Patent Publication Number: US-2013250829-A1

Title: Method for controlling communication system, communication system, and communication apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of International Application PCT/JP2010/070331 filed on Nov. 16, 2010 and designated the U.S., the entire contents of which are incorporated herein by reference. 
    
    
     FIELD 
     The present invention relates to a method for controlling a communication system, a communication system, and a communication apparatus. 
     BACKGROUND 
     In networking, a link aggregation function is known in which communication may be performed at the communication rate with which the communication rate of the bundled physical links is combined by virtually bundling a plurality of physical links as one link (link aggregation group). The link aggregation function is one of the Ethernet (registered trademark) functions, and is defined in IEEE (The Institute of Electrical and Electronics Engineers, Inc.) 802.3ad. As a similar function, a trunking function is also known in which communication may be performed at the communication rate with which the communication rate of the bundled physical ports is combined by virtually bundling a plurality of physical ports as one port in fiber channels. 
     Moreover, a method for making link aggregation redundant has been proposed in which links are made redundant by using spare links in the communication among network relay devices. In the method for making link aggregation redundant, a link aggregation group is created by using two or more specified links as operational links, and specified links are blocked and made to be pseudo-spare links; the state of the operational links in a link aggregation group is monitored; when a portion of the operational links reaches an unusable state, the portion of the operational links in an unusable state is detached from the link aggregation group, and the pseudo-closed state of the spare links is released; and the spare links are attached to the link aggregation group as operational links. 
     Further, a radio base station apparatus having a plurality of radio units has been proposed in which a data amount detection unit compares the amount of upstream data, which is the amount of communication data from radio terminals to the radio units, with the amount of downstream data, which is the amount of communication data from the radio units to the radio terminals. In the radio base station apparatus, when the amount of upstream data is greater than the amount of downstream data, a controller controls a power source controller to terminate the power supply to the transmitter of one of the radio units; when the amount of downstream data is greater than the amount of upstream data, the controller controls the power source controller to terminate the power supply to the receiver of one of the radio units; and when both the amount of upstream data and the amount of downstream data is smaller than a specified threshold, the controller controls the power source controller to terminate the power supply to both the transmitter and receiver of one of the radio units. 
     CITATION LIST 
     Patent Document 
     
         
         [Patent Document 1] Japanese Laid-open Patent Publication No. 2004-349764 
         [Patent Document 2] Japanese Laid-open Patent Publication No. 2008-252282 
       
    
     SUMMARY 
     A method for controlling a communication system disclosed herein uses a plurality of links that connect a plurality of first ports of a first communication apparatus with a plurality of second ports of a second communication apparatus, as one virtual link that is a link aggregation group, to perform communication between the first communication apparatus and the second communication apparatus. The method for controlling a communication system includes: linking down, by using the first communication apparatus, at least one of the plurality of first ports by cutting off a power supply to at least one of the plurality of first ports according to available bandwidth information of the link aggregation group; transmitting, by using the first communication apparatus, a link-down completion notice to the second communication apparatus after the linking down; and cutting off, by using the second communication apparatus that has received the link-down completion notice, a power supply to at least one of the plurality of second ports that is linked down. 
     According to a method for controlling a communication system disclosed herein, it becomes possible to reduce the power consumption in the communication where a link aggregation group is used by terminating the power supply to the linked-down ports at both ends of a link in a link aggregation group. 
     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. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates an example of the communication system including network switching apparatuses according to an embodiment. 
         FIG. 2  illustrates an example of the configuration of a network switching apparatus. 
         FIG. 3  illustrates an example of the configuration of a link aggregation controller. 
         FIG. 4  depicts an example of the LAG management table. 
         FIG. 5  depicts an example of the port state management table. 
         FIG. 6  depicts examples of the LA-ECO frame. 
         FIG. 7A  is a flow of a link aggregation monitoring process (1). 
         FIG. 7B  is a flow of a link aggregation monitoring process (2). 
         FIG. 8  is a flowchart of a port adding process. 
         FIG. 9  is a flowchart of a port deleting process. 
         FIG. 10  illustrates the flowchart of an ECO frame reception process. 
         FIG. 11  is the flowchart of a link updating process. 
         FIG. 12  is an explanatory schematic diagram of a link deleting process. 
         FIG. 13  is an explanatory schematic diagram of a link adding process. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
     Hereinafter, an example of the embodiment of a method for controlling a communication system, a communication system, and a communication apparatus will be described. 
     Generally, as long as a power is provided, a PHY module (Physical layer module), which is implemented at ports of a network switching apparatus and is defined by the first layer of the OSI reference model in which the communication functions of a network device or the like are defined by using a hierarchical structure, consumes a certain amount of power regardless of whether communication is actually being performed. Accordingly, when the available bandwidth secured by a link aggregation group is excessive, the PHY module of a linked-up port consumes power even though the PHY module does not perform any communication. 
     However, if the power supply to a linked-up port is cut in a link aggregation group, a link error due to the termination of an established link is output. Thus, the power cut is dealt with as a hardware failure or abnormality in a transmission line. For this reason, it is not possible to cut the power supply to a linked-up port in a link aggregation group. 
     As discussed above, at ports where links are established, it is necessary to turn on the power source of the PHY module at all times such that the established link will be maintained. Accordingly, when the available bandwidth secured by a link aggregation group is excessive, the power consumption at PHY modules of the ports at both ends of the link is wasted. 
     It may be an option to delete the links from a link aggregation group when the available bandwidth secured by a link aggregation group is excessive, and to add links to a link aggregation group when there is insufficient available bandwidth. 
     However, even if links are deleted from a link aggregation group, the power consumption will not be reduced unless the power supply to PHY modules of the ports at both ends of the link is cut. In other words, an excess of available bandwidth may be reduced by deleting links from one link aggregation group, but it is not possible to reduce the power consumption. In reality, it is not possible to cut the power supply to a linked-up port in a link aggregation group, as described above. 
     According to a method for controlling a communication system, a communication system, and a communication apparatus disclosed herein, it becomes possible to reduce the power consumption in a communication apparatus by terminating the power supply to the linked-down ports at both ends of a link in a link aggregation group, while maintaining the redundancy in a link aggregation group. 
       FIG. 1  illustrates an example of the communication system including network switching apparatuses. 
     The communication system includes a first network switching apparatus  1  as a communication apparatus, a second network switching apparatus  2  as a communication apparatus, and a plurality of links  3  and  4  that connect the first network switching apparatus  1  to the second network switching apparatus  2 . The first network switching apparatus  1  is a network relay device for relaying frames, and includes a link aggregation controller  11 , a plurality of first ports  12 , and a third port  18 . The second network switching apparatus  2  is a network relay device for relaying frames, and includes a link aggregation controller  21 , a plurality of second ports  22 , and a fourth port  28 . 
     The first ports  12  are connected to the second ports  22 , respectively, through the links  3 . The links  3  are, for example, transmission paths such as cables or optical fibers. The numbers of the first ports  12  and the second ports  22  are not limited to “3”. 
     The communication system uses a LAG  5  to perform communication between the first network switching apparatus  1  and the second network switching apparatus  2 . In the following description, the term “link aggregation group” will be referred to as “LAG”. The LAG  5  is a unit of links in the communication by the link aggregation. The LAG  5  is one virtual link including a plurality of links  3  that connect the first ports  12  of the first network switching apparatus  1  to the second ports  22  of the second network switching apparatus  2 . 
     The communication in which the LAG  5  is used is controlled by the link aggregation controller  11  and the link aggregation controller  21 . The communication in which the LAG  5  is used is performed, for example, in accordance with the link aggregation control protocol (LACP). The link aggregation control protocol is defined, for example, by IEEE 802.3ad. In other words, the link aggregation controller  11  and the link aggregation controller  21  control the LAG  5  in accordance with the link aggregation control protocol, and the communication in which the LAG  5  is used is performed. 
     The third port  18  is included in the first network switching apparatus  1 , and is not a port other than the first ports  12 . The third port  18  does not belong to the LAG  5  that connects the first ports  12  with the second ports  22 . 
     The fourth port  28  is included in the second network switching apparatus  2 , and is not a port other than the second ports  22 . The fourth port  28  does not belong to the LAG  5  that connects the first ports  12  with the second ports  22 . 
     The third port  18  is connected with the fourth port  28  through a link  4 . The link  4  is a transmission path such as a cable or optical fiber. The number of the third ports  18  and the number of the fourth ports  28  is not limited to “1”, and may be plural. When a plurality of third ports  18  and a plurality of fourth ports  28  are provided, a plurality of links that connect between the third ports  18  and the fourth port  28  may be used as a link aggregation group other than the LAG  5  to which the first ports  12  and the seconds ports  22  belong. 
     The third port  18  may be connected with a communication apparatus other than the second network switching apparatus  2 . The fourth port  28  may be connected with a communication apparatus other than the first network switching apparatus  1 . 
       FIG. 2  illustrates an example of the configuration of a network switching apparatus. 
     The first network switching apparatus  1  includes a data switching unit  13 , an aggregator  14 , a transmission/reception controller  15 , a power source controller  16 , and a monitoring unit  17 , in addition to the link aggregation controller  11 , the first ports  12 , and the third port  18 . The link aggregation controller  11  includes a switch management unit  111  and an aggregation controller  112 . Each of the first ports  12  includes a MAC unit  121 , a PHY module  122 , and an LED (Light Emitting Diode)  123 . The third port  18  includes a MAC unit  181 , a PHY module  182 , and an LED  183 . The third port  18  may have a similar structure to the first port  12 , or may have a different structure from the first port  12 . 
     Next, the second network switching apparatus  2  has a similar structure, for example, to the first network switching apparatus  1 . 
     The second port  22  has a similar structure to the first port  12 , and the fourth port  28  has a similar structure to the third port  18 . The second network switching apparatus  2  may have a different structure from the first network switching apparatus  1 . 
     For example, when the switch management unit  111  of the first network switching apparatus  1  is to be distinguished from the switch management unit  111  of the second network switching apparatus  2 , the switch management unit of the first network switching apparatus  1  is expressed as “ 111 A”, and the switch management unit of the second network switching apparatus  2  is expressed as “ 111 B”. In other words, a reference sign A will be given to the elements of the first network switching apparatus  1 , and a reference sign B will be given to the elements of the second network switching apparatus  2 . 
     The PHY module  122  is a physical device that performs data transmission, data reception, and signal conversion in a physical layer, where the physical layer is located at the first layer of the seventh layer of the OSI reference model in which the functions of the network are expressed in a hierarchical structure, regarding a transmission/reception frame performing transmission and reception. The position at which the PHY module  122  is implemented is indicated by a port number. The port number is the identification information by which the first port  12  is uniquely identified in the first network switching apparatus  1 . The PHY module  122  transmits the reception frame received through a cable, an optical cable, or the like to the MAC unit  121 . Moreover, the PHY module  122  transmits the transmission frame provided by the MAC unit  121  to the destination PHY module through a cable or an optical cable. A similar configuration applies to the PHY module  182 . 
     The MAC unit  121  is provided for the PHY module  122  on a one-to-one basis, and performs transmission control for a transmission/reception frame. As transmission control, for example, frame format detection or error detection is performed. A similar configuration applies to the MAC unit  181 . 
     The MAC unit  121  transmits the reception frame provided by the PHY module  122  to the transmission/reception controller  15 . Moreover, the MAC unit  121  transmits the transmission frame provided by the transmission/reception controller  15  to the PHY module  122 . On the other hand, the MAC unit  181  transmits the reception frame provided by the PHY module  182  to the data switching unit  13 . Moreover, the MAC unit  181  transmits the transmission frame provided by the data switching unit  13  to the PHY module  182 . 
     An LED  123  indicates the state of the first port  12  by lighting up in accordance with the control made by the aggregation controller  112 . For example, the state of the first port  12  is indicated by “green”, “light off”, and “flashing orange”, which correspond to a linked-up state, a linked-down state, and an error state of the first port  12 , respectively. A similar configuration applies to the LED  183 . 
     The power source controller  16  controls the power supply to the PHY modules  122  and the PHY module  182 . In particular, the power source controller  16  supplies or cuts power to the PHY modules  122  and the PHY module  182  in accordance with the control made by the switch management unit  111 . 
     The transmission/reception controller  15  is a controller, parser, multiplexer, or the like that is provided for the MAC unit  121  on a one-to-one basis. When an LACP frame is received from the MAC unit  121 , the transmission/reception controller  15  transmits the LACP frame to the aggregation controller  112 . When a frame other than an LACP frame is received from the MAC unit  121 , the transmission/reception controller  15  transmits a frame other than an LACP frame to the aggregator  14 . The transmission/reception controller  15  transmits the frame transmitted from the aggregator  14  to the corresponding MAC unit  121  just as it is. 
     The aggregator  14  is provided for each of the LAGs  5 . The aggregator  14  transmits the reception frame provided by the corresponding first port  12  to the data switching unit  13 . Moreover, the aggregator  14  distributes the transmission frame provided by the data switching unit  13  to the first ports  12  that belong to the LAG  5  in accordance with a specified communication protocol. 
     The data switching unit  13  switches between frames. In other words, the data switching unit  13  specifies the destination aggregator  14  according to the destination MAC of the frame received from the aggregator  14 , and transmits the received frame to the specified destination aggregator  14 . 
     The monitoring unit  17  retains the available bandwidth information collected from each of the first ports to be monitored. The available bandwidth information includes, for example, the amount of transmission and reception at each of the first ports  12 , the amount of transmission and reception at the LAG  5 , the number of errors at each of the first ports  12 , and the number of errors at the LAG  5 . Moreover, the monitoring unit  17  retains the available bandwidth information collected from the third port  18  to be monitored. The available bandwidth information includes, for example, the amount of transmission and reception at the third port  18 , and the number of errors at the third port  18 . When there are two or more LAGs  5 , the amount of transmission and reception or the number of errors is collected from each link aggregation group. 
     The switch management unit  111  uses a port state management table  113 , which will be described later, to manage each of the first ports  12  and the third port  18  included in the first network switching apparatus  1 . In particular, the switch management unit  111  monitors the PHY modules  122  and the PHY module  182  that are implemented at the first ports  12  and the third port  18 . Moreover, the switch management unit  111  assesses the transmission line state (link status) according to the results of a monitoring process, and controls the power supply to the PHY modules  122  and the PHY module  182  through the power source controller  16  according to the requests from the aggregation controller  112 . 
     The aggregation controller  112  uses a LAG management table  114 , which will be described later, to manage the LAG  5 . In particular, the aggregation controller  112  uses an LACP frame to exchange link aggregation information related to the LAG  5  with the other aggregation controller  112 . The LACP frame complies with the link aggregation control protocol described as above. 
       FIG. 3  illustrates an example of the configuration of a link aggregation controller. 
     The link aggregation controller  11  includes the switch management unit  111 , the aggregation controller  112 , the port state management table  113 , and the LAG management table  114 . The aggregation controller  112  includes a LAG monitoring unit  115 , a deletion unit  116 , an addition unit  117 , and an update unit  118 . 
     The link aggregation controller  21  has a similar structure to the link aggregation controller  11 . For example, when the aggregation controller  112  of the link aggregation controller  11  is to be distinguished from the aggregation controller  112  of the link aggregation controller  21 , the aggregation controller of the link aggregation controller  11  is expressed as “ 112 A”, and the aggregation controller of the link aggregation controller  21  is expressed as “ 112 B”. In other words, a reference sign A will be given to the elements of the link aggregation controller  11 , and a reference sign B will be given to the elements of the link aggregation controller  21 . 
     When the first network switching apparatus  1  to which the aggregation controller  112  belongs is the master unit in the link aggregation control, the aggregation controller  112  performs a deleting process or adding process for the link of the LAG  5  according to the results of the monitoring of the LAG  5 . In the ECO processing mode, the master unit in the link aggregation control reduces or increases the number of links of the LAG  5 . The monitoring of the LAG  5  is performed by the LAG monitoring unit  115 . The deleting process for the links of the LAG  5  is performed by the deletion unit  116 . The adding process for the links of the LAG  5  is performed by the addition unit  117 . 
     When the first network switching apparatus  1  to which the aggregation controller  112  belongs is the slave unit in the link aggregation control, the aggregation controller  112  performs an updating process for the links of the LAG  5  in accordance with the results of the deleting process or adding process for the links of the LAG  5  performed by the second network switching apparatus  2  that is the master unit. In the ECO processing mode, the slave unit in the link aggregation control follows the deleting process or increasing process for the number of the links of the LAG  5 , which is performed by the master unit. The updating process for the links, which follows the results of the deleting process or adding process for the links performed by the master unit, is performed by the update unit  118 . 
     In the examples of  FIGS. 1-3 , the first network switching apparatus  1  is the master unit in the link aggregation control, and the second network switching apparatus  2  is the slave unit in the link aggregation control. In the LAG management table  114 , which of the network switching apparatuses  1  and  2  is to become the master unit is determined in advance. The LAG management table  114  will be described later with reference to  FIG. 4 . 
     The aggregation controller  112  calculates the desired number of links according to the available bandwidth information of the LAG  5 , and compares the desired number of links with the number of the links that are currently linked up in the LAG  5 . The available bandwidth information of the LAG  5  is acquired from the monitoring unit  17 . When the desired number of links is less than the number of the links that are currently linked up in the LAG  5 , the aggregation controller  112  performs a linking down process as a process of deleting links in accordance with the results of the comparison between the desired number of links and the number of the links that are currently linked up in the LAG  5 . When the desired number of links is greater than the number of the links that are currently linked up, the aggregation controller  112  performs a linking-up process as a process of adding links in accordance with the results of the comparison between the desired number of links and the number of the links that are currently linked up in the LAG  5 . 
     The aggregation controller  112  performs a deleting process for the links of the LAG  5  according to the available bandwidth information of the LAG  5 . In particular, the aggregation controller  112  cuts off the power supply to at least one of the first ports  12 , and performs a linking down process. 
     Any of the process of cutting the power supply to the first port  12  and the linking down process of the first port  12  may be performed first. In the example of  FIG. 3 , the aggregation controller  112  performs a linking down process for at least one of the first ports  12  by cutting off the power supply to at least one of the first ports  12 . 
     After the linking down process is performed, the aggregation controller  112  transmits to the second network switching apparatus  2  a link-down completion notice that indicates the completion of a linking down process. The second network switching apparatus  2  that has received the link-down completion notice cuts off the power supply to at least one of the second ports  22  that is linked down. Accordingly, it becomes possible to cut off the power supply to the first port  21  and the second port  22  at both ends of the link. 
     The aggregation controller  112  excludes at least one of the first ports  12  in which the power supply is cut off from the target to which an error indicating link-down is sent according to the available bandwidth information. Accordingly, it becomes possible for the first network switching apparatus  1  to distinguish between a link-down caused by the cutting off of the power supply and a true link-down caused by an error according to the available bandwidth information. The information indicating whether or not to send an error notice is set in the port state management table  113 . The port state management table  113  will be described later with reference to  FIG. 5 . 
     In the second network switching apparatus  2 , the aggregation controller  112 B excludes the second port  22  connected to the first port  12  that is linked down due to the cutting off of the power supply through a cable or the like from the target to which an error indicating link-down is sent according to the available bandwidth information. Accordingly, it becomes possible for the second network switching apparatus  2  to distinguish between a link-down caused by the cutting off of the power supply and a true link-down caused by an error according to the available bandwidth information. 
     The aggregation controller  112  sends a link-down start notice to the second network switching apparatus  2  prior to the process of deleting links, i.e., the linking down process. The second network switching apparatus  2  that has received the link-down start notice turns on the power source of all the second ports  22 , and sends a reply to the link-down start notice to the aggregation controller  112 . The aggregation controller  112  that has received the reply to the link-down start notice performs a linking down process. 
     Accordingly, the power source of all the second ports  22  is turned on, and then only the power source of the second ports  22  in which the links are not linked up is cut off. For this reason, it is possible to perform a linking down process for the second ports  22  of the second network switching apparatus  2  independently from a linking down process performed by the aggregation controller  112  for the first ports  12 . Moreover, it becomes no longer necessary for a user to link-down the second ports  22  of the second network switching apparatus  2  according to the monitoring of the first ports  12  of the aggregation controller  112  by a user or a network monitoring device. 
     The aggregation controller  112  performs an adding process for the links of the LAG  5  according to the available bandwidth information of the LAG  5 . In particular, the aggregation controller  112  turns on the power source of at least one of the first ports  12  and links up the first port  12 . Note that the first port  12  that is linked up is the port in which the power source is turned on. 
     After the linking-up process is completed, the aggregation controller  112  transmits a link-up completion notice to the second network switching apparatus  2 . The second network switching apparatus  2  that has received the link-up completion notice cuts off the power supply to the second port that is linked down from among the second ports  22 . Accordingly, it becomes possible to cut off the power supply to the first port  21  and the second port  22  at both ends of the link. 
     The aggregation controller  112  sends a link-up start notice to the second network switching apparatus  2  prior to the process of adding links. The second network switching apparatus  2  that has received the link-up start notice turns on the power source of all the second ports  22 , and sends a reply to the link-up start notice to the aggregation controller  112 . The aggregation controller  112  that has received the reply to the link-up start notice performs a linking-up process. 
     Accordingly, the power source of all the second ports  22  is turned on, and then only the power source of the second ports  22  in which the links are not linked up is cut off. For this reason, it is possible to perform a linking-up process for the second ports  22  of the second network switching apparatus  2  independently from a linking-up process performed by the aggregation controller  112  for the first ports  12 . Moreover, it becomes no longer necessary to link up the second ports  22  of the second network switching apparatus  2  according to the monitoring of the first ports  12  of the aggregation controller  112  by a user or a network monitoring device. 
     The aggregation controller  112  creates and uses an LA-ECO frame in the linking-up process and linking down process. The LA-ECO frame is used to cut off the power supply to a port to reduce the power consumption. In other words, the LA-ECO frame is used to realize an ECO processing mode that will be described later. In particular, the LA-ECO frame is a kind of LACP frame, and is a frame for a link-up start notice, a link-up completion notice, a link-down start notice, and a link-down completion notice. Moreover, the LA-ECO frame is used for a reply to the link-up start notice, a reply to the link-up completion notice, a reply to the link-down start notice, and a reply to the link-down completion notice. 
     For example, one or a plurality of OCTETs at specified positions among a plurality of OCTETs included in a frame (hereinafter, referred to as a specific OCTET) is used to determine whether or not the frame is an LA-ECO frame. In other words, a specific OCTET in an LA-ECO frame involves a specified value. Further, a frame is determined to belong to one of the link-up start notice, the link-up completion notice, the link-down start notice, and the link-down completion notice, or to belong to a response to one of these notices, according to a value of a specific OCTET in an LA-ECO frame. The LA-ECO frame will be described later with reference to  FIG. 6 . 
     When the first network switching apparatus  1  to which the aggregation controller  112  belongs is the master unit in the link aggregation control, the aggregation controller  112  performs a shifting process for a port (i.e., link) that is linked up in the LAG  5  according to the results of a monitoring process for the LAG  5 . In particular, the aggregation controller  112  determines whether the period of a link up of a port that is currently linked up exceeds a specified threshold, according to the available bandwidth information of the LAG  5 . The period of a link up is indicated as “link-up operating time” in the port state management table  113 , as will be described later. 
     When there is a port in which the period of a link up exceeds a threshold, the aggregation controller  112  performs a linking down process for the port in which the period of a link up exceeds a threshold after a linking-up process is performed. Moreover, the aggregation controller  112  selects the same number of ports as the number of ports for which a linking down process is performed from a plurality of ports that are linked down, and performs a linking-up process for the selected ports. 
     Due to the above shifting process of ports, even if the available bandwidth secured by a link aggregation group is excessive in the LAG  5 , it is possible to prevent the situation in which a specific link is linked down and the power source of the specific link remains cut off. For example, in cases where a specific link is linked down and the power source remains cut off, even if a hardware failure or the like occurs at the specific link, it is not possible to detect such a hardware failure or the like until that specific link is linked up due to the lack of available bandwidth in the LAG  5 . However, it is possible to prevent a situation in which the power source of a specific link remains cut off. Accordingly, it is possible to detect a hardware failure or the like as needed basis, and an instantaneous lack of bandwidth may be prevented. Moreover, it is possible to prevent the operating time of only the components of a specific link from increasing, and the hardware failure may be equalized. 
       FIG. 4  depicts an example of the LAG management table. 
     The LAG management table  114  stores the information about the current status of the LAG  5  on an item-by-item basis. The items for the management of the status of the LAG  5  include “Actor System ID”, “Actor Aggregation Key”, “Partner System ID”, “Partner Aggregation Key”, “PORT”, “Actor ECO processing mode”, “Partner ECO processing mode”, “ECO Processing Role”, “ECO processing execution flag”, “Number of Transmission OCTETs”, and “Number of Reception OCTETs”. 
     Here, “Actor” indicates that the item is related to its own switching apparatus. “Partner” indicates that the item is related to a switch to be connected with. Accordingly, in the LAG management table  114  provided for the first network switching apparatus  1 , “Actor” indicates the first network switching apparatus  1  and “Partner” indicates the second network switching apparatus  2 . 
     “Actor System ID” indicates the MAC address used by the LAG  5  in its own switching apparatus, i.e., the first network switching apparatus  1 . “Actor Aggregation Key” indicates the identification information used to identify the LAG  5  in its own switching apparatus, i.e., the first network switching apparatus  1 . 
     “Partner System ID” indicates the MAC address used by the LAG  5  of the switch to be connected with, i.e., the second network switching apparatus  2 , in the first network switching apparatus  1 . “Partner Aggregation Key” indicates the identification information used to identify the LAG  5  in the switch to be connected with, i.e., the second network switching apparatus  2 . 
     “PORT” indicates the port number of its own switching apparatus, i.e., the first port  12 , which belongs to the LAG  5  in the first network switching apparatus  1 . 
     “Actor ECO processing mode” indicates whether the ECO processing mode is valid or invalid in the LAG  5  of its own switching apparatus, i.e., the first network switching apparatus  1 . “Partner ECO processing mode” indicates whether the ECO processing mode is valid or invalid in the LAG  5  of the switch to be connected with, i.e., the second network switching apparatus  2 . 
     Here, “ECO processing mode” indicates a mode in which the power supply to a port is cut off to reduce the power consumption. In other words, “ECO processing mode” indicates a mode realized by an LA-ECO frame. Accordingly, the first network switching apparatus  1  is set to an ECO state when “Actor ECO processing mode” is “valid”, and the first network switching apparatus  1  is set to a state other than the ECO state, i.e., a normal state, when “Actor ECO processing mode” is “invalid”. The ECO state will be described later with reference to  FIG. 5 . Also, the second network switching apparatus  2  is set to the ECO state when “Partner ECO processing mode” is “valid”, and the second network switching apparatus  2  is set to states other than the ECO state, i.e., a normal state, when “Partner ECO processing mode” is “invalid”. 
     “ECO Processing Role” indicates whether its own switching apparatus, i.e., the first network switching apparatus  1 , is the master unit or the slave unit in the ECO processing mode. When the first network switching apparatus  1  is the master unit, the ECO Processing Role is set to “MASTER”. Hence, the first network switching apparatus  1  increases or decreases the number of links of the LAG  5  in the ECO processing mode according to the available bandwidth information. When the first network switching apparatus  1  is the slave unit, the ECO Processing Role is set to “SLAVE”. Hence, in the ECO processing mode, the first network switching apparatus  1  follows the ECO processing performed by the second network switching apparatus  2  that is the master unit. 
     “ECO processing execution flag” indicates whether its own switching apparatus, i.e., the first network switching apparatus  1 , is performing an ECO processing or is at rest. In other words, “ECO processing execution flag” indicates whether or not the first network switching apparatus  1  is in the ECO processing mode. 
     “Number of Transmission OCTETs” indicates the number of OCTETs that have been transmitted through the LAG  5 . “Number of Reception OCTETs” indicates the number of OCTETs that have been received through the LAG  5 . 
     The LAG management table  114  is created by the aggregation controller  112 . When the LAG  5  is configured, each of “Actor System ID”, “Actor Aggregation Key”, “Partner System ID”, “Partner Aggregation Key”, and “PORT” in the LAG management table  114  is input, for example, from a computer connected to the first or second network device to the aggregation controller  112 . When the ECO processing mode is performed, each of “Actor ECO processing mode”, “Partner ECO processing mode”, “ECO Processing Role”, and “ECO processing execution flag” is set by the aggregation controller  112 . In the ECO processing mode, “Number of Transmission OCTETS” and “Number of Reception OCTETS” are set by the aggregation controller  112  that has acquired “Number of Transmission OCTETS” and “Number of Reception OCTETS” from the monitoring unit  17 . 
     The link aggregation controller  11  includes the LAG management table  114 A that manages the LAG  5 . In the LAG management table  114 A of the first network switching apparatus  1 , the first network switching apparatus  1  is registered as the master unit, and the second network switching apparatus  2  is registered as the slave unit. Hence, the aggregation controller  112 A of the first network switching apparatus  1 , as the master unit, determines whether or not each of the linking down process and linking-up process is to be performed for a plurality of ports that belong to the LAG  5 . 
     The link aggregation controller  21  includes the LAG management table  114 B that manages the LAG  5 . In the LAG management table  114 B of the second network switching apparatus  2 , the first network switching apparatus  1  is registered as the master unit, and the second network switching apparatus  2  is registered as the slave unit. Hence, the aggregation controller  112 B of the second network switching apparatus  2 , as the slave unit, performs the updating process for links of the LAG  5  in accordance with the results of the deleting process or adding process for the links of the LAG  5 , which is performed by the first network switching apparatus  1  as the master unit. 
       FIG. 5  depicts an example of the port state management table. 
     The port state management table  113  stores the information of the state of ports for every port. In the port state management table  113 , the ports are indicated by port numbers. In  FIG. 5 , it is assumed that there are six ports with port numbers  1 - 6  that belong to the LAG  5 , and that each of the six ports is in a different port state. 
     The port state information includes “Link Up”, “Link-down”, “Power OFF”, “no PHY module”, “Link state unknown”, and “ECO auto-poweroff”. 
     “Link Up” indicates that a port is linked up. When the port state indicates that a port is linked up, link-up operating time is also stored. The link-up operating time indicates the period during which a port keeps operating since the port is linked up. The unit of the link-up operating time is, for example, a second. 
     “Link-down” indicates that a port is linked down. 
     “Power OFF” indicates that the power supply to a port is cut off. “no PHY module” indicates that the state of a PHY module is unknown because, for example, the PHY module of a port is not recognizable. “Link state unknown” indicates that the state of the link of a port is unknown. 
     “ECO auto-poweroff” indicates that a port is in the ECO state. “ECO auto-poweroff”, in other words, the ECO state, indicates that the power supply to a port is cut off so as to reduce the power consumption. For this reason, when a port is linked down because the port is in the ECO state, it becomes possible to recognize that the link-down is not caused by a hardware failure or line failure but is caused by the cutting off of the power supply to the port for the reduction of the power consumption. 
     The port state management table  113  is created by the switch management unit  111 . In the port state management table  113 , “Link Up”, “Link-down”, “Power OFF”, and “no PHY module” are updated by the switch management unit  111 . Moreover, “Link Up”, “Link-down”, “Link state unknown”, and “ECO auto-poweroff” are updated by the switch management unit  111  according to a request from the aggregation controller  112  at a specified timing, as will be described later. 
     The link aggregation controller  11  includes the port state management table  113 A that manages the state of the first port  12 . In the first network switching apparatus  1 , the state of at least one of the first ports  12  in which the power supply is cut off according to the available bandwidth information is registered in the port state management table  113 A as the ECO state, in other words, as “ECO auto-poweroff”. The ECO state indicates a state where the power supply to a port is cut off so as to reduce the power consumption, and indicates a state realized by an LA-ECO frame. The ECO state is distinguished from the cutting off of the power source, which is irrelevant to the available bandwidth information. Hence, the aggregation controller  112 A excludes the occurrence of a link-down at at least one of the first ports  12  that is registered in the port state management table  113 A as being in the ECO state from the target of an error notice. 
     The link aggregation controller  21  includes the port state management table  113 B that manages the state of the second port  22 . In the second network switching apparatus  2 , the state of at least one of the second ports  22  that is linked down according to the link-down of at least one of the first ports  12  in which the power supply is cut off according to the available bandwidth information is registered in the port state management table  113 B as the ECO state, in other words, as “ECO auto-poweroff”. Hence, the aggregation controller  112 B excludes the link-down at at least one of the second ports  22  that is registered in the second port state management table  113 B as being in the ECO state from the target of an error notice. 
       FIG. 6  depicts examples of the LA-ECO frame. In fact,  FIG. 6  depicts LA-ECO frames, i.e., the description of fields, the number of OCTETs in the fields, and the information stored in the fields in an ECO state notification frame. 
     “Destination Address” is a field in which the destination MAC address of an LA-ECO frame is stored. “Source Address” is a field in which the source MAC address of an LA-ECO frame is stored. “Length/Type” is a field in which the type value indicating that the LA-ECO frame is an LA-ECO frame is stored. “Subtype” is a field that is not used in the network switching apparatuses  1  and  2  of the examples in  FIGS. 1 and 2 . “Version Number” is a field in which the version number of a link aggregation control protocol is stored. 
     “Actor System” indicates a field in which the MAC address of the LAG  5  of its own switching apparatus, i.e., the first network switching apparatus  1 , is stored. Hence, “Actor System” stores “Actor System ID”, which is stored in the LAG management table  114  of  FIG. 4 . “Actor Key” indicates a field in which the identification information used to identify the LAG  5  is stored, in its own switching apparatus, i.e., the first network switching apparatus  1 . Hence, “Actor Key” stores “Actor Aggregation Key”, which is stored in the LAG management table  114  of  FIG. 4 . 
     “Partner System” indicates a field in which the MAC address used by the switch to which the first network switching apparatus  1  is connected at the LAG  5 , i.e., the MAC address used by the second network switching apparatus  2  at the LAG  5 , is stored. Accordingly, “Partner System” stores “Partner System ID” stored in the LAG management table  114  of  FIG. 4 . “Partner Key” indicates a field in which identification information is stored, which is used to identify the LAG  5  in a switch to be connected with, i.e., the second network switching apparatus  2 . Accordingly, “Partner Key” stores “Partner Aggregation Key”, which is stored in the LAG management table  114  of  FIG. 4 . 
     “ECO mode” is a field in which the information is stored that indicates whether or not the frame is an LA-ECO frame. When the value of “ECO mode” is “1”, the frame is an LA-ECO frame, and thus the ECO processing mode is valid. When the value of “ECO mode” is “0”, the ECO processing mode is invalid. “Link Training” is a field in which the information is stored that indicates whether or not the port for linking is being changed in the ECO processing mode. When the value of “Link Training” is “1”, this indicates that the first network switching apparatus  1  is changing the port for linking. When the value of “Link Training” is “0”, this indicates that the first network switching apparatus  1  is not changing the port for linking. 
     “ACK” and “NACK” are fields in which responses to the firstly received frame are stored. When the value of “ACK” is “1”, this indicates that the notification of the firstly received frame is acknowledged. When the value of “NACK” is “1”, this indicates that the notification of the firstly received frame is not acknowledged. “reserve” indicates a reserved field that is not used in the network switching apparatuses  1  and  2  of the examples in  FIGS. 1 and 2 . “FCS” is a field in which the checksum of the frame is stored. 
     Hereinafter, the aggregation controller  112  will be described in detail with reference to  FIGS. 7A-13 . 
     A combination of  FIGS. 7A and 7B  illustrates the flow of a link aggregation monitoring process performed by the aggregation controller  112 A of the first network switching apparatus  1 . 
     In the aggregation controller  112 A, the LAG monitoring unit  115 A refers to the LAG management table  114 A to determine whether “Actor ECO processing mode” and “Partner ECO processing mode” are valid or invalid (step S 1 ). In other words, the LAG monitoring unit  115 A determines whether or not the first network switching apparatus  1  as “Actor” and the second network switching apparatus  2  as “Partner” belong to the ECO processing mode. 
     When it is determined that “Actor ECO processing mode” and “Partner ECO processing mode” are invalid in the LAG management table  114 A, the LAG monitoring unit  115 A repeats step S 1  after a specified length of time has passed (step S 110 ). 
     When it is determined that “Actor ECO processing mode” and “Partner ECO processing mode” are valid in the LAG management table  114 A, the LAG monitoring unit  115 A refers to the LAG management table  114 A to assess “ECO Processing Role” (step S 2 ). In other words, the LAG monitoring unit  115 A determines whether or not the first network switching apparatus  1  to which the LAG monitoring unit  115 A belongs is the master unit. 
     When “ECO Processing Role” is the “slave unit (indicated as “SLAVE” in FIG.  7 A)”, the LAG monitoring unit  115 A repeats step S 1  after a specified length of time has passed (step S 110 ). 
     When “ECO Processing Role” is the “master unit (indicated as “MASTER” in FIG.  7 A)”, the LAG monitoring unit  115 A initializes a LAG check status storage area in which LAG check status is stored (step S 3 ), and searches the LAG  5  (step S 4 ). In other words, in step S 4 , the LAG monitoring unit  115 A determines whether a LAG  5  exists for which a link aggregation monitoring process has not been performed. The LAG check status storage area is arranged, for example, within a memory of the LAG monitoring unit  115 A. 
     When a LAG  5  does not exist for which a link aggregation monitoring process has not been performed, the LAG monitoring unit  115 A repeats step S 1  after a specified length of time has passed (step S 110 ). 
     When a LAG  5  exists for which a link aggregation monitoring process has not been performed, the LAG monitoring unit  115 A selects one of the LAGs  5  from among LAGs  5  for which a link aggregation monitoring process has not been performed, and acquires statistical information about the selected LAG  5  from the monitoring unit  17 A (step S 5 ). As the statistical information, “Number of Transmission OCTETs” and “Number of Reception OCTETs” are acquired. “Number of Transmission OCTETs” acquired from the monitoring unit  17 A is “Current Number of Transmission OCTETs”, and “Number of Reception OCTETs” acquired from the monitoring unit  17  is “Current Number of Reception OCTETs”. “Current Number of Transmission OCTETs” and “Current Number of Reception OCTETs” are stored in the LAG check status storage area. 
     After that, the LAG monitoring unit  115 A calculates the desired number of links for transmission (step S 6 ). In particular, the LAG monitoring unit  115 A refers to the LAG management table  114 A to acquire “Number of Transmission OCTETs”. “Number of Transmission OCTETs” acquired from the LAG management table  114 A is equivalent to “Previous Number of Transmission OCTETs”. “Previous Number of Transmission OCTETs” is stored in the LAG check status storage area. After that, the LAG monitoring unit  115 A uses the value stored in the LAG check status storage area to divide {(Current Number of Transmission OCTETs)-(Previous Number of Transmission OCTETs)} by (reference band of one link per unit time). Accordingly, “desired number of links for transmission” is calculated. The reference band of one link per unit time is empirically known, and thus is specified in advance. The “desired number of links for transmission” is stored in the LAG check status storage area. 
     Next, the LAG monitoring unit  115 A calculates the desired number of links for reception (step S 7 ). In particular, the LAG monitoring unit  115 A refers to the LAG management table  114 A to acquire “Number of Reception OCTETs”. “Number of Reception OCTETs” acquired from the LAG management table  114 A is equivalent to “Previous Number of Reception OCTETs”. “Previous Number of Reception OCTETs” is stored in the LAG check status storage area. Subsequently, the LAG monitoring unit  115 A uses the value stored in the LAG check status storage area to divide {(Current Number of Reception OCTETs)-(Previous Number of Reception OCTETs)} by (reference band of one link per unit time). By so doing, “desired number of links for reception” is calculated. The “desired number of links for reception” is stored in the LAG check status storage area. 
     Further, the LAG monitoring unit  115 A uses the value stored in the LAG check status storage area to compare the desired number of links for transmission with the desired number of links for reception. Then, the LAG monitoring unit  115 A determines the greater number to be “desired number of links” in the LAG  5  (step S 8 ). When the “desired number of links” becomes “0”, the “desired number of links” is changed to “1” to prevent the communication between the first network switching apparatus  1  and the second network switching apparatus  2  from being completely blocked. The “desired number of links” is stored in the LAG check status storage area. 
     After that, the LAG monitoring unit  115 A refers to the port state management table  113 A to count the number of the first ports  12  that are linked up at that time from among the first ports  12  that belong to the LAG  5  (step S 9 ). In other words, the LAG monitoring unit  115 A extracts the number of the first ports  12  in which the port state is “Link Up” in the port state management table  113 A as “current number of links”. The extracted “current number of links” is stored in the LAG check status storage area. 
     Then, the LAG monitoring unit  115 A uses the value stored in the LAG check status storage area to determine whether or not “current number of links” is less than “desired number of links” (step S 10 ). When “current number of links” is less than “desired number of links” (“Yes” in step S 10 ), the LAG monitoring unit  115 A performs a process of adding the first port  12  (step S 11 ). In reality, the addition unit  117 A that is started by the LAG monitoring unit  115 A performs the process of adding the first port  12 . The process of adding the first port  12  will be described later with reference to  FIG. 8 . After step S 11 , step S 14  is executed. 
     When “current number of links” is greater than “desired number of links” (“No” in step S 10 ), the LAG monitoring unit  115 A further uses the value stored in the LAG check status storage area to determine whether or not “current number of links” is greater than “desired number of links” (step S 12 ). When “current number of links” is greater than “desired number of links” (“Yes” in step S 12 ), the LAG monitoring unit  115 A performs the process of deleting the first port  12  (step S 13 ). In reality, the deletion unit  116 A that is started by the LAG monitoring unit  115 A performs the process of deleting the first port  12 . The process of deleting the first port  12  will be described later with reference to  FIG. 9 . After step S 13 , step S 14  is executed. 
     When “current number of links” is less than “desired number of links” (“No” in step S 12 ), i.e., when “current number of links” is equal to “desired number of links”, the LAG monitoring unit  115 A performs the process of shifting the first port  12 . In particular, the LAG monitoring unit  115 A refers to the LAG management table  114 A to extract the first ports  12  that belong to the LAG  5  (step S 14 ), and refers to the port state management table  113 A to determine whether or not the link-up operating time of any of the first ports  12  exceeds a reference time from among the first ports  12  that belong to the LAG  5  (step S 15 ). The reference time is set in advance. 
     When none of the link-up operating times of the first ports  12  exceeds a reference time (“No” in step S 15 ), the LAG monitoring unit  115 A repeats step S 1  after a specified length of time has passed (step S 110 ). 
     When the link-up operating time of any of the first ports  12  does exceed a reference time (“Yes” in step S 15 ), the LAG monitoring unit  115 A refers to the port state management table  113 A to determine whether or not the port state of any of the first ports  12  from among the first ports  12  that belong to the LAG  5  is “ECO auto-poweroff” (step S 16 ). 
     When none of the port states of the first ports  12  is “ECO auto-poweroff” (“No” in step S 16 ), the LAG monitoring unit  115 A repeats step S 1  after a specified length of time has passed (step S 110 ). 
     When the port state of any of the first ports  12  is “ECO auto-poweroff” (“Yes” in step S 16 ), the LAG monitoring unit  115 A performs a process of adding the first port  12  (step S 17 ). As mentioned above, the process of adding the first port  12  will be described later with reference to  FIG. 8 . 
     After that, the LAG monitoring unit  115 A checks the state of the first ports  12  that belong to the LAG  5  to confirm whether the number of the links that are linked up has become greater than “current number of links” (step S 18 ). When the number of the links that are linked up has not become greater than “current number of links”, the LAG monitoring unit  115 A repeats step S 17 . After that, the LAG monitoring unit  115 A designates the first port  12  in which the link-up operating time exceeds a reference time, which is extracted in step S 15 , and performs the process of deleting the first port  12  (step S 19 ). As mentioned above, the process of deleting the first port  12  will be described later with reference to  FIG. 9 . After that, the LAG monitoring unit  115 A repeats step S 4 . 
       FIG. 8  illustrates the flow of the port adding process performed by the aggregation controller  112 A of the first network switching apparatus  1 . 
     In the aggregation controller  112 A, the addition unit  117 A refers to the port state management table  113 A to extract the first port  12  in which the port state is “ECO auto-poweroff” from the first ports  12  that belong to the LAG  5  (step S 21 ). Then, the addition unit  117 A determines whether the number of the first ports  12  in which the port state is “ECO auto-poweroff” is “0” or “1 or more” (step S 22 ). When the number of the first ports  12  in which the port state is “ECO auto-poweroff” is “0”, the addition unit  117 A terminates the process. 
     When the number of the first ports  12  in which the port state is “ECO auto-poweroff” is “1 or more”, the addition unit  117 A specifies the first port  12  to be newly linked up among the first ports  12  in which the port state is “ECO auto-poweroff” (step S 23 ). The first port  12  to be additionally linked up is selected, for example, from the first ports  12  in which the port state is “ECO auto-poweroff”, on a random basis. 
     Subsequently, the addition unit  117 A sends a frame that indicates that an updating process is being performed for a link of the LAG  5 , i.e., an LA-ECO frame, to the aggregation controller  112 B of the second network switching apparatus  2  (step S 24 ). At this time, “Link Training” is set to “1” in the LA-ECO frame. The LA-ECO frame is equivalent to a link-up start notice. 
     After that, the addition unit  117 A awaits a response to the sent LA-ECO frame from the aggregation controller  112 B of the second network switching apparatus  2  (step S 25 ). In other words, the addition unit  117 A awaits the reception of a frame where “ACK” indicating acknowledgement in the sent LA-ECO frame is set to “1”. 
     When a frame is received where “NACK” indicating a negative acknowledgement in the sent LA-ECO frame is set to “1”, or when a timeout has occurred without receiving anything, the addition unit  117 A terminates the process. The timeout period is set in advance. 
     When a frame is received where “ACK” in the sent LA-ECO frame is set to “1”, the addition unit  117 A raises the ECO processing execution flag of the LAG  5  in the LAG management table  114 A (step S 26 ). Further, the addition unit  117 A changes the state of the first port  12  to be additionally linked up, as determined in step S 23 , from “ECO auto-poweroff” to “Link state unknown” in the port state management table  113 A (step S 27 ). Further, the addition unit  117 A changes the LED  123  of the first port  12  to be linked up to indicating the state of link-down (step S 28 ). 
     After that, the addition unit  117 A requests a power source controller  16 A through the switch management unit  111 A to supply power to the PHY module  122  of the first port  12  to be linked up (step S 29 ). In other words, the addition unit  117 A issues to the switch management unit  111 A a request to supply power to the power source controller  16 A, and the switch management unit  111 A transfers the received request for a power supply to the power source controller  16 A. Then, the power source controller  16 A actualizes the power supply as received. By so doing, power is supplied from the power source controller  16 A to the PHY module  122  of the first port  12  to be linked up. Accordingly, the first port  12  to be linked up is linked up. Moreover, the addition unit  117 A changes the state of the first port  12  to be additionally linked up from “Link state unknown” to “Link Up”, in the port state management table  113 A. 
     After that, the addition unit  117 A sends a frame that indicates the completion of the updating process for links of the LAG  5 , i.e., an LA-ECO frame, to the aggregation controller  112 B of the second network switching apparatus  2  (step S 210 ). At this time, “Link Training” is set to “0” in the LA-ECO frame. 
     The LA-ECO frame is equivalent to a link-up completion notice. Further, the addition unit  117 A drops the ECO processing execution flag of the LAG  5  from “1” to “0” in the LAG management table  114 A (step S 211 ). 
     After that, the addition unit  117 A examines the state of the first ports  12  that belong to the LAG  5 . When it is found that the first port  12  to be linked up has not been linked up as a result of the examination, the addition unit  117 A changes the state of the first port  12  to be additionally linked up from “Link state unknown” to “Link-down” in the port state management table  113 A, and requests the switch management unit  111 A to send an error notice (step S 212 ). 
       FIG. 10  illustrates the flow of the port deleting process performed by the aggregation controller  112 A of the first network switching apparatus  1 . 
     In the aggregation controller  112 A, the deletion unit  116 A refers to the port state management table  113 A to extract the first port  12  in which the port state is “Link Up” from the first ports  12  that belong to the LAG  5  (step S 31 ). Then, the deletion unit  116 A determines whether the number of the first ports  12  in which the port state is “Link Up” is “1” or “2 or more” (step S 32 ). When the number of the first ports  12  in which the port state is “Link Up” is “1”, the deletion unit  116 A terminates the port deleting process. 
     When the number of the first ports  12  in which the port state is “Link Up” is “2 or more”, the deletion unit  116 A specifies the first port  12  to be linked down from among the first ports  12  in which the port state is “Link Up” (step S 33 ). In other words, the deletion unit  116 A determines the first port  12  to be deleted from among the first ports  12  that are being linked up. The first port  12  to be deleted from the link up is selected, for example, from the first ports  12  that are being linked up, on a random basis. Note that the first port  12  whose port transition timer value is small may be selected. The port transition timer value is calculated by subtracting “link-up operating time” from “period given to the port”, and the port transition timer value indicates the remaining time during which the port is linked up. The “period given to the port” is set sufficiently long that the port transition timer value does not become a negative value. When the first port  12  to be deleted from the link up is designated by a host computer, the designated first port  12  is deleted from the link up. 
     Subsequently, the deletion unit  116 A sends a frame that indicates that an updating process is being performed for a link of the LAG  5 , i.e., an LA-ECO frame, to the aggregation controller  112 B of the second network switching apparatus  2  (step S 34 ). At this time, “Link Training” is set to “1” in the LA-ECO frame. The LA-ECO frame is equivalent to a link-down start notice. 
     After that, the deletion unit  116 A awaits a response to the sent LA-ECO frame from the aggregation controller  112 B of the second network switching apparatus  2  (step S 35 ). In other words, the deletion unit  116 A awaits the reception of a frame where “ACK”, indicating acknowledgement in the sent LA-ECO frame, is set to “1”. 
     When a frame is received where “NACK”, indicating negative acknowledgement in the sent LA-ECO frame, is set to “1”, or when a timeout has occurred without receiving anything, the deletion unit  116 A terminates the process. The timeout period is set in advance. 
     When a frame is received where “ACK” in the sent LA-ECO frame is set to “1”, the deletion unit  116 A raises the ECO processing execution flag of the LAG  5  from “0” to “1” in the LAG management table  114 A (step S 36 ). 
     After that, the deletion unit  116 A requests the power source controller  16 A through the switch management unit  111 A to cut off the power supply to the PHY module  122  of the first port  12  to be deleted from link up (step S 37 ). In other words, the deletion unit  116 A issues to the switch management unit  111 A a request to cut off the power supply to the power source controller  16 A, and the switch management unit  111 A transfers the received request to cut off the power supply to the power source controller  16 A. Then, the power source controller  16 A actualizes the cutting off of the power supply as received. By so doing, the power supply to the PHY module  122  of the first port  12  to be deleted from the link up is cut off. Accordingly, the first port  12  to be deleted from the link up is linked down. Moreover, the deletion unit  116 A changes the state of the first port  12  to be deleted from the link up from “Link Up” to “ECO auto-poweroff”, in the port state management table  113 A. Further, the deletion unit  116 A changes the LED  123  of the first port  12  to be deleted from the link up to indicating the ECO state (step S 38 ). 
     After that, the deletion unit  116 A sends a frame that indicates the completion of the updating process for links of the LAG  5 , i.e., an LA-ECO frame, to the aggregation controller  112 B of the second network switching apparatus  2  (step S 39 ). At this time, “Link Training” is set to “0” in the LA-ECO frame. The LA-ECO frame is equivalent to a link-down completion notice. Further, the deletion unit  116 A drops the ECO processing execution flag of the LAG  5  from “1” to “0” in the LAG management table  114 A (step S 310 ). 
     After that, the deletion unit  116 A changes the link-up operating time of the first port  12  to be deleted from link up to “0” in the port state management table  113 A (step S 311 ), and terminates the process. 
       FIG. 10  illustrates the flow of an ECO frame reception process performed by the aggregation controller  112 B of the second network switching apparatus  2 . 
     In the aggregation controller  112 B of the second network switching apparatus  2 , when an LA-ECO frame is received from the aggregation controller  112 A of the first network switching apparatus  1 , the LAG monitoring unit  115 B searches the LAG  5  designated by the received LA-ECO frame (step S 41 ). In other words, in step S 41 , the LAG monitoring unit  115 B determines whether or not the LAG  5  designated by the received LA-ECO frame exists. 
     When the LAG  5  that is designated by the received LA-ECO frame does not exist, the LAG monitoring unit  115 B terminates the process. 
     When the LAG  5  that is designated by the received LA-ECO frame exists, the LAG monitoring unit  115 B determines whether or not there is any ongoing updating process for the links in the relevant LAG  5  (step S 42 ). 
     When there is no ongoing updating process for the links in the LAG  5  designated by the received LA-ECO frame, the LAG monitoring unit  115 B determines whether “Link Training” of the received LA-ECO frame is “1” or “0” (step S 43 ). 
     When “Link Training” is “1”, the LAG monitoring unit  115 B determines whether “ACK” of the received LA-ECO frame is “1” or “0” (step S 44 ). When “ACK” is “0”, the LAG monitoring unit  115 B starts the update unit  118 B that performs an updating process for the links (step S 45 ), and then the LAG monitoring unit  115 B terminates the process. The updating process for the links will be described later with reference to  FIG. 11 . When “ACK” is “1”, the LAG monitoring unit  115 B terminates the process. 
     When “Link Training” is “0” in step S 43 , the LAG monitoring unit  115 B refers to “ECO mode” in the received LA-ECO frame, and records the value of “ECO mode” in “Partner ECO processing mode” of the LAG management table  114 B (step S 46 ). Then, the LAG monitoring unit  115 B terminates the process. 
     When it is determined in step S 42  that an updating process is being performed for the links in the LAG  5  designated by the received LA-ECO frame, the LAG monitoring unit  115 B determines whether “Link Training” of the received LA-ECO frame is “1” or “0” (step S 47 ). 
     When “Link Training” is “1”, the LAG monitoring unit  115 B terminates the process. When “Link Training” is “0”, the LAG monitoring unit  115 B determines whether “ACK” of the received LA-ECO frame is “1” or “0” (step S 48 ). When “ACK” is “1”, the LAG monitoring unit  115 B terminates the process. When “ACK” is “0”, the LAG monitoring unit  115 B notifies the update unit  118 B that is performing an updating process for the links of the reception of the LA-ECO frame where “Link Training” is “0” (step S 49 ), and then the LAG monitoring unit  115 B terminates the process. 
       FIG. 11  illustrates the flow of a link updating process performed by the aggregation controller  112 B of the second network switching apparatus  2 . 
     In the aggregation controller  112 B, the update unit  118 B instructs the power source controller  16 B to supply power to the PHY modules of all the second ports  22  through the switch management unit  111 B (step S 51 ). In other words, the update unit  118 B instructs the switch management unit  111 B to request the power source controller  16 B to supply power, and the switch management unit  111 B sends the received request related to power supply to the power source controller  16 B. Then, the power source controller  16 B supplies power in accordance with the received request. Accordingly, power is supplied to the PHY modules of all the second ports  22 . 
     Subsequently, the update unit  118 B refers to the port state management table  113 B to change the LED of the second port  22  where the state is “ECO auto-poweroff” to indicating “Link-down” (step S 52 ). Further, the update unit  118 B changes the state of the second port  22  where the state is “ECO auto-poweroff” to “Link state unknown” in the port state management table  113 B (step S 53 ). 
     After that, the update unit  118 B raises the ECO processing execution flag of the LAG  5  from “0” to “1” in the LAG management table  114 B (step S 54 ). 
     After that, the update unit  118 B transmits a frame that permits the execution of an updating process for links of the LAG  5  to the aggregation controller  112 A of the first network switching apparatus  1  (step S 55 ). The frame that permits the execution of an updating process for links of the LAG  5  is the received LA-ECO frame in which “ACK” is “1”. 
     After that, the update unit  118 B await the reception of an LA-ECO frame where “Link Training” is “0” from the aggregation controller  112 A of the first network switching apparatus  1  (step S 56 ). In reality, an LA-ECO frame where “Link Training” is “0” is detected in step S 49  as described above. 
     When a timeout has occurred without receiving anything, the update unit  118 B terminates the process. The timeout period is set in advance. 
     When an LA-ECO frame where “Link Training” is “0” is received, the update unit  118 B extracts a link that is linked down by checking the state of the second ports  22  that belong to the LAG  5  (step S 57 ). 
     After that, the update unit  118 B changes the state of the linked-down second port  22  to “ECO auto-poweroff” in the port state management table  113 B (step S 58 ). 
     Further, the update unit  118 B instructs the power source controller  16 B to cut off the power supply to the PHY module of the linked-down second port  22  through the switch management unit  111 B (step S 59 ). In other words, the update unit  118 B instructs the switch management unit  111 B to request the power source controller  16 B to cut off the power supply, and the switch management unit  111 B sends the received request for the termination of power supply to the power source controller  16 B. Then, the power source controller  16 B cuts off the power supply in accordance with the received request. Accordingly, the power supply to the PHY module of the linked-down second port  22  is cut off. 
     After that, the update unit  118 B changes the LED of the linked-down second port  22 , i.e., the LED of the second port  22  that has been deleted from link up, to indicating the ECO state (step S 510 ). Further, the update unit  118 B drops the ECO processing execution flag of the LAG  5  from “1” to “0” in the LAG management table  114 B (step S 511 ). Then, the update unit  118 B changes the link-up operating time of the linked-down second port  22  to “0” in the port state management table  113 B (step S 512 ), and terminates the process. 
       FIG. 12  is an explanatory schematic diagram of the link deleting process. 
     The first network switching apparatus  1  transmits an LA-ECO frame to the second network switching apparatus  2  prior to the linking down process. At this time, “Link Training” is set to “1” in the LA-ECO frame. As “Link Training” is “1” and “ACK” is “0”, the LA-ECO frame is a link-down start notice. 
     When the second network switching apparatus  2  has received the LA-ECO frame as a link-down start notice, the power source of the second ports  22  is turned on, and the ECO processing execution flag of the LAG  5  is raised in accordance with the received LA-ECO frame. 
     After that, the second network switching apparatus  2  transmits the LA-ECO frame to the first network switching apparatus  1 . At this time, “Link Training” and “ACK” are set to “1” in the LA-ECO frame. As “Link Training” and “ACK” are “1”, this LA-ECO frame is a reply to the link-down start notice. 
     When the LA-ECO frame as a reply to the link-down start notice is received, the first network switching apparatus  1  performs a linking down process in accordance with the received LA-ECO frame. 
     In particular, the first network switching apparatus  1  raises the ECO processing execution flag of the LAG  5 . Subsequently, the first network switching apparatus  1  cuts off the power supply to at least one of the first ports  12  from among the first ports  12  that belong to the LAG  5 , according to the available bandwidth information of the LAG  5 . By so doing, the first network switching apparatus  1  links down at least one of the first ports  12 . 
     Further, the first network switching apparatus  1  excludes at least one of the first ports  12  in which the power supply is cut off according to the available bandwidth information from the target to which an error indicating link-down is sent. Accordingly, the first network switching apparatus  1  changes the LED  123  of at least one of the first ports  12  in which the power supply is cut off according to the available bandwidth information to indicating the ECO state. 
     After this linking down process, the first network switching apparatus  1  transmits the LA-ECO frame to the second network switching apparatus  2 . At this time, “Link Training” is set to “0” in the LA-ECO frame. As “Link Training” and “ACK” are “0”, this LA-ECO frame is a link-down completion notice. 
     After that, the first network switching apparatus  1  drops the ECO processing execution flag of the LAG  5 . 
     When the link-down completion notice is received, the second network switching apparatus  2  checks the state of the second ports  22  that belong to the LAG  5  to extract at least one of the second ports  22  that is linked down according to the received link-down completion notice. Then, the second network switching apparatus  2  cuts off the power supply to at least one of the second ports  22  that is linked down due to the link-down of the first port  12 . Accordingly, the power supply to the linked-down ports at both ends of a link is cut off. 
     Further, the second network switching apparatus  2  excludes at least one of the second ports  22  in which the power supply is cut off due to the link-down that corresponds to the link-down of the first port  12  from the target to which an error indicating link-down is sent. Accordingly, the second network switching apparatus  2  changes the LED of at least one of the second ports  22  in which the power supply is cut off due to the link-down that corresponds to the link-down of the first port  12  to indicating the ECO state. 
     After that, the second network switching apparatus  2  drops the ECO processing execution flag of the LAG  5 . 
       FIG. 13  is an explanatory schematic diagram of the link adding process. 
     The first network switching apparatus  1  transmits an LA-ECO frame to the second network switching apparatus  2  prior to the linking-up process. At this time, “Link Training” is set to “1” in the LA-ECO frame. As “Link Training” is “1” and “ACK” is “0”, the LA-ECO frame is a link-up start notice. 
     When the second network switching apparatus  2  has received the LA-ECO frame as a link-up start notice, the power source of the second ports  22  is turned on, and the ECO processing execution flag of the LAG  5  is raised from “0” to “1” in accordance with the received LA-ECO frame. 
     After that, the second network switching apparatus  2  transmits the LA-ECO frame to the first network switching apparatus  1 . At this time, “Link Training” and “ACK” are set to “1” in the LA-ECO frame. As “Link Training” and “ACK” are “1”, this LA-ECO frame is a reply to the link-up start notice. 
     When the LA-ECO frame as a reply to the link-down start notice is received, the first network switching apparatus  1  performs a linking-up process in accordance with the received LA-ECO frame. 
     In particular, the first network switching apparatus  1  raises the ECO processing execution flag of the LAG  5 . Subsequently, the first network switching apparatus  1  supplies power to at least one of the first ports  12  from among the first ports  12  that belong to the LAG  5 , according to the available bandwidth information of the LAG  5 . By so doing, the first network switching apparatus  1  links up at least one of the first ports  12 . 
     Further, the first network switching apparatus  1  includes at least one of the first ports  12  to which power is supplied according to the available bandwidth information to the target to which an error indicating link-down is sent. Accordingly, the first network switching apparatus  1  changes the LED  123  of at least one of the first ports  12  to which power is supplied according to the available bandwidth information, from indicating the ECO state to indicating link up. 
     After this linking-up process, the first network switching apparatus  1  transmits the LA-ECO frame to the second network switching apparatus  2 . At this time, “Link Training” is set to “0” in the LA-ECO frame. As “Link Training” and “ACK” are “0”, this LA-ECO frame is a link-up completion notice. 
     After that, the first network switching apparatus  1  drops the ECO processing execution flag of the LAG  5  from “1” to “0”. 
     When the link-up completion notice is received, the second network switching apparatus  2  checks the state of the second ports  22  that belong to the LAG  5  to extract the linked-down second port  22  according to the received link-up completion notice. Then, the second network switching apparatus  2  cuts off the power supply to the second port  22  that is linked down due to the link-down of the first port  12 . Accordingly, the power supply to the linked-down ports at both ends of a link, excluding linked-up ports including a newly linked-up port, is cut off. 
     Further, the second network switching apparatus  2  excludes the second port  22  in which the power supply is cut off due to the link-down that corresponds to the link-down of the first port  12  from the target to which an error indicating link-down is sent. Accordingly, the second network switching apparatus  2  changes the LED of the second port  22  in which the power supply is cut off due to the link-down that corresponds to the link-down of the first port  12  to indicating the ECO state. 
     Subsequently, the second network switching apparatus  2  drops the ECO processing execution flag of the LAG  5  from “1” to “0”. 
     All examples and conditional language provided herein are intended for the pedagogical purposes of aiding the reader in understanding the invention and the concepts contributed by the inventor to further the art, and are not to be construed as limitations 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 one or more 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.