Patent Publication Number: US-7590126-B2

Title: Frame transfer processing method and device

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a frame transfer processing method and device, and in particular to a method and device for transferring a frame by a flooding method through a layer 2 switch which determines a transfer destination port based on a VLAN (Virtual Local Area Network) and a MAC (Medical Access Control) address. 
   2. Description of the Related Art 
   A layer 2 (hereinafter, occasionally abbreviated as L2) switch having a plurality of input/output communication interfaces (hereinafter, referred to as physical ports) has a function of voluntarily learning path information indicating a transfer destination of a MAC frame. Namely, this is a function of learning the source MAC address of a MAC frame and the location of a port having received the MAC frame, and of then determining a port to which the MAC frame should be transmitted by using the information already learned when another port receives a frame having a destination MAC address which is the same MAC address as one previously learned. 
   Also, the L2 switch having a VLAN function learns the MAC address and VLAN identifying information (hereinafter, occasionally referred to as VLAN-ID) in combination. For such a system, a specification is described in IEEE802.1d as a method of learning and forwarding (transferring) by the VLAN and the MAC address. 
   For another technology of the layer 2 switch, there is mentioned a function of binding a plurality of physical links to be used as a single logical transmission line. This is called a link aggregation function. By using this function, the transfer of data amounts exceeding a bandwidth of a single physical link between two L2 switches is made possible. Also, even if any physical link among the bound aggregated links fails, the arrangement of the logical link is changed or switched to use remaining normal physical links, thereby enabling an operation to be continued. The specification of this system is described in IEEE802.3ad. 
   Thus, when no frame is transferred between certain end stations (terminals) and when a fixed time elapses after learning and the learned contents become disabled, the L2 switch assumes a state where nothing has been learned. In such a state where no MAC address has been learned, a frame is required to be transferred to all of the ports belonging to the same VLAN. This function is called “flooding”. How to perform the “flooding” function will now be described by referring to operations of “learning” and “frame forwarding by learned information”. 
   Network Arrangement (1) 
   It is supposed that a network shown in  FIG. 3 , for example, is structured. An L2 switch  100  has ports represented by #Pn, where n is supposed to be a port No. It is to be noted that hereinafter, a word “port” is used as a synonym of “physical link”. In this example, it is arranged that among five physical ports shown by #P1-#P5, the ports #P1, #P2, and #P4 belong to a VLAN 501 indicated by VLAN-ID=501, and the ports #P3 and #P5 similarly belong to a VLAN 601 indicated by VLAN-ID=601. It is to be noted that a terminal  11  (MAC address=11) is connected to the port #P1, a terminal  12  (MAC address=12) is connected to the port #P2, a terminal  13  (MAC address=13) is connected to the port #P3, a terminal  14  (MAC address=14) and a terminal  15  (MAC address=15) are connected to the port #P4, and a terminal  16  (MAC address=16) is connected to the port #P5 respectively. 
   The arrangement of the VLAN is stored in a memory (not shown) within the L2 switch  100  in the form of representing with 1 bit whether or not each port belongs to the VLAN for every VLAN-ID, as shown by the following table T101 and a bitmap arrangement of a port in  FIG. 4A . 
   
     
       
         
             
             
             
           
             
                 
               TABLE T101 
             
             
                 
                 
             
             
                 
               VLAN-ID 
               BITMAP OF PORT 
             
             
                 
                 
             
           
          
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               501 
               11010 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               601 
               00101 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
                 
             
          
         
       
     
   
   Also, in order to obtain a VLAN to which a frame is attributed when a frame is received at each port, a VLAN-ID allocated to the received frame is set for every port as shown by the following table T102. 
   
     
       
         
             
             
             
           
             
                 
               TABLE T102 
             
             
                 
                 
             
             
                 
                 
               VLAN-ID ALLOCATED TO 
             
             
                 
               PORT NO. 
               RECEIVED FRAME 
             
             
                 
                 
             
           
          
             
                 
               1 
               501 
             
             
                 
               2 
               501 
             
             
                 
               3 
               601 
             
             
                 
               4 
               501 
             
             
                 
               5 
               601 
             
             
                 
                 
             
          
         
       
     
   
   A frame transfer processing operation by the L2 switch  100  will now be described referring to a flowchart of  FIG. 5 . It is to be noted that this flowchart is executed by an exclusive logical circuit, CPU, or the like (not shown) installed in the L2 switch  100 , which applies to the following description. 
   Firstly, the terminal  11  transmits a frame whose destination address is “14” and source address is “11” in a format shown in  FIG. 6A . The L2 switch  100  monitors a frame received at step S 1  to receive the frame at port #P1 (at step S 2 ). A correspondence between the reception port of the frame and the VLAN-ID is preset in the L2 switch  100  by the table T102 described above. 
   At step S 3 , the VLAN to which the frame belongs is obtained based on the reception port by using the table T102. Since the table T102 indicates that the frame received at the port #P1 belongs to the VLAN 501 of the VLAN-ID=501, the VLAN-ID=501 is derived at step S 3 . 
   At step S 4 , the No. “1” of the port having received the frame is learned with the VLAN-ID obtained at step S 3  and the source address of the frame as keys. The result of the learning is prepared as following table T103. 
   
     
       
         
             
             
             
           
             
               TABLE T103 
             
             
                 
             
             
               VLAN-ID 
               MAC ADDRESS 
               PORT NO. 
             
             
                 
             
           
          
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               501 
               11 
               1 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
                 
             
          
         
       
     
   
   Namely, since the received frame has the VLAN-ID=501, the source address=11, and the reception port=1, a single entry in the table as shown in the above table T103 is prepared. 
   At step S 5 , whether or not the port No. is learned is searched or checked with the VLAN-ID=501 and the destination address=14 of the received frame as keys. For this search, the above-mentioned table T103 is searched again to determine whether or not any hit is found (at step S 6 ). Since the port No. has not been learned yet, no hit is found, so that the process proceeds to step S 7 . 
   At step S 7 , in order to check the port composing the VLAN, bitmaps of all the ports corresponding to the VLAN-ID=501 are obtained referring to the above-mentioned table T101, whereby it is recognized that the ports #P1, #P2, and #P4 belong to the VLAN 501. 
   At step S 8 , the frame is outputted from the obtained ports one after another. However, since the port having received the frame does not transmit the frame, the frame is transmitted from the ports #P2 and #P4. Thus, when the destination has not been learned, the frame is transferred to all of the ports belonging to the same VLAN except for the port having received the frame (at step S 9 ). This is called a “flooding” function. As a result, the destination terminal  14  connected to the port #P4 can receive the frame transmitted by the source terminal  11 . Although other terminals  12  and  15  receive the same frame, the destination address is not coincident with the MAC address of their own. Therefore, the frame is discarded. 
   An operation when the L2 switch  100  transfers a frame in the direction opposite (from right to left in  FIG. 3 ) to the above-mentioned description will now be described. This is an operation in a case where the destination address is learned. 
   Firstly, when the terminal  14  transmits a frame having the destination address=11 and the source address=14, the L2 switch  100  monitors the frame reception at step S 1  to receive the frame at the port #P4 (at step S 2 ). The L2 switch  100  learns the port No. in the same way as mentioned above (at steps S 3  and S 4 ), and a table T103a, which is the table T103 in which the second entry is prepared, can be obtained as follows: 
   
     
       
         
             
             
             
           
             
               TABLE T103a 
             
             
                 
             
             
               VLAN-ID 
               MAC ADDRESS 
               PORT NO. 
             
             
                 
             
           
          
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               501 
               11 
               1 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               501 
               14 
               4 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
                 
             
          
         
       
     
   
   At step S 5 , whether or not the port No. has been learned is searched with the VLAN-ID and the destination address of the received frame as keys. For this search, the above-mentioned table T103a is used again. Since the entry has been already learned with the VLAN-ID=501 and the MAC address=11 as keys, a hit is found, so that the process proceeds from step S 6  to step S 10 . 
   Since it is recognized at step S 10  that the output destination port=#P1 from the result of learning, the frame is outputted from the port #P1. Thus, the terminal  11  connected to the port #P1 receives the frame transmitted from the terminal  14 . 
   On the other hand, the terminals  13  and  16  are also connected to the L2 switch  200 , the frame transferred between the terminals  13  and  16  is transferred only to the terminal belonging to the VLAN 601, but not to the terminal belonging to a different VLAN, the VLAN 501. This is because a flooding destination is limited based on the setting of the above-mentioned table T101. By this function, data leakage to a different VLAN is prevented and data security is attained. 
   Network Arrangement (2) 
   How the flooding process is performed when a network is arranged by using a link aggregation function as shown in  FIG. 7  will now be described. 
   Firstly, it is supposed that the network arrangement (2) of  FIG. 7  is equivalent to the network arrangement (1) of  FIG. 3  in which the L2 switch  100  is composed of L2 switches  200  and  300 , and the arrangement of the terminals and the VLANs to which the terminals belong are the same as those in  FIG. 3 . Between the L2 switches  200  and  300 , frames are transferred by using a logical link aggregation-transmission line LA in which physical links are bound between the ports #P4-#P6 of the L2 switch  200  and the ports #P1-#P3 of the L2 switch  300  by the link aggregation function. The locations of the ports and the number of ports form one example, so that it is possible to freely select the ports by the arrangement of the switch and to bind the links. This logical link aggregation-transmission line LA belongs to both of the VLANs 501 and 601, so that the VLAN frames for both of the VLANs 501 and 601 are transferred. Which VLAN the frame passing through the logical link belongs to is indicated by using the frame with a VLAN tag as shown in  FIG. 6B . On the transmission side of the logical link, the VLAN tag is inserted upon outputting the frame, while on the reception side the VLAN tag is deleted after identifying the VLAN. 
   Frame Transferring Process by the L2 Switches  200  and  300  will now be Described along a Flowchart of  FIG. 8 . 
   The terminal  11  transmits the frame having the format as shown in  FIG. 6B  where the destination address=14 and the source address=11. The L2 switch  200  monitors the frame reception at step S 1  to receive the frame at the port #P1 (at step S 2 ). In the L2 switch  200 , the reception port is associated with the VLAN-ID by the following table T104. 
   
     
       
         
             
             
             
             
           
             
               TABLE T104 
             
             
                 
             
             
                 
               VLAN-ID 
                 
                 
             
             
                 
               ALLOCATED 
               PORT/TRUNK 
                 
             
             
                 
               TO RECEIVED 
               IDENTIFICATION 
                 
             
             
               PORT NO. 
               FRAME* 
               (0: PORT, 1: TRUNK) 
               TRUNK NO. 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
               1 
               501 
               0 
               0 
             
             
               2 
               501 
               0 
               0 
             
             
               3 
               601 
               0 
               0 
             
             
               4 
               0 
               1 
               20 
             
             
               5 
               0 
               1 
               20 
             
             
               6 
               0 
               1 
               20 
             
             
                 
             
             
               *WHEN 0 IS SET TO VLAN-ID CORRESPONDING TO RECEIVED FRAME, IT IS INDICATED THAT THE PORT USES FRAME WITH VLAN TAG &amp; VLAN IS NOT SPECIFIED. 
             
          
         
       
     
   
   However, a disabling value “0” is set as the VLAN-ID corresponding to the ports #P4-#P6 in the table T104. This set value “0” assumes that the frame transmitted/received at the ports #P4-#P6 of the L2 switch  200  uses the VLAN tag within the frame shown in  FIG. 6B , and the VLAN is not specified corresponding to the ports. 
   At step S 3 , the VLAN to which the frame belongs is obtained by using the above-mentioned table T104. Since the table T104 indicates that the frame received at the port #P1 belongs to the VLAN 501 of the VLAN-ID=501, the VLAN-ID=501 is derived at step S 3 . 
   At step S 4 , the No. “1” of the port having received the frame is learned with the VLAN-ID obtained at step S 3  and the source address within the frame as keys. Being the same arrangement as the above table T103, the result of the learning is omitted. 
   At step S 5 , whether or not the port No. is learned is searched with the VLAN-ID and the destination address of the received frame as keys. For this search, the same table as the table T103 is used again. Since the port No. has not been learned yet, no hit is found (at step S 6 ), so that the process proceeds to step S 7 . 
   At step S 7 , in order to check the ports composing the VLAN, the following table T105 is referred to. 
   
     
       
         
             
             
             
           
             
                 
               TABLE T105 
             
             
                 
                 
             
             
                 
               VLAN-ID 
               BITMAP OF PORT 
             
             
                 
                 
             
           
          
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               501 
               110100 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               601 
               001100 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
                 
             
          
         
       
     
   
   From this table T105, a bitmap of a port corresponding to the VLAN-ID=501 is obtained, whereby it is recognized that the ports #P1, #P2, and #P4 belong to the VLAN 501 of the VLAN-ID=501. 
   At step S 8 , the frame is outputted from the obtained ports one after another. However, the frame is not outputted from the port having received the frame. The frame is outputted from the ports #P2 and #P4 (at step S 9 ). Since it is recognized from the above table T104 that the frame tag is to be inserted into the frame at the port #P4 (VLAN-ID=0), the VLAN tag is inserted into the frame outputted. As a result, the frame is transferred to the L2 switch  300  connected to the port #P4, and the terminal  14  can receive the frame transmitted by the terminal  11 . 
   The above-mentioned flooding process is the same as that of  FIG. 5  already described, so that usage of the link aggregation function is unnoticed. The reason why the same flooding process is enabled is that the above-mentioned table T105 is preliminarily composed by the method mentioned as follows. 
   The L2 switch  200  has a table defining the logical link arrangement composed by the link aggregation function as in the following table T106. 
   
     
       
         
             
             
             
           
             
               TABLE T106 
             
             
                 
             
             
                 
                 
               PORT LIST 
             
             
                 
                 
               (ARRANGED IN 
             
             
                 
               NUMBER OF 
               ASCENDING 
             
             
               TRUNK NO. 
               PORTS 
               ORDER) 
             
             
                 
             
           
          
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               20 
               3 
               4, 5, 6 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
                 
             
          
         
       
     
   
   A single logical link is called a trunk, to which a number is allocated for identification. In the network arrangement (2) of  FIG. 7 , for the L2 switch  200 , the entries shown in the table T106 is prepared when the link aggregation-transmission line LA in which the trunk No. is “20” and the links of the physical ports #P4, #P5, and #P6 are bound are composed. 
   The ports #P1, #P2, #P4, #P5, and #P6 belong to the VLAN 501. If the bitmap of the ports is generated as it is when the above table T105 is composed, “110111” is obtained. However, since the ports #P4, #P5, and #P6 compose trunks, frames are overlapped if all of the ports #P4, #P5, and #P6 are made the flooding destination. Since the overlapping is against the operation of the link aggregation function, a mechanism for avoiding overlapping is required. A physical port composing a trunk and its trunk No. can be identified by referring to a port type (port/trunk identification) and the trunk No. in the above table T104. In order to avoid the overlapping of the frames at the ports #P4, #P5, and #P6, a list of the ports composing the trunk is checked by referring to the entry of the above table T106 based on the trunk No. “20”, and the entry of the bitmap “110100” corresponding to the VLAN 501 is prepared in the table T105 so that only the top physical port of the list may become the flooding destination. 
   Thus, the flooding process can be done in the same way as the case of a normal port even if the link aggregation-transmission line LA is used. It is to be noted that since the trunk  20  also belongs to the VLAN 601, the entry of the VLAN-ID=601 is similarly arranged in the table T105 so that only the port #P4 within the trunk may assume the flooding destination. 
   Hereinafter, an operation after the frame transmitted from the terminal  11  is flooded to the port #P4 of the L2 switch  200  to be received at the port #P1 of the L2 switch  300  will be described referring to the flowchart of  FIG. 8  again. 
   The L2 switch  300  monitors the frame reception at step S 1 , and receives the frame at port #P1 (at step S 2 ). The L2 switch  300  preliminarily associates the reception port with the VLAN-ID by the following table T107. 
   
     
       
         
             
             
             
             
           
             
               TABLE T107 
             
             
                 
             
             
                 
               VLAN-ID 
               PORT/TRUNK 
                 
             
             
                 
               ALLOCATED TO 
               IDENTIFICATION 
               TRUNK 
             
             
               PORT NO. 
               RECEIVED FRAME 
               (0: PORT, 1: TRUNK) 
               NO. 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
               1 
               0 
               1 
               30 
             
             
               2 
               0 
               1 
               30 
             
             
               3 
               0 
               1 
               30 
             
             
               4 
               501 
               0 
               0 
             
             
               5 
               601 
               0 
               0 
             
             
                 
             
          
         
       
     
   
   At step S 3 , the VLAN to which the frame belongs is obtained by using this table T107. Since this table T107 exhibits that the VLAN-ID corresponding to the port #P1 is set to “0” and the frame received from the L2 switch  200  is determined by the VLAN-tag (see  FIG. 6B ) within the frame, it is recognized that the received frame belongs to the VLAN 501 of the VLAN-ID=501 referring to the VLAN-tag of the received frame, and then the VLAN tag is deleted. 
   At step S 4 , a path having received the frame is learned with the VLAN-ID obtained at step S 3  and the source address of the frame as keys. The result of this learning is “30” that is the No. of a trunk having received the frame as shown in the following table T103b. 
   
     
       
         
             
             
             
             
           
             
               TABLE T103b 
             
             
                 
             
             
                 
                 
                 
               PORT/TRUNK 
             
             
                 
                 
               PORT/ 
               IDENTIFICATION 
             
             
               VLAN-ID 
               MAC ADDRESS 
               TRUNK NO. 
               (0: PORT, 1: TRUNK) 
             
             
                 
             
           
          
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               501 
               11 
               30 
               1 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               501 
               14 
               4 
               0 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
                 
             
          
         
       
     
   
   Also, information “1” for identifying that the trunk has received the frame is entered in the table T103b. 
   At step S 5 , whether or not the port No. has been learned is searched with the VLAN-ID and the destination address of the received frame as keys. For this search, the above table T103b is used again. Since the port No. has not been learned yet, no hit is found (at step S 6 ), so that the process proceeds to step S 7 . 
   At step S 7 , in order to check the port composing the VLAN, the bitmap of the port corresponding to the VLAN-ID=501 is obtained referring to the following table T108, whereby it is recognized that the ports #P1 and #P4 belong to the VLAN 501. 
   
     
       
         
             
             
             
           
             
                 
               TABLE T108 
             
             
                 
                 
             
             
                 
               VLAN-ID 
               BITMAP OF PORT 
             
             
                 
                 
             
           
          
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               501 
               10010 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               601 
               10001 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
                 
             
          
         
       
     
   
   At step S 8 , the frame is outputted from the obtained ports one after another. However, when the trunk has received the frame and the output destination port belongs to the same trunk, the frame is not outputted from the port. Therefore, the frame is not outputted from the port #P1. This determination is performed by using the setting of the above table T107. Namely, it is stored as the information of the frame that the received port is a trunk  30 . As mentioned above, the ports #P1 and #P4 are set as the output destinations for the VLAN-ID=501 from the bitmap of the table T108. When the output destination of the frame is the port #P1, it is recognized that the port belongs to the trunk  30  by referring to the table T107. Since it is the same trunk, the frame is not outputted from the port #P1. Thus, the received frame is outputted from the port #P4 of the L2 switch  300 . As a result, the terminal  14  receives the frame transmitted by the terminal  11 . 
   An operation when the L2 switches  200  and  300  transfer a frame in the opposite direction (from right to left in  FIG. 7 ) to the above-mentioned description will now be described. This is an operation in a case where the destination address is learned. 
   Firstly, when the terminal  14  transmits a frame whose destination address=11 and source address=14, the L2 switch  300  monitors the frame reception at step S 1  to receive the frame at the port #P4 (at step S 2 ). At step S 3 , the VLAN of the frame is obtained based on the reception port No. 
   At step S 4 , the L2 switch  300  learns the port No. in the same way as described above, and the second entry is prepared as shown in the above table T103b. 
   At step S 5 , whether or not the destination has been learned is searched with the VLAN-ID and the destination address of the received frame as keys. For this search, the above table T103b is used again. Since the entry with the VLAN-ID=501 and the MAC address=11 as keys has already been learned, a hit is found, the process proceeds from step S 6  to step S 11 , and the learned port is obtained from the table T103b. By the result of this learning, it is recognized that the output destination is a trunk (port/trunk identifying information=“1”), and its No. is “30” (at steps S 11  and S 12 ). In the case of a trunk, the process proceeds to step S 13 , where hashing operation is performed in order to select a single physical port as an output destination from among physical ports composing the trunk. For this purpose, the following table T109 is used to obtain that the number of ports in the trunk is “3”. 
   
     
       
         
             
             
             
           
             
               TABLE T109 
             
             
                 
             
             
                 
                 
               PORT LIST 
             
             
                 
                 
               (ARRANGED IN 
             
             
                 
               NUMBER OF 
               ASCENDING 
             
             
               TRUNK NO. 
               PORTS 
               ORDER) 
             
             
                 
             
           
          
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               30 
               3 
               1, 2, 3 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
               . 
               . 
               . 
             
             
                 
             
          
         
       
     
   
   As an example of the hashing operation, such a method is used that a remainder calculation is performed by the number of ports to the result of the addition of the destination address and the source address of the frame, in which the first port is made an output destination if the result is “0”, and the second port is made an output destination if it is “1”. Since frames transferred between certain specific end stations are to be transferred through the same physical link at all times by this method, the order of frames can be prevented from changing. 
   As a result, the frame is outputted from any one of the ports #P1-#P3 of the L2 switch  300  (at step S 14 ) to reach the L2 switch  200 . If the port No. is one of “1”-“3” in this example, it is recognized from the above table T107 that the VLAN tag is inserted into the frame to be transmitted. Being the same as that already described, the subsequent operation of the L2 switch  200  is omitted. However, the table used at steps S 11  and S 12  in the L2 switch  200  at this time is the following table T103c. 
   
     
       
         
             
             
             
             
           
             
               TABLE T103c 
             
             
                 
             
             
                 
                 
                 
               PORT/TRUNK 
             
             
                 
               MAC 
               PORT/ 
               IDENTIFICATION 
             
             
               VLAN-ID 
               ADDRESS 
               TRUNK NO. 
               (0: PORT, 1: TRUNK) 
             
             
                 
             
           
          
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               501 
               11 
                1 
               0 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               501 
               14 
               20 
               1 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
                 
             
          
         
       
     
   
   As a result, the terminal  11  receives the frame transmitted by the terminal  14 . 
   On the other hand, there are a method and device for trunking multiple ports in a network switch by which a network switch configured for switching data packets across multiple ports uses an address table to generate frame forwarding information, a decision-making engine checks the frame forwarding information to determine whether the frame is to be forwarded to a port that is part of a trunk, and the decision-making engine determines the port which is to become the transmitting destination of the frame when the frame is to be outputted to a trunk port (see e.g. patent document 1). 
   [Patent Document 1] 
   
       
       Japanese Translation of PCT International Application No. 2003-500926
 
Problem 1
 
     
  
   As described above, when the flooding process is performed to the trunk, a bit of only a single port among the ports composing the trunk is made “1” as shown in table T105, only to which the flooding is performed. A port of the trunk set for the flooding port is set as follows: A port list in which port Nos. are arranged in ascending order is prepared, and the smallest No. among the ports in a state where the communication with the opposing terminal is possible (link-up state) is set to be used. Since such a setting has a complicated logic, it is general that the L2 switch mounts thereon a CPU and a memory to operate the firmware for setting the port. If the trunk accommodates a plurality of VLANs, setting for each VLAN is performed. 
   When a failure occurs at the flooding port and the communication is disabled, a flame to be flooded is not transferred if the failure remains unaddressed. Therefore, it is required to reset another port of the trunk for the flooding. This setting is performed by firstly making the bit, which is corresponding to the port having been set, “0”, and by making a bit corresponding to a port newly used as a representative port “1”. 
   Processing upon port&#39;s failure will now be described referring to flowchart shown in  FIG. 9 . 
   Firstly, the L2 switches  200  and  300  monitor a port&#39;s failure at steps S 21  and S 22 . If a failure occurs, whether or not the port belongs to the trunk is checked at step S 23 . For this purpose, setting as shown in the above tables T104 and T107 is used. 
   If the port belongs to the trunk, whether or not the port is coincident with the flooding port is checked at step S 24 . For this purpose, the above table T106 is used. 
   If the port is coincident with the top port arranged in ascending order, it is the flooding port. Therefore, bitmap setting for flooding at an object port for every VLAN is canceled in the table T105 (step S 25 ). This processing is repeated for every VLAN to which the trunk belongs (at steps S 26  and S 25 ). Which VLAN the trunk belongs to can be obtained by referring to e.g. the setting of the following table T110. 
   
     
       
         
             
             
             
           
             
                 
               TABLE T110 
             
             
                 
                 
             
             
                 
                 
               BITMAP BELONGING TO 
             
             
                 
               TRUNK NO. 
               VLAN 
             
             
                 
                 
             
           
          
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               20 
               00 . . . 010 . . . 010 . . . 00 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
                 
             
             
                 
               (502nd &amp; 602nd ONES FROM LEFT ARE “1”) 
             
          
         
       
     
   
   An example of this table T110 exhibits that the trunk  20  belongs to the VLANs 501 and 601. It is to be noted that the following table T110a can be obtained in the case of the L2 switch  300 . 
   
     
       
         
             
             
             
           
             
                 
               TABLE T110a 
             
             
                 
                 
             
             
                 
                 
               BITMAP BELONGING TO 
             
             
                 
               TRUNK NO. 
               VLAN 
             
             
                 
                 
             
           
          
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               30 
               00 . . . 010 . . . 010 . . . 00 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
                 
             
             
                 
               (502nd &amp; 602nd ONES FROM LEFT ARE “1”) 
             
          
         
       
     
   
   In order to remove the port in which a failure has occurred from the trunk, the port list is updated at step S 27 . This updating is performed to the above table T106. The top port after the updating is obtained at step S 28 , and a new flooding port is set for every VLAN at step S 29 . This setting is performed at the table T105 in the same way as the canceling. This is performed to all of the VLANs accommodated in the trunk (at step S 30 ). 
   On the other hand, when the failed port is not coincident with the flooding port at step S 24 , the port list is updated at step S 31  in the same way as step S 27  to end the process. 
   Since the above-mentioned setting at steps S 25  and S 29  is performed by firmware for every VLAN, it takes much time for setting if the trunk belongs to numerous VLANs. When there are numerous VLANs, from several hundreds to several thousands of VLANs are accommodated in some cases. Until the time comes when such a setting is changed, a frame whose destination has not been learned is discarded without being transferred to the new flooding port. Since the communication is disabled during the setting for a user&#39;s standpoint, it has been an object to minimize a setting time. 
   Problem 2 
   Another problem for switching the flooding port is as follows: 
   Network Arrangement (3) 
   Firstly, supposing that the L2 switch as shown in  FIG. 7  has a plurality of slots in a chassis, and a single line unit is inserted per slot to be used, the single line unit can have a plurality of physical ports and can use a plurality of line units, while a single trunk can be arranged across a plurality of line units. 
     FIG. 10  shows such a network arrangement (3). In this example, L2 switches  400  and  500  respectively have three line units U1-U3.The L2 switches  400  and  500  are connected with trunks  20  and  30  of the link aggregation-transmission line LA composed of five physical links, the 1st-3rd physical links among them connect the ports #P1-#P3 of the line unit U2 in the L2 switch  400  to the ports #P1-#P3 of the line unit U1 in the L2 switch  500 , and the 4th and 5th physical links connect the ports #P1 and #P2 of the line unit U3 in the L2 switch  400  to the ports #P1 and #P2 of the line unit U2 in the L2 switch  500 . 
   When a failure is detected at the port #P1 of the line unit U2 in the L2 switch  400  in this example, upper digits as a unit No. and lower digits as a port No. within the unit are arranged in ascending order and the port in subsequent operation is selected, thereby obtaining the port #P2 of the line unit U2 as a port newly used for flooding. 
   If a failure has occurred in the line unit U1 itself in the opposing L2 switch  500  at this time, a failure is also subsequently detected in the port #P2 of the line unit U2 in the L2 switch  400  newly selected for the flooding port. Therefore, the switching of the flooding port is required again. Such a switching is repeated until the port #P1 of another line unit U3 in the L2 switch  400  is selected for the flooding port. Thus, since it takes much time until the flooding frame is transferred again and the communication is interrupted in the meantime, it has been an object that the setting time is reduced. 
   SUMMARY OF THE INVENTION 
   It is accordingly an object of the present invention to provide a frame transfer processing method and device by which an interruption time of communication is minimized by shortening a switching time of flooding ports when a port&#39;s failure occurs in a link aggregation-transmission line, in view of the above-mentioned problems. 
   In order to achieve the object for the above-mentioned problem 1, in the present invention, a frame transfer processing method for transferring a frame through a link aggregation-transmission line which aggregates a plurality of physical links connecting switches to be used as a single logical transmission line, comprises: a first step of setting a single physical link among the link aggregation-transmission line to be used for flooding for every physical link and every VLAN, upon accommodating a plurality of VLANs in the link aggregation-transmission line; and a second step of changing a flooding destination per link upon failure of the link without changing the flooding destination for every VLAN. 
   The second step may be further provided with a step of setting the failed link disabled when the failed link belongs to a trunk composing the link aggregation-transmission line and is a present flooding destination, and of setting another link composing the trunk as the flooding destination. 
   Also, in order to achieve the object for the above-mentioned problem 2, in the frame transfer processing method of the present invention, when each switch is composed of a plurality of line units, and the link aggregation-transmission line is used across a plurality of line units, the second step may be further provided with a step of setting the failed link disabled when the failed link belongs to a trunk composing the link aggregation-transmission line and is a present flooding destination, and of setting a link of another line unit as a link newly used for the flooding destination. 
   Furthermore, the above-mentioned second step may be further provided with a step of decreasing a number of links composing the trunk by deleting the failed link upon setting the failed link disabled. 
   Namely, in the above-mentioned method of the present invention, it has become possible to select enabling/disabling of flooding-setting for all of the VLANs per physical link (port) in addition to flooding-setting per VLAN of the trunk. In other words, the flooding is set per VLAN in all of the links composing the trunk, and aside from this, the selection of enabling/disabling of the flooding-setting can be done for every physical link. This setting only makes the physical link which should become a flooding link enable and others corresponding to remaining physical links of the trunk disable. 
   When the flooding link is changed, the selection of enabling/disabling per physical link is changed, thereby enabling the flooding from a single link among the trunk. Accordingly, since it is not necessary to change the flooding-setting per VLAN, but is necessary only to set enabling/disabling per physical link, the switching time of a representative link can be minimized. 
   Also, the method of the present invention has a logic that when the flooding links are switched, a subsequent flooding link is selected so as to shift across the line unit. Namely, when a failure occurs in the flooding link, instead of arranging Nos. of the line units as well as links in e.g. the ascending order to make the link of the subsequent position the subsequent flooding link, it is selected from different line units. Thus, when a failure of the line unit occurs in an opposing device, a change of the flooding link can be completed by a one-time change, so that the interruption time of the communication by the switching can be minimized. 
   In a frame transfer processing device for realizing the above-mentioned method of the present invention, the device for transferring a frame through a link aggregation-transmission line which aggregates a plurality of physical links connecting switches to be used as a single logical transmission line, comprises: a first means of setting a single physical link among the link aggregation-transmission line to be used for flooding for every physical link and every VLAN, upon accommodating a plurality of VLANs in the link aggregation-transmission line; and a second means of changing a flooding destination per link upon failure of the link without changing the flooding destination for every VLAN. 
   The second means may be further provided with means of setting the failed link disabled when the failed link belongs to a trunk composing the link aggregation-transmission line and is a present flooding destination, and of setting another link composing the trunk as the flooding destination. 
   Also, when each switch is composed of a plurality of line units, and the link aggregation-transmission line is used across a plurality of line units, the second means may be further provided with means of setting the failed link disabled when the failed link belongs to a trunk composing the link aggregation-transmission line and is a present flooding destination, and of setting a link of another line unit as a link newly used for the flooding destination. 
   Also, the above-mentioned second means may be further provided with means of decreasing a number of links composing the trunk by deleting the failed link upon setting the failed link disabled. 
   As described above, according to the present invention, when the flooding is performed to a trunk in a network using a trunk of a link aggregation, time required for switching to a new link when a failure has occurred in a flooding link (port) can be minimized. 
   Namely, even if numerous VLANs are accommodated in a trunk, only the setting of enabling/disabling for every physical port has to be operated in order to change the flooding link, whereby it becomes possible to switch the flooding links in a short time since it is not necessary to change the setting for flooding per VLAN. 
   Also, when a single trunk is composed from a link of a plurality of line units, and even if a failure occurs per line unit at the opposing device, changing a flooding link can be performed with a single operation. Therefore, it becomes possible to switch the flooding link in a shorter time than the flooding link is shifted only in ascending order. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The above and other objects and advantages of the invention will be apparent upon consideration of the following detailed description, taken in conjunction with the accompanying drawings, in which the reference numerals refer to like parts throughout and in which: 
       FIG. 1  is a flowchart of frame transfer processing in network arrangements (2) and (3) of the present invention; 
       FIG. 2  is a flowchart of processing when a failure occurs in a port of a trunk and a flooding port is changed in network arrangements (2) and (3) of the present invention; 
       FIG. 3  is a block diagram showing a conventionally known network arrangement (1) using an L2 switch; 
       FIGS. 4A and 4B  are diagrams showing an arrangement of a bitmap of a port in an L2 switch used in the present invention; 
       FIG. 5  is a flowchart of frame transfer processing by an L2 switch in a network arrangement (1); 
       FIGS. 6A and 6B  are format diagrams of a conventionally known MAC frame; 
       FIG. 7  is a diagram showing a conventionally known network arrangement (2); 
       FIG. 8  is a flowchart of frame transfer processing by a conventional L2 switch in a network arrangement (2); 
       FIG. 9  is a flowchart of conventional processing when a failure occurs at a port of a trunk and a flooding port is changed in a network arrangement (2); and 
       FIG. 10  is a block diagram showing a conventionally known network arrangement (3). 
   

   DESCRIPTION OF THE EMBODIMENTS 
   Operational Embodiment in Network Arrangement (2) (Problem 1) 
   As an arrangement of a network to which the present invention is applied,  FIG. 7  already described can be mentioned similarly. The arrangements of terminals, VLANs and a link aggregation to which the terminals belong are the same as those having been already described referring to  FIG. 7 , wherein the logical link aggregation-transmission line LA binding three physical ports exists. The logical link aggregation-transmission line LA belongs to both VLANs 501 and 601, and frames of both VLANs 501 and 601 are transferred. Which VLAN the frame flowing through the logical link aggregation-transmission line LA belongs to is indicated by using a frame with a VLAN tag shown in  FIG. 6B . The transmitting side of the logical link aggregation-transmission line LA inserts the VLAN tag into a frame upon outputting the frame, and the receiving side deletes the VLAN tag after identifying the VLAN. 
   Firstly, an example of frame transfer processing by the L2 switch  200  will now be described by referring to the flowchart shown in  FIG. 1 . 
   The terminal  11  transmits a frame with a format shown in  FIG. 6A  in which a destination address=14 and a source address=11. The L2 switch  200  monitors the frame reception at step S 1  to receive the frame at the port #P1 (at step S 2 ). Setting for associating the reception port with the VLAN-ID in the L2 switch  200  is the same as the above-mentioned table T104. 
   At step S 3 , the VLAN to which the frame belongs is obtained by using the table T104. Since the table T104 indicates that the frame received at the port #P1 belongs to the VLAN of the VLAN-ID=501, the VLAN-ID=501 is derived at step S 3 . At step S 4 , No. “1” of the port having received the frame is learned with the VLAN-ID derived at step S 3  and the source address of the frame as keys. The result of the learning is entered as shown in the above-mentioned table T103. 
   At step S 5 , it is searched whether or not the port No. has been learned with the VLAN-ID of the received frame and the destination address as keys. For this search, the table T103 is used again. Since it has not been learned yet at the moment, no hit is found, so that the process proceeds from step S 6  to step S 7 . 
   At step S 7 , the following table T111 is referred to in order to check the ports composing the VLAN. 
   
     
       
         
             
             
             
           
             
                 
               TABLE T111 
             
             
                 
                 
             
             
                 
               VLAN-ID 
               BITMAP OF PORT 
             
             
                 
                 
             
           
          
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               501 
               110111 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               601 
               001111 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
               . 
               . 
             
             
                 
                 
             
          
         
       
     
   
   This table T111 has the same arrangement as the tables T101 and T105 already described, which is preset for indicating a flooding destination for every VLAN. While only a single port is set as the flooding destination within the ports composing the link aggregation-transmission line LA in the table T105, “1” is set to all of the ports within the concerned trunk as the flooding destination in the above-mentioned table T111. This is different from the prior art example described above. Namely, the bitmap of the ports corresponding to the VLAN-ID=501 is obtained, so that it is indicated that the ports #P1, #P2, #P4, #P5, and #P6 belong to the VLAN 501. 
   At step S 8 - 1 , a frame is outputted from the obtained ports one after another. However, a frame is not outputted from a port having received the frame. Accordingly, a frame is not outputted from the first port #P1. When the obtained flooding destination port is different from the port having received the frame, the setting of “flooding enable” of the port is checked by the following table T112. 
   
     
       
         
             
             
             
           
             
                 
               TABLE T112 
             
             
                 
                 
             
             
                 
               PORT NO. 
               FLOODING ENABLE* 
             
             
                 
                 
             
           
          
             
                 
               1 
               1 
             
             
                 
               2 
               1 
             
             
                 
               3 
               1 
             
             
                 
               4 
               1 
             
             
                 
               5 
               0 
             
             
                 
               6 
               0 
             
             
                 
                 
             
             
                 
               BEFORE PORT 4 FAILS 
             
             
                 
               *FLOODING ENABLE (0: DISABLE, 1: ENABLE) 
             
          
         
       
     
   
   This table T112 is preset as follows: When the physical port is used as normal port, “1” is always set as “enable”. When it is used as a port of the link aggregation-transmission line LA, ports composing the link aggregation-transmission line LA in which no failure occurs are arranged in the ascending order, and enable “1” is preset to only the top port. Thus, it is possible to use a table having an arrangement of the table T106 already described for determining a port. In the example of the table T112, only the port #P4 is enabled among the trunk  20 . 
   Thus, the frames are outputted from the ports #P2 and #P4 by steps S 8 - 1 -S 8 - 3  and S 9 . Since it is recognized that for the port #P4 the VLAN tag is to be inserted into the frame from the table T104, it is inserted into the outputted frame. As a result, the frame is transferred to the L2 switch  300  located ahead of the port #P4. 
   The output of the frame from the L2 switch  300  to the terminal  14  is basically the same operation as that already described. However, as for the operation of the L2 switch  300 , step S 8 - 2  within the operation of the L2 switch  200  mentioned above referring to  FIG. 1  is different, in which the following table T112a is used. 
   
     
       
         
             
             
             
           
             
                 
               TABLE T112a 
             
             
                 
                 
             
             
                 
               PORT NO. 
               FLOODING ENABLE* 
             
             
                 
                 
             
           
          
             
                 
               1 
               1 
             
             
                 
               2 
               0 
             
             
                 
               3 
               0 
             
             
                 
               4 
               1 
             
             
                 
               5 
               1 
             
             
                 
                 
             
             
                 
               *FLOODING ENABLE (0: DISABLE, 1: ENABLE) 
             
          
         
       
     
   
   Namely, the frame outputted from the port #P4 of the L2 switch  200  is received at the port #P1 of the L2 switch  300 . Since the frame is outputted to the ports #P4 and #P5 of “flooding enable” which are not reception ports based on the above table T112a, the frame transmitted by the terminal  11  can be received by the terminal  14 . 
   Steps S 11 -S 14  of  FIG. 1  indicate processings in a case where the destination address has already been learned. Since they are the same as the processing of  FIG. 8  already described, the description will be omitted. 
   The processing in a case where a failure occurs in the flooding port, communication is disabled, and setting is changed so as to perform the flooding from another port will now be described referring to the flowchart of  FIG. 2 . 
   The L2 switch  200  monitors a port&#39;s failure at steps S 21  and S 22 , so that it is supposed that a failure has occurred at the port #P4 in the L2 switch  200  shown in  FIG. 7 . At step S 23 , whether or not the port belongs to a trunk is checked. For this purpose, the above table T104 is referred to. When the port belongs to the trunk, whether the port is coincident with the flooding port is checked by the entry of the following table T106a. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE T106a 
             
             
                 
                 
             
             
                 
                 
               NUMBER 
                 
                 
             
             
                 
               TRUNK 
               OF 
                 
               FLOODING 
             
             
                 
               NO. 
               PORTS 
               PORT LIST 
               PORT 
             
             
                 
                 
             
           
          
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               20 
               3 
               4, 5, 6 
               4 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
                 
             
          
         
       
     
   
   When the port #P4 has failed, the port is coincident with the top of the ports arranged in the ascending order, whereby it is recognized that the port is the flooding port, so that “disable” is set to the entry corresponding to the port #P4 of the table T112 at step S 41  as shown by the following table T113. 
   
     
       
         
             
             
             
           
             
                 
               TABLE T113 
             
             
                 
                 
             
             
                 
               PORT NO. 
               FLOODING ENABLE* 
             
             
                 
                 
             
           
          
             
                 
               1 
               1 
             
             
                 
               2 
               1 
             
             
                 
               3 
               1 
             
             
                 
               4 
               0 
             
             
                 
               5 
               1 
             
             
                 
               6 
               0 
             
             
                 
                 
             
             
                 
               AFTER PORT 4 FAILS 
             
             
                 
               *FLOODING ENABLE (0: DISABLE, 1: ENABLE) 
             
          
         
       
     
   
   It is to be noted that in the case of the L2 switch  300 , the setting is performed as shown by the following table T113a. 
   
     
       
         
             
             
             
           
             
                 
               TABLE T113a 
             
             
                 
                 
             
             
                 
               PORT NO. 
               FLOODING ENABLE* 
             
             
                 
                 
             
           
          
             
                 
               1 
               1 
             
             
                 
               2 
               0 
             
             
                 
               3 
               0 
             
             
                 
               4 
               1 
             
             
                 
               5 
               1 
             
             
                 
                 
             
             
                 
               *FLOODING ENABLE (0: DISABLE, 1: ENABLE) 
             
          
         
       
     
   
   At step S 42 , the number of ports is changed to “2” in the entry corresponding to the trunk  20  of the table T106, and the port #P4 is deleted from the port list. 
   At step S 43 , the top port after the updating is obtained, and at step S 44 , the port #P5 is set to flooding “enable” as shown by the above table T113. 
   Thus, the port #P5 operates as a new flooding destination. 
   When the failed port is not coincident with the flooding port at step S 24 , the flooding port remains available. Therefore, the process proceeds to step S 31 , the number of ports is reduced in the entry corresponding to the trunk  20  of the table T106, and the failed port is deleted from the list. 
   Thus, in order to change the flooding port, the change per VLAN is not required different from the conventional technology, but the table T112 has only to be changed to the table T113. Even if the trunk belongs to numerous VLANs, the time required for changing is minimized, and the number of frames discarded while the flooding port is being changed can be extensively reduced. 
   Operational Embodiment for Network Arrangement (3) (Problem 2) 
   As an arrangement of another network to which the present invention is applied, the arrangement of  FIG. 10  already described can be mentioned similarly. The arrangements of the terminals and the VLANs to which the terminals belong are the same as those of  FIG. 10 . The L2 switches  400  and  500  are respectively composed of three line units U1-U3, the ports #P1-#P3 of the line unit U2 and the ports #P1 and #P2 of the line unit U3 in the L2 switch  400  compose the trunk  20 , and the ports #P1-#P3 of the line unit U1 and the ports #P1 and #P2 of the line unit U2 in the L2 switch  500  compose the trunk  30 . The trunks  20  and  30  are connected with the link aggregation-transmission line LA. 
   Firstly, the frame transfer processing by the L2 switches  400  and  500  in the network arrangement (3) is also performed alone the flowchart of  FIG. 1 . This transfer processing is different from that by the L2 switches  200  and  300  in the above-mentioned network arrangement (2) in that the following table T103d is used at step S 5  of  FIG. 1  for the L2 switch  400 . 
   
     
       
         
             
             
             
             
           
             
               TABLE T103d 
             
             
                 
             
             
                 
                 
                 
               PORT/TRUNK 
             
             
                 
               MAC 
               LINE UNIT/ 
               IDENTIFICATION 
             
             
               VLAN-ID 
               ADDRESS 
               PORT/TRUNK NO. 
               (0: PORT, 1: TRUNK) 
             
             
                 
             
           
          
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               501 
               11 
               0101 
               0 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               501 
               14 
                20 
               1 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
                 
             
          
         
       
     
   
   Namely, in the table T103d, the port, line unit or trunk No. is set instead of the port No. in the table T103, and a code (“0”/“1”) of the port/trunk identification is set to be referred to. 
   Also, at step S 7 , the following table T105a is used. 
   
     
       
         
             
             
           
             
                 
               TABLE T105a 
             
           
          
             
                 
                 
             
             
                 
               BITMAP OF LINE UNIT/PORT 
             
          
         
         
             
             
             
             
          
             
               VLAN-ID 
               LINE UNIT U1 
               LINE UNIT U2 
               LINE UNIT U3 
             
             
                 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               501 
               110 
               111 
               11 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               601 
               001 
               111 
               11 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
                 
             
          
         
       
     
   
   Namely, in this table T105a, the bitmap of the line unit or port is set instead of the bitmap of the port in the table T111 to be referred to. As for the line unit U2, for example, it is indicated that ports #P1-#P3 belong to the VLAN 501. 
   Also, as for the L2 switch  500 , the following table T104a is used at step S 3  of  FIG. 1 . 
   
     
       
         
             
             
             
             
           
             
               TABLE T104a 
             
             
                 
             
             
               UNIT 
               VLAN-ID 
               PORT/TRUNK 
                 
             
             
               NO. &amp; 
               ALLOCATED TO 
               IDENTIFICATION 
               TRUNK 
             
             
               PORT NO. 
               RECEIVED FRAME* 
               (0: PORT, 1: TRUNK) 
               NO. 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
               0101 
               0 
               1 
               30 
             
             
               0102 
               0 
               1 
               30 
             
             
               0103 
               0 
               1 
               30 
             
             
               0201 
               0 
               1 
               30 
             
             
               0202 
               0 
               1 
               30 
             
             
               0301 
               501 
               0 
               0 
             
             
               0302 
               601 
               0 
               0 
             
             
                 
             
          
         
       
     
   
   This is a table corresponding to the table T104 concerning the L2 switch  400 . 
   Also, the following table T103e is used at steps S 4  and S 5 . 
   
     
       
         
             
             
             
             
           
             
               TABLE T103e 
             
             
                 
             
             
                 
                 
                 
               PORT/TRUNK 
             
             
                 
               MAC 
               LINE UNIT/ 
               IDENTIFICATION 
             
             
               VLAN-ID 
               ADDRESS 
               PORT/TRUNK NO. 
               (0: PORT, 1: TRUNK) 
             
             
                 
             
           
          
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               501 
               11 
                30 
               1 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               501 
               14 
               0301 
               0 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
                 
             
          
         
       
     
   
   This corresponds to the above-mentioned table T103d. 
   Also, the following table T105b is used at step S 7 . 
   
     
       
         
             
             
           
             
                 
               TABLE T105b 
             
           
          
             
                 
                 
             
             
                 
               BITMAP OF LINE UNIT/PORT 
             
          
         
         
             
             
             
             
          
             
               VLAN-ID 
               LINE UNIT U1 
               LINE UNIT U2 
               LINE UNIT U3 
             
             
                 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               501 
               111 
               11 
               10 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               601 
               111 
               11 
               01 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
               . 
               . 
               . 
               . 
             
             
                 
             
          
         
       
     
   
   This corresponds to the above-mentioned table T105a. 
   Also, the following table T113b is used at step S 8 - 2 . 
   
     
       
         
             
             
             
           
             
                 
               TABLE T113b 
             
             
                 
                 
             
             
                 
               UNIT NO.&amp; PORT NO. 
               FLOODING ENABLE* 
             
             
                 
                 
             
           
          
             
                 
               0101 
               1 
             
             
                 
               0102 
               0 
             
             
                 
               0103 
               0 
             
             
                 
               0201 
               0 
             
             
                 
               0202 
               0 
             
             
                 
               0301 
               1 
             
             
                 
               0302 
               1 
             
             
                 
                 
             
             
                 
               *FLOODING ENABLE (0: DISABLE, 1: ENABLE) 
             
          
         
       
     
   
   The following table T106a is used at step S 13 . 
   
     
       
         
             
             
             
             
           
             
               TABLE T106a 
             
             
                 
             
             
               TRUNK 
               NUMBER 
                 
               FLOODING 
             
             
               NO. 
               OF PORTS 
               PORT LIST 
               PORT 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               30 
               5 
               0101, 0102, 0103, 0201, 
               0101 
             
             
                 
                 
                 
               0202 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
                 
             
          
         
       
     
   
   It is noted that the tables in the L2 switch  400  corresponding to the above-mentioned tables T113b and T106a are omitted in the above and  FIG. 1 . 
   The operation on the side of the L2 switch  400  when a failure has occurred in the line unit U1 of the L2 switch  500  will now be described referring to the flowchart of  FIG. 2 . Also, as the table managing the port of the trunk, the following table T114 is used. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE T114 
             
             
                 
                 
             
             
                 
                 
               NUM- 
                 
                 
             
             
                 
                 
               BER 
                 
               FLOOD- 
             
             
                 
               TRUNK 
               OF 
               PORT 
               ING 
             
             
                 
               NO. 
               PORTS 
               LIST* 
               PORT 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
               BEFORE DETECTING 
               . 
               . 
               . 
               . 
             
             
               FAILURE OF PORT 
               . 
               . 
               . 
               . 
             
             
               #P1 IN LINE UNIT U2 
               . 
               . 
               . 
               . 
             
             
                 
               20 
               5 
               0201, 0202, 
               0201 
             
             
                 
                 
                 
               0203, 0301, 
             
             
                 
                 
                 
               0302 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
             
             
               *UPPER 2 DIGITS = LINE UNIT NO.; LOWER 2 DIGITS = PORT NO. 
             
          
         
       
     
   
   Although this table T114 is similar to the above-mentioned table T106, in the list of the port composing the trunk, each port is managed by the numerical values composed of the upper two digits for a unit No. and the lower two digits for a port No., whereby the flooding port indicating the flooding destination of the trunk can be stored. In this example, the state before the failure occurs in the line unit U1 of the L2 switch  500  is shown by the table T114. 
   The L2 switch  400  monitors the port&#39;s failure at step S 21 . Since detecting the failure per link when the failure occurs at the line unit U1 in the L2 switch  500 , the L2 switch  400  specifically detects the failure at the port #P1 of the line unit U2 (at step S 22 ). 
   At step S 23 , whether or not the port belongs to the trunk is checked. For the check, the entry of the following table T115 can be used. 
   
     
       
         
             
             
             
             
           
             
               TABLE T115 
             
             
                 
             
             
               UNIT 
               VLAN-ID 
               PORT/TRUNK 
                 
             
             
               NO. &amp; 
               ALLOCATED TO 
               IDENTIFICATION 
               TRUNK 
             
             
               PORT NO. 
               RECEIVED FRAME* 
               (0: PORT, 1: TRUNK) 
               NO. 
             
             
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
          
             
               0101 
               501 
               0 
               0 
             
             
               0102 
               501 
               0 
               0 
             
             
               0103 
               601 
               0 
               0 
             
             
               0201 
               0 
               1 
               20 
             
             
               0202 
               0 
               1 
               20 
             
             
               0203 
               0 
               1 
               20 
             
             
               0301 
               0 
               1 
               20 
             
             
               0302 
               0 
               1 
               20 
             
             
                 
             
          
         
       
     
   
   When the port belongs to the trunk, whether or not the port is coincident with the flooding port is checked from the table T114 at step S 24 . 
   When the port #P1 of the line unit U2 fails, the port is coincident with the flooding port, based on the table T114. Therefore, at step S 41 , “disable” is set in the entry corresponding to the port #P1 of the line unit U2 in the following table T116 as shown in the following table T117. 
   
     
       
         
             
             
             
           
             
                 
               TABLE T116 
             
             
                 
                 
             
             
                 
               UNIT NO.&amp;PORT NO. 
               FLOODING ENABLE* 
             
             
                 
                 
             
           
          
             
                 
               0101 
               1 
             
             
                 
               0102 
               1 
             
             
                 
               0103 
               1 
             
             
                 
               0201 
               1 
             
             
                 
               0202 
               0 
             
             
                 
               0203 
               0 
             
             
                 
               0301 
               0 
             
             
                 
               0302 
               0 
             
             
                 
                 
             
             
                 
               *FLOODING ENABLE (0: DISABLE, 1: ENABLE) 
             
          
         
       
     
   
   
     
       
         
             
             
             
           
             
                 
               TABLE T117 
             
             
                 
                 
             
             
                 
               UNIT NO.&amp;PORT NO. 
               FLOODING ENABLE* 
             
             
                 
                 
             
           
          
             
                 
               0101 
               1 
             
             
                 
               0102 
               1 
             
             
                 
               0103 
               1 
             
             
                 
               0201 
               0 
             
             
                 
               0202 
               0 
             
             
                 
               0203 
               0 
             
             
                 
               0301 
               1 
             
             
                 
               0302 
               0 
             
             
                 
                 
             
             
                 
               *FLOODING ENABLE (0: DISABLE, 1: ENABLE) 
             
          
         
       
     
   
   At step S 42 , the number of ports is changed to “4” in the entry corresponding to the trunk  20  in the table T114, and “0201” is deleted from the port list, thereby preparing the following table T118. 
   
     
       
         
             
             
             
             
             
           
             
                 
               TABLE T118 
             
             
                 
                 
             
             
                 
               TRUNK 
               NUMBER 
               PORT 
               FLOODING 
             
             
                 
               NO. 
               OF PORTS 
               LIST* 
               PORT 
             
             
                 
                 
             
           
          
             
                 
             
          
         
         
             
             
             
             
             
          
             
               AFTER 
               . 
               . 
               . 
               . 
             
             
               DETECTING 
               . 
               . 
               . 
               . 
             
             
               FAILURE OF 
               . 
               . 
               . 
               . 
             
             
               PORT #P1 IN 
               20 
               4 
               0202, 0203, 
               0301 
             
             
               LINE UNIT U2 
                 
                 
               0301, 0302 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
               . 
               . 
               . 
               . 
             
             
                 
             
             
               *UPPER 2 DIGITS = LINE UNIT NO.; LOWER 2 DIGITS = PORT NO. 
             
          
         
       
     
   
   As shown by dotted lines in  FIG. 2 , the port different from the line unit of the present flooding port is obtained based on the updated list at step S 50 , whereby the port #P1 of the line unit U3 is stored as a new flooding port as shown in the above-mentioned table T118. 
   As step S 44 , the port #P1 of the line unit U3 is set to flooding “enable” as shown in the above-mentioned table T117. Thus, the frame is outputted from the new flooding destination. 
   Since the failure has occurred in the line unit U1 in the L2 switch  500  in this example, the L2 switch  400  subsequently detects a failure at the port #P2 of the line unit U2 at steps S 21  and S 22  of the flowchart of  FIG. 2 . Since the failed port is not coincident with the flooding port at this time, it can be used as the flooding port. At this time, the process proceeds from step S 24  to step S 31 , and the number of ports has only to be reduced and the failed port has only to be deleted from the port list in the entry corresponding to the trunk  20  of the table T114. Similarly, if a failure is detected at the port #P3 of the line unit U2 at step S 21 , the process has only to proceed to step S 31 , and no change of the flooding port is required. 
   Thus, by using a logic of selecting the port which becomes the next flooding port so as to shift across the line unit when the flooding port is changed, change of the flooding port can be completed only once upon an occurrence of a failure per line unit, so that the switching time can be minimized. 
   It is to be noted that the entry of each table of the L2 switch can be generally provided by a command from a supervisory control terminal, a software operation, or the like which operates on a CPU mounted on the switch.