored in the name server  210  in the center switch  2  to the overall control unit  101  (operation S 107 ). The overall control unit  101  acquires the information in the address table  211  stored in the name server  210  from the routing control unit  103  (operation S 108 ). The overall control unit  101  then reflects the information in the address table  211  stored in the name server  210  into the address table  111  stored in the name server  110  in the overall control unit  101  (operation S 109 ). Furthermore, the overall control unit  101  sends the information in the address table  111  stored in the name server  110  to the overall control unit  102  (operation S 110 ). 
     The overall control unit  102  stores information about the device connected to the center switch  1  and information about the device connected to the center switch  2  in the extended address table  121 . Furthermore, the overall control unit  102  creates a switching PID from the PID assigned to the device connected to the center switch  2 , stores the created switching PID in the extended address table  121  together with a native PID and the like, creating information about the extended name server  120  (operation S 111 ). 
     Steps S 112  to S 122  indicate processing executed to send data from the server  5  to the storage unit  4 . The server  5  sends an FC frame to be sent to the storage unit  4  to the port unit  205  (operation S 112 ). 
     The port unit  205  receives the FC frame and sends it to the routing control unit  203  (operation S 113 ). 
     The routing control unit  203  acquires the DID of the FC frame and uses the acquired DID to inquire the transfer destination of the FC frame of the overall control unit  201  (operation S 114 ). 
     The overall control unit  201  receives the DID from the routing control unit  203  and uses the received DID to acquire the PID of the storage unit  4 , which is the transfer destination, from the name server  210 , (operation S 115 ). The overall control unit  201  then sends the acquired PID of the transfer destination to the routing control unit  203  (operation S 116 ). 
     The routing control unit  203  receives the PID of the transfer destination from the overall control unit  201 . The routing control unit  203  then makes a decision as to transmission to the center switch  1  from the received PID and sends the FC frame to the routing control unit  103  in the center switch  1  through the ICL  3  (operation S 117 ). 
     The routing control unit  103  receives the FC frame, acquires the DID from the received FC frame, and uses the acquired DID to inquire the transfer destination of the FC frame of the overall control unit  101  (operation S 118 ). 
     The overall control unit  101  receives the DID from the routing control unit  103  and uses the received DID to acquire the PID of the storage unit  4 , which is the transfer destination, from the name server  110  (operation S 119 ). The overall control unit  101  then sends the acquired PID of the transfer destination to the routing control unit  103  (operation S 120 ). 
     The routing control unit  103  acquires the PID of the transfer destination, which has been sent from the overall control unit  101 . The routing control unit  103  then sends the FC frame to the port unit  105  to which the device assigned the PID of the transfer destination is connected (operation S 121 ). 
     The port unit  105  receives the FC frame from the routing control unit  103  and sends the received FC frame to the storage unit  4 , which is the device at the transfer destination, (operation S 122 ). 
     Next, processing executed by the storage system in the first embodiment at the occurrence of a center switch down will be described with reference to  FIG. 8 .  FIG. 8  is a sequence diagram of processing executed by the storage system in the first embodiment at a time when one center switch goes down. A case in which both the overall control units  201  and  202  have caused a failure and the center switch  2  has gone down will be described here. 
     The routing control units  203  and  103  exchange information through the ICL  3 . Even at a time when a failure occurs, the routing control units  203  and  103  are exchanging information (operation S 201 ). After the overall control units  201  and  202  have caused a failure, the routing control unit  103  becomes unable to receive information from the routing control unit  203 . 
     The routing control unit  103  notifies the overall control unit  101  that the routing control unit  103  is unable to receive information from the routing control unit  203  (operation S 202 ). 
     Upon receipt of a notification from the routing control unit  203 , the overall control unit  101  detects a down of the center switch  2  (operation S 203 ). The overall control unit  101  then commands the overall control unit  102  to reflect the information about the name server  210  in the center switch  2  (operation S 204 ). 
     The overall control unit  102  receives a command to reflect the information about the name server  210  in the center switch  2  from the overall control unit  101  (operation S 205 ). In the extended address table  121  stored in the extended name server  120  in the overall control unit  102 , the overall control unit  102  indicates x in the access flag field corresponding to the device connected to the center switch  2 . The overall control unit  102  decides whether communication using a switching PID is possible depending on whether the switching PID that uses the PID of the device for which x is indicated in the access flag field as a native PID is indicated in the extended address table  121  (operation S 206 ). In the description below, it is assumed that communication using a switching PID is possible. The overall control unit  102  notifies the routing control unit  103  that communication using a switching PID is possible (operation S 207 ). 
     Upon receipt of the notification from the overall control unit  102 , the routing control unit  103  notifies the routing control unit  203  of communication using a switching PID through the ICL  3  (operation S 208 ). 
     Upon receipt of the notification from the routing control unit  103 , the routing control unit  203  sets the port connected to the routing control unit  103  as a prioritized port. That is, the routing control unit  103  performs prioritized port setting in which the transfer destination of the FC frame received from the device is set to the center switch  1  (operation S 209 ). The routing control unit  203  also performs switching port setting in which output of the FC frame received from the routing control unit  103  to the specified port is set (operation S 210 ). The routing control unit  203  then notifies the routing control unit  103  that a witching port has been set (operation S 211 ). 
     Upon receipt of the notification from the routing control unit  203 , the routing control unit  103  performs switching port setting so that the FC frame transferred to the center switch  2  is received and routing to the switching port may be carried out (operation S 212 ). The routing control unit  103  then notifies the overall control unit  102  that switching port setting has been completed (operation S 213 ). 
     Upon receipt of the notification from the routing control unit  103 , the overall control unit  102  defines connection preservation or performance preservation as operation carried out when the standby side (overall control unit  102 ) goes down (operation S 214 ). When performance preservation is defined, subsequent processing is not carried out. When connection preservation is defined, the overall control unit  102  notifies the overall control unit  101  of connection preservation (operation S 215 ). 
     Upon receipt of the notification of connection preservation, the overall control unit  101  obtains synchronization with the extended name server  120  (operation S 216 ). That is, the overall control unit  101  makes a setting so that when the standby side (overall control unit  102 ) goes down, routing using the extended address table  121  stored in the extended name server  120  is taken over. 
     Next, flow of FC frame transfer processing with one center switch in the storage system in the first embodiment placed in a down state will be described with reference to  FIG. 9 .  FIG. 9  is a sequence diagram of transfer processing executed by the storage system in the first embodiment while the one center switch is in a down state. 
     The server  5  connected to the center switch  2  outputs an FC frame to be sent to another device to the port unit  205  of the center switch  2  (operation S 301 ). 
     The port unit  205  receives the FC frame from the server  5  and sends the FC frame to the routing control unit  203  (operation S 302 ). 
     The routing control unit  203  receives the FC frame from the port unit  205  and transfers the received FC frame to a prioritized port (operation S 303 ). Thus, the FC frame output from the server  5  is transferred from the routing control unit  203  in the center switch  2  to the routing control unit  103  in the center switch  1  (operation S 304 ). 
     The routing control unit  103  decides that the FC frame received from the routing control unit  203  has been transferred by using a prioritized port. The routing control unit  103  acquires a DID from the FC frame. The routing control unit  103  then uses the acquired DID to inquire the transfer destination of the FC frame of the overall control unit  102 , which is on the standby side, (operation S 305 ). 
     The overall control unit  102  receives the inquiry, which has been made by using the DID about the transfer destination, from the routing control unit  103  (operation S 306 ). The overall control unit  102  then acquires the PID of the transfer destination of the FC frame that the server  5  has output from the extended name server  120  in the overall control unit  102  (operation S 307 ). The overall control unit  102  notifies the routing control unit  103  of the acquired PID (operation S 308 ). The notification indicates whether the PID is a switching PID or the PID assigned by the center switch  1  to the device. 
     The routing control unit  103  receives the PID and decides whether it is a switching PID (operation S 309 ). If the received PID is the PID that the center switch  1  has assigned to the device (the result in operation  309  is No), the routing control unit  103  sends the FC frame to the port unit  105  to which the device assigned the PID of the transfer destination is connected (operation S 310 ). 
     The port unit  105  sends the FC frame received from the routing control unit  103  to the storage unit  4 , which is the device at the transfer destination, (operation S 311 ). 
     If the received PID is a switching PID (the result in operation S 309  is Yes), the routing control unit  103  references the switching PID and determines a transport port to which to transport the FC frame (operation S 312 ). The routing control unit  103  then sends the FC frame to the routing control unit  203  through the route indicated in the switching PID (operation S 313 ). 
     The routing control unit  203  references the switching PID, and transfers the FC frame to the port unit  205  at the destination specified by the switching PID (operation S 314 ). Thus, the FC frame sent from the routing control unit  103  is transferred to the port unit  205  (operation S 315 ). 
     The port unit  205  outputs the FC frame from the port specified by the switching PID (operation S 316 ). 
     If the destination of the FC frame is the server  5 , for example, the FC frame sent from the routing control unit  103  is sent to the server  5  because the server  5  is connected to the port specified by the switching PID (operation S 317 ). 
     When one center switch goes down, the extended name server in the other center switch performs processing. This processing will be described next with reference to  FIG. 10 .  FIG. 10  is a flowchart of processing executed by an extended name server at a time when one center switch goes down. In an example described below, the center switch  2  goes down and the extended name server  120  performs processing. 
     The extended name server  120  checks whether the DID in the FC frame received from the routing control unit  203  in the center switch  2  is found in the extended address table  121  (operation S 401 ). The extended name server  120  then decides whether the received DID is found in the PIDs in the extended address table  121  (operation S 402 ). 
     If the received DID is not found in the extended address table  121  (the result in operation S 402  is No), the extended name server  120  notifies the overall control unit  102  that the DID, which is the destination of the FC frame, is not indicated in the extended address table  121 . The overall control unit  102  commands the routing control unit  103  to discard the FC frame (operation S 408 ). 
     If the received DID is found in the extended address table  121  (the result in operation S 402  is Yes), the extended name server  120  decides whether ◯ is indicated in the access flag field on the row including the PID (operation S 403 ). If ◯ is indicated in the access flag field (the result in operations D 403  is Yes), the extended name server  120  commands the routing control unit  103  to carry out normal FC frame transfer (operation S 404 ). 
     If x is indicated in the access flag field (the result in operations S 403  is No), the extended name server  120  decides whether the PID is indicated in the native PID field in the extended address table  121  (operation S 405 ). If the PID is not indicated in the native PID field (the result in operation S 405  is No), the overall control unit  102  commands the routing control unit  103  to discard the FC frame (operation S 408 ). 
     If the PID is indicated in the native PID field (the result in operation S 405  is Yes), the overall control unit  102  decides whether ◯ is indicated in the access flag field on the row including the PID in the native PID field (operation S 406 ). If x is indicated in the access flag field (the result in operations S 406  is No), the overall control unit  102  commands the routing control unit  103  to discard the FC frame (operation S 408 ). 
     If ◯ is indicated in the access flag field (the result in operations S 406  is Yes), the extended name server  120  commands the routing control unit  103  to transfer the FC frame by using the switching PID. Then, the overall control unit  102  and routing control unit  103  transfer the FC frame by using the switching PID (operation S 407 ). 
     As described above, the fiber channel switch, which is the center switch in this embodiment, continues routing processing by using an extended name server if another center switch connected to the fiber channel switch goes down, the processing having been carried out by the other center switch. Thus, even if a center switch with a redundant configuration causes failures in both the active side and the standby side and thereby goes down, communication with a device connected to the down center switch may be continued. 
     Second Embodiment 
     Next, a storage system in a second embodiment will be described. The storage system in this embodiment differs from the storage system in the first embodiment in that a switch that simply performs routing is connected to a center switch and another device is further connected to the switch. The switch that simply performs routing will be referred to below as the transfer switch. The center switch in this embodiment has the same structure as in  FIGS. 2 and 3 . 
       FIG. 11  illustrates the states of name servers and extended name servers in the second embodiment at normal times. As illustrated in  FIG. 11 , the transfer switch  6  is connected to the center switch  2 . A server  7  is further connected to the transfer switch  6 . 
     The center switch and the transfer switch connected to it exchange the contents of address tables stored in their name servers. Each switch that has exchanged the contents of address tables adds, to its address table, information stored in the name server in the other switch with which the contents of the address tables have been exchanged. The center switch  2  and transfer switch  6  are mutually connected as an example in this embodiment. In this embodiment, therefore, the center switch  2  acquires information stored in the name server in the transfer switch  6 , that is, information about the server  7 , and stores the information in the address table  211  in the name server  210 . Conversely, the transfer switch  6  stores information stored in the name server  210  in the address table stored in the name server in the transfer switch  6 . 
     Since the center switch  2  and center switch  1  exchange information in the address tables stored in their name servers, the center switch  1  also acquires information in the address table stored in the name server in the transfer switch  6 . Thus, the name servers in the all center switches and the transfer switch in the storage system have the same contents. 
     This embodiment assumes that the WWN of the server  7  connected to the transfer switch  6  is 10:00:00:05:1e:9d:a2:00. In this case, the address table  111  in the name server  110  and the address table  211  in the name server  210  have the same contents, as illustrated in  FIG. 11 . Specifically, the address tables  111  and  211  include the PID, WWN, and Info of the storage unit  4 , server  5 , and server  7 . The name server in the transfer switch  6  also has an address table having the same contents as the address tables  111  and  211 . 
     In this embodiment, the extended address table  221  stored in the extended name server  220  includes two switching PIDs for the storage unit  4  connected to the center switch  1 . The extended address table  121  stored in the extended name server  120  includes two switching PIDs for the server  5  connected to the center switch  2  and two switching PIDs for the server  7  connected to the center switch  2  through the transfer switch  6 . PIDs  124  and  125  in  FIG. 11  indicate the switching PIDs of the server  7 , for example. Specifically, the PID  124  is a switching PID used to connect to the server  7  by using a route passing through the ICL  3  with port number  1  on the core blade inserted into the third slot. the PID  125  is a switching PID used to connect to the server  7  by using a route passing through the ICL  3  with port number  1  on the core blade inserted into the sixth slot. In the switching PID used to connect to the server  7 , the port number of a port, on the center switch  2 , to which the transfer switch  6  is connected is used as the port number of the port to which the server  7  is connected. The switching PID used to connect to the server  7  connected to the center switch  2  through the transfer switch  6  is an example of route information on a device connected to the adjacent connection control apparatus through another connection control apparatus. 
       FIG. 12  illustrates a state in which one of the center switches in the second embodiment goes down, and particularly illustrates the state of the expanded name server in the other center switch. A case in which the center switch  2  goes down in the state illustrated in  FIG. 11  will be described with reference to  FIG. 12 . 
     When the center switch  2  goes down, the overall control unit  101  commands the overall control unit  102  to reflect information stored in the name server  210  in the center switch  2 . The extended name server  120  in the overall control unit  102  indicates x in the access flag field, in the extended address table  121 , corresponding to the device directly connected to the center switch  2  and the device connected to the center switch  2  through the transfer switch  6 . In this example, x is indicated in the access flag field corresponding to two servers in the extended address table  121 , as illustrated in  FIG. 12 . 
     If the overall control unit  102  receives a request to transfer FC data to the server  5  or  7  in this state, the overall control unit  102  acquires a switching PID, which indicates a transfer destination, from the extended name server  120 . If the overall control unit  102  receives a request to transfer FC data to the server  7 , for example, the overall control unit  102  acquires the PID  124  or  125  from the extended name server  120 . The overall control unit  102  then notifies the routing control unit  103  of the PID  124  or  125 , which is the acquired switching PID. The routing control unit  103  uses the switching PID, of which the routing control unit  103  has been notified, to send a FC frame to the relevant port, on the center switch  2 , to which the transfer switch  6  is connected. The transfer switch  6  receives the FC frame from the port, on the center switch  2 , indicated by the switching PID and transfers the received FC frame to the server  7 , which is the destination indicated by the DID in the FC frame. 
     The FC frame is output from the device connected directly to the center switch  2  or connected through the transfer switch  6  to the center switch  2  and is transferred by the routing control unit  203  to the routing control unit  103  according to the prioritized port setting. The routing control unit  103  receives the FC frame and uses the PID specified by the extended name server  120  to transfer the FC frame. 
     As described above, even if a center switch goes down in a configuration in which the center switch and a device are mutually connected through a transfer switch, an FC frame may be transferred by an extended name server to the device connected to the center switch through the transfer switch. 
     How information is exchanged between name servers in center switches, each of which includes a transfer switch, when the center switch  2  goes down will be described with reference to  FIG. 13 .  FIG. 13  is a sequence diagram of processing executed at a time when one center switch in the storage system in the second embodiment goes down. In  FIG. 13 , a transfer switch  8  is connected to the center switch  1  and a transfer switch  9  is connected to the center switch  2 , these transfer switches being denoted SW. 
     The transfer switch  9  sends information stored in its name server to the port unit  205  (operation S 501 ). 
     The port unit  205  receives the information in the name server from the transfer switch  9  and transfers the received information to the routing control unit  203  (operation S 502 ). 
     The routing control unit  203  receives the information in the name server from the port unit  205  and transfers the received information to a prioritized port (operation S 503 ). The information in the name server, which has been output from the transfer switch  9  in this way, is sent to the routing control unit  103  (operation S 504 ). 
     Similarly, the transfer switch  8  sends information stored in its name server to the port unit  105  (operation S 505 ). 
     The port unit  105  receives the information in the name server from the transfer switch  8  and transfers the received information to the routing control unit  103  (operation S 506 ). 
     Since the center switch  2  is in a down state, the routing control unit  103  decides that the transfer destination of the information in the name server is the overall control unit  102  on the standby side (operation S 507 ). The routing control unit  103  then sends the information in the name server to the overall control unit  102  (operation S 508 ). 
     The overall control unit  102  receives the information in the name server and reflects the received information into the extended name server  120  in the overall control unit  102  (operation S 509 ). That is, the extended name server  120  stores, in the extended address table  121 , the received information in the name server. The overall control unit  102  transfers the received information in the name server to the overall control unit  101  (operation S 510 ). 
     The overall control unit  101  receives the information in the name server and reflects the received information into the name server  110  in the overall control unit  101  (operation S 511 ). That is, the name server  110  stores, in the address table  111 , the received information in the name server. 
     Then, the overall control unit  101  starts processing to respond to the exchanging of the information in the name server. The overall control unit  101  first decides whether the transfer switch with which the information in the name server is to be exchanged is connected to the down center switch (operation S 512 ). 
     If the information exchange destination is connected to the down center switch (the result in operation S 512  is Yes), the overall control unit  101  inquires a transfer destination of the overall control unit  102 . The overall control unit  102  receives the inquiry from the overall control unit  101  and inquires a switching PID, which is the PID of the transfer destination, of the extended name server  120  (operation S 513 ). The overall control unit  102  then sends the transfer destination PID acquired from the extended name server  120  to the routing control unit  103 . 
     If the information exchange destination is not connected to the down center switch (the result in operation S 512  is No), the overall control unit  101  sends the transfer destination PID acquired from its name server  110  to the routing control unit  103 . The routing control unit  103  receives the PID from the overall control unit  101  or  102  and decides whether the received PID is a switching PID (operation S 514 ). 
     If the received PID is not a switching PID (the result in operation S 514  is No), the routing control unit  103  sends a response to the port unit  105  connected to the device to which the PID of the transfer destination has been assigned (operation S 515 ). 
     The port unit  105  receives the response from the routing control unit  103  and sends the received response to the transfer switch  8  (operation S 516 ). 
     If the received PID is a switching PID (the result in operation S 514  is Yes), the routing control unit  103  references the switching PID and determines a transfer port to which to transfer the FC frame (operation S 517 ). The routing control unit  103  then sends the FC frame to the routing control unit  203  through the route indicated in the switching PID (operation S 518 ). 
     The routing control unit  203  references the switching PID and transfers the response to the port unit  205 , which is the destination specified in the switching PID, (operation S 519 ). Accordingly, the response sent from the routing control unit  103  is transferred to the port unit  205  (operation S 520 ). 
     The port unit  205  outputs a response from a port specified by the switching PID (operation S 521 ). 
     If the destination of the response is the transfer switch  9 , for example, the response sent from the routing control unit  103  is sent to the transfer switch  9  because the transfer switch  9  is connected to the port specified by the switching PID (operation S 522 ). 
     As described above, the fiber channel switch, which is the center switch in this embodiment, may continue communication with a device connected to a center switch, which is connected to the fiber channel switch, through a switch that simply performs routing if the center switch causes failures in both the active side and the standby side and thereby goes down. That is, even in a storage system having a very large structure in which, for example, a switch is connected to a center switch and a device is further connected to the switch, if the center switch causes failures in both the active side and the standby side and thereby goes down, communication may be continued. 
     In the above example, a case in which two center switches are used have been described. Even if N center switches are used, however, when they are connected in, for example, a star, they may operate. That is, even in a storage system in which N center switches are placed as described above, communication may be continued if a center switch causes failures in both the active side and the standby side and thereby goes down. 
     All examples and conditional language recited herein are intended for pedagogical purposes to aid the reader in understanding the invention and the concepts contributed by the inventor to furthering the art, and are to be construed as being without limitation to such specifically recited examples and conditions, nor does the organization of such examples in the specification relate to a showing of the superiority and inferiority of the invention. Although the embodiments of the present invention have been described in detail, it should be understood that the various changes, substitutions, and alterations could be made hereto without departing from the spirit and scope of the invention.