Abstract:
Provided is a method of managing a network system including a computer device which processes data, a storage system which is used in the computer device to store data, and a switch which connects the computer device and the storage system to each other. The storage system sets VLANs and the priority of each of the VLANs on the switch, VLANs including at least one of a control VLAN transferring mainly control data, and at least one of a user VLAN transferring data other than the control data. The switch transfers data in a manner that allows the VLANs of higher priority to precede the VLANs of lower priority. Therefore the storage system manages a network in accordance with the level of load or the like of respective VLANs.

Description:
CLAIM OF PRIORITY  
       [0001]     The present application claims priority from Japanese application P2004-281253 filed on Sep. 28, 2004, the content of which is hereby incorporated by reference into this application.  
       BACKGROUND  
       [0002]     This invention relates to a network system having network-attached storage and, more specifically, a technology of setting priority to plural VLANs.  
         [0003]     Network systems having network-attached storage (NAS) have recently come into existence. The emergence of such network systems has created the need for a system that is capable of managing a complicated network.  
         [0004]     An example of the management system for a complicated network is a network distributed management system (see JP 2001-144761 A, for example). The network distributed management system divides a network into network groups and provides a network monitoring server or a network monitoring terminal for each network group, so that the network is managed by plural network monitoring servers or network monitoring terminals.  
         [0005]     Another example is network management that uses the VLAN (Virtual Local Area Network) technology. With the VLAN technology, no special hardware is necessary in virtually constructing a number of LANs.  
       SUMMARY  
       [0006]     According to the network distributed management system of JP 2001-144761 A, an increase in load at some point on the network prolongs the response time of other network terminals, which lowers the quality of service. This is because lines dedicated to communications of control data including information of load of each network are not connected to the network management servers and network management terminals. Thus a delay of control information in a network that is under a great load causes a prolonged response time in other network terminals.  
         [0007]     In application of the VLAN technology to a network system with NAS, an increase in load of one VLAN affects the rest of VLANs. This is because, in NAS, the same CPU, memory and others are shared in processing of data input/output from different VLANs.  
         [0008]     This invention has been made in view of the above problems, and it is therefore an object of this invention to provide a method of managing a network system for a storage system in which NAS manages a network in accordance with the level of load or the like of respective VLANs.  
         [0009]     According to a embodiment of this invention, there is provided a method of managing a network system including a computer device which processes data, a storage system which is used in the computer device to store data, and a switch which connects the computer device and the storage system to each other, in which the storage system sets a control VLAN, a user VLAN, and the priority of each of the VLANs on the switch, the control VLAN transferring mainly control data, the user VLAN transferring data other than the control data, and in which the switch transfers data in a manner that allows the VLANs of higher priority to precede the VLANs of lower priority.  
         [0010]     According to a embodiment of this invention, it is possible to facilitate management of a network by providing the network with a control VLAN, which is dedicated to communications of control information of user VLANs. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     The present invention can be appreciated by the description which follows in conjunction with the following figures, wherein:  
         [0012]      FIG. 1  is a system configuration diagram of a network system according to a first embodiment of this invention.  
         [0013]      FIG. 2  is an explanatory diagram outlining processing of the network system according to the first embodiment of this invention.  
         [0014]      FIG. 3  is an explanatory diagram outlining processing of the network system according to the first embodiment of this invention.  
         [0015]      FIG. 4  is a block diagram of NAS according to the first embodiment of this invention.  
         [0016]      FIG. 5  is a block diagram of a directly connected L 2  switch according to the first embodiment of this invention.  
         [0017]      FIG. 6  is a configuration diagram of a VLAN priority table stored in the NAS according to the first embodiment of this invention.  
         [0018]      FIG. 7  is an explanatory diagram of processing of L 2  switches based on priority in the first embodiment of this invention.  
         [0019]      FIG. 8  is a configuration diagram of a monitoring table stored in the NAS according to the first embodiment of this invention.  
         [0020]      FIG. 9  is a configuration diagram of an event packet according to the first embodiment of this invention.  
         [0021]      FIG. 10  is a configuration diagram of a traffic packet according to the first embodiment of this invention.  
         [0022]      FIG. 11  is a configuration diagram of a priority modify packet according to the first embodiment of this invention.  
         [0023]      FIG. 12  is a relation table of addresses used in the event packet and traffic packet according to the first embodiment of this invention.  
         [0024]      FIG. 13  is a relation table of addresses used in the priority modify packet according to the first embodiment of this invention.  
         [0025]      FIG. 14  is a flow chart of processing of the network system for when an event A occurs in the first embodiment of this invention.  
         [0026]      FIG. 15  is a flow chart of processing of the NAS for when the event A occurs in the first embodiment of this invention.  
         [0027]      FIG. 16  is a flow chart of processing of the network system for when there is a change of traffic information in a user VLAN 1  in the first embodiment of this invention.  
         [0028]      FIG. 17  is a flow chart of processing of the NAS for when there is a change of traffic information in the user VLAN 1  in the first embodiment of this invention.  
         [0029]      FIG. 18  is a configuration diagram of a VLAN priority table according to a second embodiment of this invention.  
         [0030]      FIG. 19  is a configuration diagram of a VLAN priority table according to a third embodiment of this invention.  
         [0031]      FIG. 20  is a configuration diagram of a VLAN priority table according to a fourth embodiment of this invention.  
         [0032]      FIG. 21  is a configuration diagram of a VLAN priority table that is used when a control VLAN is unavailable in a fifth embodiment of this invention.  
         [0033]      FIG. 22  is a flow chart of processing of NAS that makes a control VLAN unavailable in the fifth embodiment of this invention.  
         [0034]      FIG. 23  is a system configuration diagram of a network system according to a sixth embodiment of this invention.  
         [0035]      FIG. 24  is a flow chart of processing of control VLAN for when a fault occurs in the sixth embodiment of this invention. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       [0036]     Embodiments of this invention will be described below with reference to the accompanying drawings.  
       First Embodiment  
       [0037]      FIG. 1  is a system configuration diagram of a network system according to a first embodiment of this invention. The network system of the first embodiment is composed of NAS  1 , a directly connected L 2  switch  2 , a network  3 , L 2  switches  10 - 0  to n 0 - 0 , and terminals  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n.    
         [0038]     The network attached storage (NAS)  1  is a storage system directly connected to the network to exchange data with the network. The NAS  1  is composed of a disk drive and a control unit which controls data input/output to the disk drive (see  FIG. 4  for details). The NAS  1  is a dedicated file server having a file sharing function, which allows the terminals  10 - 1  to  10 - n  to access common files, and other functions.  
         [0039]     The following description on embodiments of this invention deals with a case in which NAS controls VLANs set to a network. However, this invention is also applicable in the manner described below to an NAS head or like other storage system that has a control unit but not a disk drive.  
         [0040]     The directly connected L 2  switch  2  is a switch directly connected to the NAS  1  to judge the destination of a packet in a data link layer of the OSI reference model and transfer the packet. The NAS  1  and the directly connected L 2  switch  2  may be integrated with each other.  
         [0041]     The L 2  switches  10 - 0  to n 0 - 0  are switches directly connected to the terminals  10 - 1  to  10 - n  to judge the destination of a packet in a data link layer of the OSI reference model and transfer the packet. The directly connected L 2  switch  2  and the L 2  switches  10 - 0  to n 0 - 0  transfer packets via the network  3 .  
         [0042]     In the network system of the first embodiment, one physical LAN is divided into (n+1) VLANs. The (n+1) VLANs here are n user VLANs  10  to no and one control VLAN  4 .  
         [0043]     The user VLANs  10  to n 0  make groups of the terminals  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n , and the terminals in each group constitute a virtual LAN. This means that the terminals belonging to the same user VLAN can access each other but cannot access the terminals belonging to other VLANs.  
         [0044]     The terminals  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n  which respectively belong to the user VLANs  10  to n 0  can store data in the NAS  1 . The NAS  1  manages stored data  1 - 1  to  1 - n  separately for each of the user VLANs  10  to n 0 . The terminals in the same user VLAN can access one another&#39;s data but not data of the terminals in the rest of user VLANs.  
         [0045]     The control VLAN  4  is a virtual LAN which is connected to all of the L 2  switches  10 - 0  to n 0 - 0  and to the directly connected L 2  switch  2  and which is dedicated to communications of control information. Control information contains event information or traffic information. Event information is information of data that is about to be communicated, and refers to data type such as streaming data and backup data. Traffic information is about the data transfer amount per unit time of the user VLANs  10  to n 0 .  
         [0046]     Owing to the control VLAN  4 , the NAS  1  in the network system according to the first embodiment of this invention can quickly obtain control information of every one of the user VLANs  10  to n 0  and can therefore balance load throughout the user VLANs  10  to n 0 .  
         [0047]     Described next is the outline of the operation of the network system according to the first embodiment of this invention.  
         [0048]      FIG. 2  is an explanatory diagram outlining network system processing according to the first embodiment of this invention.  
         [0049]      FIG. 2  shows the operation of the control VLAN  4 , the NAS  1 , and the directly connected L 2  switch  2 . The NAS  1  has a control VLAN management program  111 , a VLAN priority set-up program  112 , and a monitoring table  115 .  
         [0050]     The NAS  1  first executes the control VLAN management program  111 . The control VLAN management program  111  receives event information or traffic information (control information) from the control VLAN  4  ( 200 ). The control VLAN management program  111  judges whether the received control information is event information or traffic information.  
         [0051]     In the case where the received control information is judged to be event information, the control VLAN management program  111  notifies the VLAN priority set-up program  112  of occurrence of an event ( 201 ). Notified of the event, the VLAN priority set-up program  112  sets an order of priority to the user VLANs  10  to n 0  in accordance with the event information.  
         [0052]     When there is a change in priority of the user VLANs  10  to n 0 , the VLAN priority set-up program  112  requests the directly connected L 2  switch  2  to change the order of priority of the user VLANs  10  to n 0  ( 204 ). Receiving the request of change of priority, the directly connected L 2  switch  2  changes the order of priority of the user VLANs  10  to n 0 .  
         [0053]     On the other hand, in the case where the received control information is judged to be traffic information, the control VLAN management program  111  stores the traffic information in the monitoring table  115  ( 202 ). The VLAN priority set-up program  112  monitors the monitoring table periodically ( 203 ) to set an order of priority to the user VLANs  10  to n 0  in accordance with the traffic volume.  
         [0054]     When there is a change in priority of the user VLANs  10  to n 0 , the VLAN priority set-up program  112  requests the directly connected L 2  switch  2  to change the order of priority of the user VLANs  10  to n 0  ( 204 ). Receiving the request of change of priority, the directly connected L 2  switch  2  changes the order of priority of the user VLANs  10  to n 0 .  
         [0055]     According to the outline of the first embodiment shown in  FIG. 2 , the NAS  1  sets an order of priority that suits control information received from the control VLAN  4  to the directly connected L 2  switch  2 .  
         [0056]      FIG. 3  is an explanatory diagram outlining processing of the network system according to the first embodiment of this invention.  
         [0057]      FIG. 3  shows the operation of the directly connected L 2  switch  2  and the L 2  switches  10 - 0  to n 0 - 0 .  
         [0058]     After changing the order of priority of the user VLANs  10  to n 0 , the directly connected L 2  switch  2  requests every one of the L 2  switches  10 - 0  to n 0 - 0  to change the priorities of the user VLANs  10  to n 0  ( 210 ). Receiving the request of change of priority, the L 2  switches  10 - 0  to n 0 - 0  change the priorities of the user VLANs  10  to n 0 .  
         [0059]     According to the outline of the first embodiment shown in  FIG. 3 , the user VLANs  10  to n 0  are all set to the same priority level by the directly connected L 2  switch  2  and the L 2  switches  10 - 0  to n 0 -O.  
         [0060]     Such processing is achieved by details of the network system according to the first embodiment of this invention which will be described below.  
         [0061]      FIG. 4  is a block diagram of the NAS  1  according to the first embodiment of this invention.  
         [0062]     The NAS  1  is composed of a CPU  100 , a memory  101 , an input output processor (IOP)  102 , a network interface controller (NIC)  103 , a disk controller  104 , and a disk drive  105 .  
         [0063]     The memory  101  stores an OS (Operating System)  110 , the control VLAN management program  111 , the VLAN priority set-up program  112 , a file service A  113 , a file service B  114 , the monitoring table  115 , a user VLAN management table  116 , and a VLAN priority set-up table  117 . Various information stored in the memory  101  are inputted by a terminal (management server) connected to the NAS  1  via the network  3 .  
         [0064]     The CPU  100  loads and executes the programs  110 ,  111 , and  112  stored in the memory  101  to perform various processing.  
         [0065]     The IOP  102  controls data input and output of the NIC  103 . The NIC  103  is an interface that is connected to the directly connected L 2  switch  2  via the Ethernet. The disk controller  104  controls data input/output to the disk drive  105 . The disk drive  105  stores data of the terminals  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n.    
         [0066]     The control VLAN management program  111  manages control information received from the control VLAN  4 . The VLAN priority set-up program  112  sets the priority that suits the received control information to the directly connected L 2  switch  2 .  
         [0067]     The file service A  113  and the file service B  114  are, for example, NFS and samba, and provide a file sharing service to the terminals  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n . The monitoring table  115  stores traffic information of the user VLANs  10  to n 0 .  
         [0068]     The user VLAN management table  116  stores the priorities of the user VLANs  10  to n 0 . The VLAN priority table  117  stores the association relation between control information and priority.  
         [0069]      FIG. 5  is a block diagram of the directly connected L 2  switch  2  according to the first embodiment of this invention.  
         [0070]     The directly connected L 2  switch  2  is the network&#39;s switch composed of a CPU  120 , a memory  121 , a switch control interface  122 , and an interface  123 .  
         [0071]     The memory  121  stores an OS  130 , a VLAN priority set-up program  131 , a VLAN priority management table  132 , a traffic information collecting program  133 , and an event information collecting program  134 . Various information stored in the memory  121  are inputted by a terminal connected to the directly connected L 2  switch  2  via the network  3 .  
         [0072]     The CPU  120  loads and executes the programs  130  to  134  stored in the memory  121  to perform various processing.  
         [0073]      FIG. 5  shows only one interface  123  but actually there is more than one interface  123  connected to the NAS  1  and the network  3 . The switch control interface  122  is connected to a terminal or the like, through which the directly connected L 2  switch  2  is controlled.  
         [0074]     The VLAN priority set-up program  131  sets an order of priority to the user VLANs  10  to n 0 . The VLAN priority management program  132  stores the priorities of the user VLANs  10  to n 0  in a given area of the memory  121 .  
         [0075]     The traffic information collecting program  133  collects traffic information to send the collected information to the NAS  1 . The event information collecting program  134  collects event information to send the collected information to the NAS  1 .  
         [0076]     The L 2  switches  10 - 0  to n 0 - 0  have the same configuration as the directly connected L 2  switch  2 .  
         [0077]      FIG. 6  is a configuration diagram of the VLAN priority table  117  stored in the NAS  1  according to the first embodiment of this invention.  
         [0078]     The VLAN priority table  117  is composed of a priority  141 , a control VLAN flag  142 , a user VLAN flag  143 , event information  144 , and traffic information  145 .  
         [0079]     The priority  141  is the order in which the L 2  switches  10 - 0  to n 0 - 0  send data. In this embodiment, the priority  141  has eight levels, “ 0 ” to “ 7 ”, with the level “ 7 ” being the highest priority and gradually lowering toward the level “ 0 ”. The L 2  switches  10 - 0  to n 0 -O send, as will be described with reference to  FIG. 7 , data of the highest priority first.  
         [0080]     The control VLAN flag  142  indicates how the priority of a related record is allotted to the control VLAN  4 . In this embodiment, the priority level “ 7 ” is allotted to the control VLAN  4  in order to give priority to communications of control information.  
         [0081]     The user VLAN flag  143  indicates how the priorities of related records are allotted to the user VLANs  10  to n 0 . In this embodiment, the priority levels “ 0 ” to “ 6 ” are allotted to the user VLANs  10  to n 0 .  
         [0082]     The event information  144  holds the name of an event that is associated with the priority of a related record and the code that identifies the event. In this embodiment, an “event A” with an event information code “ 0 A” is given a priority level of “ 6 ”. Similarly, a priority level of “ 5 ” is given to an “event B” with an event information code “ 0 B” and the subsequent events down to an “event G” with an event information code “ 1 A” are given their respective priority levels.  
         [0083]     For instance, stream data such as audio data is handled as the “event A” of high priority to reduce delay in data transfer. Backup data or the like is handled as the “event G” of low priority to transfer other types of data first and thereby enhance the usability of the network system.  
         [0084]     Priorities are also set to VLAN traffics. The traffic information  145  holds the data transfer amount per unit time and traffic information identifying code that are associated with the priority of a related record. In this embodiment, a traffic information code “ 0 A” which means “100 Gbit/sec or more” is associated with the priority level “ 6 ”. Similarly, a traffic information code “ 0 B” which means “10 Gbit/sec to 100 Gbit/sec” is associated with the priority level “ 5 ” and the subsequent traffic information codes down to one that means “below 1 Mbit/sec” are given their respective priorities.  
         [0085]     This embodiment uses the data transfer amount per unit time for traffic information. Alternatively, response time or the number of times data is issued may be used as traffic information. The response time refers to a time it takes for the NAS  1  to receive a packet from the L 2  switches  10 - 0  to n 0 -O and to send the result. The number of times data is issued refers to the number of times the NAS  1  is accessed.  
         [0086]     In the network system of the first embodiment, the priority levels from “ 6 ” to down are set in a manner that gives higher priority to data of larger transfer amount to thereby process user VLANs that have a heavier load first. The load is thus evened out throughout the user VLANs  10  to n 0 .  
         [0087]     As has been defined in the user VLAN flag  143 , the event information  144  and the traffic information  145  in the first embodiment are applied only to the user VLANs  10  to n 0 .  
         [0088]     In the case where event information and traffic information occur at the same time, various methods can be employed to determine the priority. For instance, the priority of either event information or traffic information that has higher priority is set as the priority of the corresponding VLAN. To give a specific example, the priority level is set to “ 5 ” when the event information code is “ 0 E” whereas the traffic information code is “ 0 B”. Alternatively, the priority of event information may be set as the priority of the corresponding VLAN irrespective of the priority of traffic information.  
         [0089]     Now, a description is given on processing of the L 2  switches  10 - 0  to n 0 -O based on priority.  
         [0090]      FIG. 7  is an explanatory diagram of processing of L 2  switches based on priority in the first embodiment of this invention.  
         [0091]     A control VLAN  4 , a user VLAN 1  ( 10 ), a user VLAN 2  ( 20 ), and a user VLAN 3  ( 30 ) are connected to the L 2  switches  10 - 0  to n 0 -O. Queues are provided in the L 2  switches  10 - 0  to n 0 -O for each VLAN connected.  
         [0092]     When data is sent from the VLANs, the L 2  switches  10 - 0  to n 0 - 0  store transmission packets in the corresponding queues. The L 2  switches  10 - 0  to n 0 - 0  then send packets in the order of priority set. In the case where packets have the same priority, the packets are sent round robin.  
         [0093]     In this explanatory diagram, the control VLAN  4  has the priority level “ 7 ”, the user VLAN 1  ( 10 ) has the priority level “ 2 ”, the user VLAN 2  ( 20 ) has the priority level “ 2 ”, and the user VLAN 3  ( 30 ) has the priority level “ 1 ”.  
         [0094]     The L 2  switches  10 - 0  to n 0 -O first send all packets stored in the queue for the control VLAN  4 , which has the highest priority level “ 7 ” ( 221 ). Then the L 2  switches  10 - 0  to n 0 -O send packets of the user VLAN 1  ( 10 ) and the user VLAN 2  ( 20 ) which have the second highest priority of the four. Since the user VLAN 1  ( 10 ) and the user VLAN 2  ( 20 ) have the same priority, their packets are sent alternately round robin ( 222 ,  223 ). The L 2  switches  10 - 0  to n 0 - 0  lastly send packets stored in the queue for the user VLAN 3  ( 30 ), which has the lowest priority of the four ( 224 ).  
         [0095]     Having the L 2  switches  10 - 0  to n 0 - 0  send packets in the order of priority makes it possible to execute processing of greater importance first. This also makes it possible to even out the load throughout the VLANs.  
         [0096]     The directly connected L 2  switch  2  sends packets based on priority similar to the L 2  switches  10 - 0  to n 0 - 0 .  
         [0097]      FIG. 8  is a configuration diagram of the monitoring table  115  stored in the NAS  1  according to the first embodiment of this invention.  
         [0098]     The monitoring table  115  is composed of a time  160  and traffic information  161  to  16   n  of the respective user VLANs.  
         [0099]     The time  160  is a time at which a record in question is stored in the monitoring table  115 . The traffic information  161  to  16   n  of the user VLAN 1  to user VLANn holds the data transfer amount per unit time of the user VLAN  1  ( 10 ) to user VLANn (n 0 ).  
         [0100]      FIG. 9  is a configuration diagram of an event packet according to the first embodiment of this invention.  
         [0101]     An event packet  180  is sent, when an event occurs, to the NAS  1  from the terminal  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n.    
         [0102]     The event packet  180  contains a destination address  181 , a sender address  182 , and event information  183 .  
         [0103]     The destination address  181  is the address to which this packet is to be sent, and corresponds to the address of the NAS  1 . The sender address  182  is the address of one of the user VLANs  10  to n 0  to which the sender of this packet (one of the terminal  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n ) belongs. The event information  183  holds a code that identifies event information of data that is about to be sent.  
         [0104]      FIG. 10  is a configuration diagram of a traffic packet according to the first embodiment of this invention.  
         [0105]     A traffic packet  170  is periodically sent to the NAS  1  from the terminal  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n.    
         [0106]     The traffic packet  170  contains a destination address  171 , a sender address  172 , and traffic information  173 .  
         [0107]     The destination address  171  is the address to which this packet is to be sent, and corresponds to the address of the NAS  1 . The sender address  172  is the address of one of the user VLANs  10  to n 0  to which the sender of this packet (one of the terminal  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n ) belongs. The traffic information  173  holds a code that identifies current traffic information.  
         [0108]      FIG. 11  is a configuration diagram of a priority modify packet according to the first embodiment of this invention.  
         [0109]     A priority modify packet  150  is sent, when the priorities of the user VLANs  10  to n 0  are to be changed, from the NAS  1  to the directly connected L 2  switch  2 , or from the directly connected L 2  switch  2  to every one of the L 2  switches  10 - 0  to n 0 - 0 .  
         [0110]     The priority modify packet  150  contains a destination address  151 , a sender address  152 , a VLAN identifier  153 , and a VLAN priority  154 .  
         [0111]     The destination address  151  is an address to which this packet is to be sent. The sender address  152  is an address from which this packet is sent. The VLAN identifier  153  is an identifier of one of the user VLANs  10  to n 0  whose priority is to be changed. The VLAN priority  154  shows a priority level set after the priority change for one of the user VLANs  10  to n 0 .  
         [0112]      FIG. 12  is a relation table of addresses used in the event packet  180  and traffic packet  170  according to the first embodiment of this invention. The table defines which address is associated with a destination/sender  190 .  
         [0113]     The address relation table is stored in the terminal  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n , the L 2  switches  10 - 0  to n 0 - 0 , and the NAS  1 .  
         [0114]     The destination address  181  and the sender address  182  in the event packet  180  are addresses listed as an address  191 . The destination address  171  and the sender address  172  in the traffic packet  170  are also addresses listed as the address  191 .  
         [0115]      FIG. 12  shows addresses associated with the NAS and the VLANs. Specifically, the address of the NAS is “ 00 ”, the address of the user VLAN  1  is “ 01 ”, and the address of the user VLANn is “ 0 n”. The address in broadcasting, where data is sent to every terminal in a segment, is “FF”.  
         [0116]     The address  191  is also used as the VLAN identifier  153  of the priority modify packet  150 .  
         [0117]      FIG. 13  is a relation table of addresses used in the priority modify packet  150  according to the first embodiment of this invention. The table defines which address is associated with a destination/sender  193 .  
         [0118]     The address relation table is stored in the directly connected L 2  switch  2  and the NAS  1 .  
         [0119]     The destination address  151  and the sender address  152  in the priority modify packet  150  are addresses listed as an address  194 .  
         [0120]     In  FIG. 13 , the address of the NAS is “ 00 ”, the address of the directly connected L 2  switch  2  is “ 01 ”, the address of the L 2  switch  10 - 0  is “ 2 ”, and the address of the L 2  switch n′ 0 - 0  is “ 0 n+1”. The address in broadcasting, where data is sent to every terminal in a segment, is “FF”.  
         [0121]     The description given next is about processing for when the event A occurs in the network system according to this invention.  
         [0122]      FIG. 14  is a flow chart of processing of the network system for when the event A occurs in the network system according to this invention.  
         [0123]     Let us assume here that the event A with the priority level “ 6 ” occurs in the terminal  10 - 1  which belongs to the user VLAN 1  ( 10 ) ( 1000 ).  
         [0124]     The terminal  10 - 1  in which the event A has occurred creates the event packet  180  that indicates occurrence of the event A. The event packet  180  contains the address “ 00 ” of the NAS  1  as the destination address  181 , the address “ 01 ” of the user VLAN 1  ( 10 ) to which the terminal  10 - 1  belongs as the sender address  182 , and the code “ 0 A” of the event A as the event information  183 .  
         [0125]     The terminal  10 - 1  sends the created event packet  180  to the L 2  switch  10 - 0  to which the terminal  10 - 1  is directly connected ( 1001 ).  
         [0126]     The L 2  switch  10 - 0  receives the event packet  180  from the terminal  10 - 1  ( 1010 ). The event packet  180  is stored in the queue for the control VLAN  4  by the L 2  switch  10 - 0 . Since the control VLAN  4  is set to the priority level “ 7 ”, the L 2  switch  10 - 0  uses the control VLAN  4  to immediately send the event packet  180  to the directly connected L 2  switch  2  ( 1011 ).  
         [0127]     The directly connected L 2  switch  2  receives the event packet  180  from the L 2  switch  10 - 0  ( 1020 ). Using the control VLAN  4 , the directly connected L 2  switch  2  then sends the event packet  180  to the NAS  1  ( 1021 ).  
         [0128]     The NAS  1  receives the event packet  180  from the directly connected L 2  switch  2  ( 1030 ). The NAS  1  then carries out processing illustrated in  FIG. 15  ( 1031 ) to create the priority modify packet  150 . The priority modify packet  150  stores the address “ 01 ” of the directly connected L 2  switch  2  as the destination address  151 , the address “ 00 ” of the NAS  1  as the sender address  152 , the address “ 01 ” of the user VLAN 1  as the VLAN identifier  153 , and the priority level “ 6 ” of the event A as the VLAN priority  154 .  
         [0129]     The NAS  1  uses the control VLAN  4  to send the created priority modify packet  150  to the directly connected L 2  switch  2  ( 1032 ).  
         [0130]     The directly connected L 2  switch  2  receives the priority modify packet  150  from the NAS  1  ( 1040 ). Referring to the priority modify packet  150 , the directly connected L 2  switch  2  changes the priority of one of the user VLANs  10  to n 0  that is associated with the address stored as the VLAN identifier  153  to the VLAN priority  154  ( 1041 ). In this example, the directly connected L 2  switch  2  changes the priority of the user VLAN 1  ( 10 ) to the level “ 6 ”. A priority change by the directly connected L 2  switch  2  is achieved by having the VLAN priority management program  132  change the priority stored in a given area of the memory  121 .  
         [0131]     Then the directly connected L 2  switch  2  changes the destination address  151  and the sender address  152  in the received priority modify packet  150  ( 1042 ). In this example, the destination address  151  is changed to the address “FF” for broadcasting and the sender address  151  is changed to the address “ 01 ,” of the directly connected L 2  switch  2 . The directly connected L 2  switch  2  uses the control VLAN  4  to send the address modify packet  150  with the addresses changed to every L 2  switch ( 1043 ).  
         [0132]     The L 2  switches  10 - 0  to n 0 - 0  receive the priority modify packet  150  ( 1050 ). The L 2  switches  10 - 0  to n 0 - 0  consult the priority modify packet  150  to change the priority of one of the user VLANs  10  to n 0  that is associated with the address stored as the VLAN identifier  153  to the VLAN priority  154  ( 1051 ). In this example, the L 2  switches  10 - 0  to n 0 -O change the priority of the user VLAN 1  ( 10 ) to the level “ 6 ”. A priority change by the L 2  switches  10 - 0  to n 0 - 0  is achieved by having the VLAN priority management program  132  changes the priority stored in a given area of the memory  121 .  
         [0133]     Through the above processing, the NAS  1  changes the priority stored in the directly connected L 2  switch  2  in accordance with event information. Furthermore, the directly connected L 2  switch  2  makes the priority of the corresponding VLAN equal in every one of the L 2  switches  10 - 0  to n 0 - 0 .  
         [0134]      FIG. 15  is a flow chart of processing of the NAS  1  for when the event A occurs in the first embodiment of this invention.  
         [0135]     The NAS  1  receives the event packet  180  from the directly connected L 2  switch  2  ( 1030  of  FIG. 14 ) and, upon reception, executes the control VLAN management program  111 .  
         [0136]     The control VLAN management program  111  chooses, from the VLAN priority table  117 , a record in which the code of the event information  183  of the received event packet  180  and the code of the event information  144  match. The control VLAN management program  111  extracts the priority  141  of the chosen record. In this example, the code of the event information  183  is “ 0 A” and therefore the level “ 6 ” of the priority  141  is extracted. The control VLAN management program  111  sets the extracted priority level “ 6 ” to the priority of the user VLAN ( 10 ) ( 1101 ).  
         [0137]     Then the control VLAN management program  111  issues an order to start the VLAN priority set-up program ( 1102 ).  
         [0138]     As the VLAN priority set-up program  112  is activated, the priority of one of the user VLANs  10  to n 0  that is associated with the sender address  182  in the event packet  180  is read from the user VLAN management table  116  ( 1109 ). In this example, the address “ 01 ” is stored as the sender address  182  in the event packet  182  and therefore the priority of the user VLAN  1  ( 10 ) is read from the user VLAN management table  116 .  
         [0139]     Next, the VLAN priority set-up program  112  judges whether there is a change in user VLAN priority or not ( 1110 ). In this example, whether the priority read in the step S 1109  is the level “ 6 ” or not is judged.  
         [0140]     When there is no change in priority, the VLAN priority set-up program  112  is terminated at this point.  
         [0141]     When there is a change in priority, on the other hand, the VLAN priority set-up program  112  creates the priority modify packet  150  ( 1111 ). The priority modify packet  150  stores the address “ 01 ,” of the directly connected L 2  switch  2  as the destination address  151 , the address “ 00 ” of the NAS  1  as the sender address  152 , the address “ 01 ” of the user VLAN 1  whose priority is changed as the VLAN identifier  153 , and the priority level “ 6 ” of the event A as the VLAN priority  154 . After creating the priority modify packet  150 , the VLAN priority set-up program  112  is terminated.  
         [0142]     Subsequently, the processing proceeds to the step S 1032  of  FIG. 14  to continue. The NAS  1  thus determines the priority and creates the priority modify packet  150  which is to be sent to the directly connected L 2  switch  2 .  
         [0143]     The description given next is about processing for when traffic information is changed in the network system according to the first embodiment of this invention.  
         [0144]      FIG. 16  is a flow chart of processing of the network system for when there is a change of traffic information in the user VLAN  1  in the first embodiment of this invention.  
         [0145]     Assume that the data transfer amount per unit time of the user VLAN 1  ( 10 ) has changed to 1.5 Gbit/sec. The L 2  switch  10 - 0  uses the traffic information collecting program  133  to measure the data transfer amount per unit time and collect the measurements as traffic information ( 1200 ).  
         [0146]     The L 2  switch then creates the traffic packet  170 . The traffic packet  170  stores the address “ 00 ” of the NAS  1  as the destination address  171 , the address “ 01 ” of the user VLAN 1  ( 10 ) as the sender address  172 , and a code “ 0 C”, which corresponds to a data transfer amount “1.5 G”, as the traffic information  173 .  
         [0147]     The L 2  switch  10 - 0  uses the control VLAN  4  to send the created traffic packet  170  to the directly connected L 2  switch  2  at a given timing (for example, periodically) ( 1201 ).  
         [0148]     The directly connected L 2  switch  2  receives the traffic packet  170  ( 1210 ). The directly connected L 2  switch  2  uses the control VLAN 4  to send the traffic packet  170  to the NAS  1  (a step S 1211 ).  
         [0149]     The NAS  1  receives the traffic packet  170  ( 1220 ). The NAS  1  then carries out processing illustrated in  FIG. 17  ( 1221 ) to create the priority modify packet  150 . The priority modify packet  150  in this example stores the address “ 01 ” of the directly connected L 2  switch  2  as the destination address  151 , the address “ 00 ” of the NAS  1  as the sender address  151 , the address “ 01 ” of the user VLAN 1  ( 10 ) as the VLAN identifier  153 , and the priority level “ 4 ”, which corresponds to the data transfer amount “1.5 G”, as the VLAN priority  154 .  
         [0150]     The NAS  1  uses the control VLAN  4  to send the created priority modify packet  150  to the directly connected L 2  switch  2  ( 1222 ).  
         [0151]     The directly connected L 2  switch  2  receives the priority modify packet  150  from the NAS  1 ( 1230 ). The directly connected L 2  switch  2  consults the priority modify packet  150  to change the priority of one of the user VLANs  10  to n 0  that is associated with the address stored as the VLAN identifier  153  to the VLAN priority  154  ( 1231 ). In this example, the directly connected L 2  switch  2  changes the priority of the user VLAN  1 ( 10 ) to the level “ 4 ”.  
         [0152]     Then the directly connected L 2  switch  2  changes the destination address  151  and the sender address  152  in the received priority modify packet  150  ( 1232 ). In this example, the destination address  151  is changed to the address “FF” for broadcasting and the sender address  151  is changed to the address “ 01 ” of the directly connected L 2  switch  2 . The directly connected L 2  switch  2  sends the address modify packet  150  with the addresses changed to every L 2  switch ( 1233 ).  
         [0153]     The L 2  switches  10 - 0  to n 0 -O receive the priority modify packet  150  ( 1240 ). The L 2  switches  10 - 0  to n 0 - 0  consult the priority modify packet  150  to change the priority of one of the user VLANs  10  to n 0  that is associated with the address stored as the VLAN identifier  153  to the VLAN priority  154  ( 1241 ). In this example, the L 2  switches  10 - 0  to n 0 -O change the priority of the user VLAN 1  ( 10 ) to the level “ 4 ”.  
         [0154]     Through the above processing, the NAS  1  changes the priority stored in the directly connected L 2  switch  2  in accordance with traffic information. Furthermore, the directly connected L 2  switch  2  makes the priority of the corresponding VLAN equal in every one of the L 2  switches  10 - 0  to n 0 - 0 .  
         [0155]      FIG. 17  is a flow chart of processing of the NAS  1  for when traffic information is changed in the user VLAN 1  ( 10 ) in the first embodiment of this invention.  
         [0156]     The NAS  1  receives the traffic packet  170  from the directly connected L 2  switch  2  ( 1220  of  FIG. 16 ) and, upon reception, executes the control VLAN management program  111 .  
         [0157]     The control VLAN management program  111  extracts, from the traffic packet  170  received, the address “ 01 ” of the sender address  172  and the code “ 0 c” of the traffic information  173 . The control VLAN management program  111  then stores the extracted traffic information  173  in a record of the monitoring table  115  that is recorded at the time the traffic packet  170  is received ( 1301 ).  
         [0158]     Then the control VLAN management program  111  issues an order to start the VLAN priority set-up program ( 1302 ).  
         [0159]     As the VLAN priority set-up program  112  is activated, the priority of one of the user VLANs  10  to n 0  that is associated with the sender address  172  in the traffic packet  170  is read from the user VLAN management table  116  ( 1309 ). In this example, the address “ 01 ” is stored as the sender address  172  in the traffic packet  172  and therefore the priority of the user VLAN  1  ( 10 ) is read from the user VLAN management table  116 .  
         [0160]     Next, the VLAN priority set-up program  112  chooses, from the VLAN priority table  117 , a record in which the code of the traffic information  173  of the traffic packet  173  and the code of the traffic information  145  match. The VLAN priority set-up program  112  extracts the priority  141  of the chosen record. In this example, the code of the traffic information  173  is “ 0 C” and therefore the level “ 4 ” of the priority  141  is extracted. The VLAN priority set-up program  112  sets the extracted priority level “ 4 ” to the priority of the user VLAN 1  ( 10 ) ( 1310 ).  
         [0161]     Next, whether there is a change in priorities in the user VLANs  10  to n 0  or not is judged ( 1311 ). In this example, whether the priority read in the step S 1309  is the level “ 4 ” or not is judged.  
         [0162]     When there is no change in priority, the VLAN priority set-up program  112  is terminated at this point.  
         [0163]     When there is a change in priority, on the other hand, the VLAN priority set-up program  112  creates the priority modify packet  150  ( 1312 ). The priority modify packet  150  stores the address “ 01 ” of the directly connected L 2  switch  2  as the destination address  151 , the address “ 00 ” of the NAS  1  as the sender address  152 , the address “ 01 ” of the user VLAN 1  whose priority is changed as the VLAN identifier  153 , and the priority level “ 4 ” determined in the step S 1310  as the VLAN priority  154 . After creating the priority modify packet  150 , the VLAN priority set-up program  112  is terminated.  
         [0164]     Subsequently, the processing proceeds to the step S 1222  of  FIG. 16  to continue. The NAS  1  thus determines the priority and creates the priority modify packet  150  which is to be sent to the directly connected L 2  switch  2 .  
         [0165]     The NAS  1  may create a priority modify packet using as traffic information the response time or the number of times data is issued.  
         [0166]     In the network system having the NAS  1  according to the first embodiment of this invention, the control VLAN  4  is provided in addition to the user VLANs  10  to n 0  as a constituent of VLANs. The control VLAN  4  is dedicated to communications of control data such as network load information. The control VLAN  4  is set to the highest priority in order to communicate control data first.  
         [0167]     The NAS  1  collects control data from the control VLAN  4  to judge whether the priorities of the user VLANs  10  to n 0  are optimum. In the case where the NAS  1  updates the priorities of the user VLANs  10  to n 0 , the directly connected L 2  switch  2 , which is directly connected to the NAS  1 , is notified of the change. The directly connected L 2  switch  2  further notifies every one of the L 2  switches  10 - 0  to n 0 - 0  of the priority change of the user VLANs  10  to n 0 , and makes the priority of the corresponding VLAN equal in all of the L 2  switches  10 - 0  to n 0 - 0 .  
         [0168]     With these functions, the network system of this embodiment manages the network in accordance with the load of the user VLANs  10  to n 0 , thus setting the optimum priority to each of the user VLANs  10  to n 0  and balancing the load throughout the user VLANs  10  to n 0 . Moreover, the SLA (response time) can be kept constant by balancing the load throughout all the VLANs.  
       Second Embodiment  
       [0169]     In a second embodiment of this invention, communications of event information by the control VLAN  4  is given the highest priority.  
         [0170]     The configuration and processing of a network system according to the second embodiment are the same as those of the first embodiment except the VLAN priority table  117  stored in the NAS  1 . Therefore descriptions on points other than the VLAN priority management table  117  are omitted here.  
         [0171]      FIG. 18  is a configuration diagram of the VLAN priority table  117  according to the second embodiment of this invention.  
         [0172]     The VLAN priority table  117  of the second embodiment is composed of the same items as those in the priority management table ( FIG. 6 ) of the first embodiment and, accordingly, descriptions on details thereof will not be repeated.  
         [0173]     In the VLAN priority table  117  of the second embodiment, the priority levels “ 7 ” and “ 6 ” are allotted to the control VLAN  4  whereas the priority levels “ 5 ” to “ 0 ” are allotted to the user VLANs  10  to n 0 . The priority level “ 7 ” is allotted to event information of the control VLAN  4 , and the priority level “ 6 ” is allotted to traffic information of the control VLAN  4 . In other words, communications of the event packet  180  is at the priority level “ 7 ” whereas communications of the traffic packet  170  is at the priority level “ 6 ”, thus giving the top priority to communications of the event packet  180 .  
         [0174]     That is, once an event occurs in the network system of the second embodiment, communications of the event packet  180  are given higher priority than in the first embodiment and the priorities of the user VLANs  10  to n 0  can be set at high speed.  
       Third Embodiment  
       [0175]     In a third embodiment of this invention, communications of important event information by the control VLAN  4  is given a high priority.  
         [0176]     The configuration and processing of a network system according to the third embodiment are the same as those of the first embodiment except the VLAN priority table  117  stored in the NAS  1 . Therefore descriptions on points other than the VLAN priority management table  117  are omitted here.  
         [0177]      FIG. 19  is a configuration diagram of the VLAN priority table  117  according to the third embodiment of this invention.  
         [0178]     The VLAN priority table  117  of the third embodiment is composed of the same items as those in the priority management table ( FIG. 6 ) of the first embodiment and, accordingly, descriptions on details thereof will not be repeated.  
         [0179]     In the VLAN priority management table  117  of the third embodiment, the priority levels “ 7 ” to “ 4 ” are allotted to the control VLAN  4  and the priority levels “ 3 ” to “ 0 ” are allotted to the user VLANs  10  to n 0 . Specifically, the priority level “ 7 ” is allotted to the events A and B of the control VLAN  4 , the priority level “ 6 ” is allotted to events C and D of the control VLAN  4 , the priority level “ 5 ” is allotted to events E, F, and G of the control VLAN  4 , and the priority level “ 4 ” is allotted to traffic information of the control VLAN  4 . In this way, the priority of event information is set in accordance with the degree of importance of the event.  
         [0180]     In the network system of the third embodiment, communications of a packet of an event in question precedes transmission of other event packet  180  and the traffic packet  170  following the priority determined based on the type of event. As a result, the priorities of the user VLANs  10  to n 0  can be set at higher speed than in the second embodiment.  
       Fourth Embodiment  
       [0181]     In a fourth embodiment of this invention, communications of high-load traffic information by the control VLAN  4  is given a high priority.  
         [0182]     The configuration and processing of a network system according to the fourth embodiment are the same as those of the first embodiment except the VLAN priority table  117  stored in the NAS  1 . Therefore descriptions on points other than the VLAN priority management table  117  are omitted here.  
         [0183]      FIG. 20  is a configuration diagram of the VLAN priority table  117  according to the fourth embodiment of this invention.  
         [0184]     The VLAN priority table  117  of the fourth embodiment is composed of the same items as those in the priority management table ( FIG. 6 ) of the first embodiment and, accordingly, descriptions on details thereof will not be repeated.  
         [0185]     In the VLAN priority management table  117  of the fourth embodiment, the priority levels “ 7 ” to “ 4 ” are allotted to the control VLAN  4  and the priority levels “ 3 ” to “ 0 ” are allotted to the user VLANs  10  to n 0 . Specifically, the priority level “ 7 ” is allotted to event information of the control VLAN  4 , the priority level “ 6 ” is allotted to traffic information “10 G or more” of the control VLAN  4 , the priority level “ 5 ” is allotted to traffic information “100 M to 10 G” of the control VLAN  4 , and the priority level “ 4 ” is allotted to traffic information “below 100 M” of the control VLAN  4 . In this way, the priority of event information is set in accordance with the degree of importance of the event.  
         [0186]     The network system of the fourth embodiment sets the priority based on traffic information to give priority to communications of the traffic packet  170  of a user VLAN with a greater load. Therefore the load can be evened out at high speed throughout the user VLANs.  
       Fifth Embodiment  
       [0187]     In a fifth embodiment of this invention, the control VLAN  4  is temporarily made disable.  
         [0188]     The configuration and processing of a network system according to the fifth embodiment are the same as those of the first embodiment except that two VLAN priority tables  117  and  118  are stored in the NAS  1 . Therefore descriptions on points other than the configuration of the priority management table  118 , which is used when the control VLAN is made disable, and processing of making the control VLAN  4  unusable are omitted here.  
         [0189]      FIG. 21  is a configuration diagram of the VLAN priority management table  118  used when the control VLAN  4  is made disable in the fifth embodiment of this invention.  
         [0190]     In the VLAN priority management table  118  for when the control VLAN  4  is made disable, the priority level “ 7 ” is allotted to none of the VLANs. The rest of the configuration is identical with that of the VLAN priority management table  117  in the first embodiment.  
         [0191]     The priority control of user VLANs by the control VLAN  4  is not necessary if for what operation the network system is used is predetermined. In this case, turning the control VLAN  4  unusable makes it possible to avoid overhead, which is caused by the use of the control VLAN  4 .  
         [0192]     Processing of making the control VLAN  4  unusable will be described next.  
         [0193]      FIG. 22  is a flow chart of processing of the NAS  1  that makes the control VLAN  4  unusable in the fifth embodiment of this invention.  
         [0194]     When the terminals  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n  do not need to use the control VLAN  4 , the terminals send the event packet  180  indicating that the control VLAN  4  is unusable to the NAS  1 .  
         [0195]     The control VLAN management program  111  of the NAS  1  reads an event in which the control VLAN  4  is unusable from the event information  183  of the received event packet  180  ( 1301 ). Then the program  111  issues an order to activate the VLAN priority set-up program ( 1302 ).  
         [0196]     As the VLAN priority set-up program  112  is activated, a packet that instructs to make the control VLAN  4  unusable is created ( 1311 ) and sent to the directly connected L 2  switch  2  ( 1312 ). Receiving the packet, the directly connected L 2  switch  2  makes the control VLAN  4  unusable. The directly connected L 2  switch  2  then sends the packet that instructs to make the control VLAN  4  unusable to every one of the L 2  switches  10 - 0  to n 0 - 0 . The control VLAN  4  is made disable in the network system through this processing.  
       Sixth Embodiment  
       [0197]     In a sixth embodiment of this invention, the control VLAN  4  of a network system is duplicated.  
         [0198]      FIG. 23  is a system configuration diagram of a network system according to the sixth embodiment of this invention.  
         [0199]     The configuration of the network system according to the sixth embodiment is the same as that of the network system in the first embodiment except that the control VLAN  4  is duplicated. Detailed descriptions are therefore omitted here.  
         [0200]     The network system of the sixth embodiment divides one physical LAN into (n+2) VLANs. The (n+2) VLANs are composed of n user VLANs  10  to n 0  and two control VLANs  4 - 1  and  4 - 2 . With this redundant configuration, a fault in one of the control VLANs  4 - 1  and  4 - 2  does not prevent processing from continuing.  
         [0201]     Although two control VLANs are set in the sixth embodiment, the network system of this embodiment may have more than two control VLANs.  
         [0202]     Processing of the control VLANs  4 - 1  and  4 - 2  upon occurrence of a fault will be described next.  
         [0203]      FIG. 24  is a flow chart of processing of the control VLANs  4 - 1  and  4 - 2  for when a fault occurs.  
         [0204]     The terminals  10 - 1  to  10 - n ,  20 - 1  to  20 - n , . . . , and n 0 - 1  to n 0 - n  and the NAS  1  usually use the duplicated control VLANs  4 - 1  and  4 - 2  by turns to communicate control information ( 1401 ,  1402 ).  
         [0205]     When a fault occurs in the control VLAN  4 - 1  ( 1403 ), the NAS  1  cannot receive control data from the control VLAN  4 - 1  and it is thus judged that a fault has occurred in the control VLAN  4 - 1 . As a fault in the control VLAN  4 - 1  is detected, the control VLAN  4 - 1  is blocked (a step  1404 ) and the other control VLAN  4 - 2  alone is used for communications ( 1405 ).  
         [0206]     After the control VLAN  4 - 1  that has failed recovers, the NAS  1  uses both of the control VLANs  4 - 1  and  4 - 2  (alternately, for example) to communicate control data ( 1406 ,  1407 ).  
         [0207]     In the sixth embodiment of this invention, the duplication of the control VLANs  4 - 1  and  4 - 2  allows one of the control VLANs  4 - 1  and  4 - 2  that has not failed to continue processing while the other that has failed recovers.  
         [0208]     This invention is applicable to, for example, a network system that has NAS and that uses a VLAN, and can balance the network load throughout the user VLANs. This invention is also applicable to a network system that has such a storage control system as NAS head which has a control unit but not a disk drive and that uses a VLAN.  
         [0209]     While the present invention has been described in detail and pictorially in the accompanying drawings, the present invention is not limited to such detail but covers various obvious modifications and equivalent arrangements, which fall within the purview of the appended claims.