Abstract:
A packet forwarding apparatus has link aggregation capability and prevents the occurrence of flooding when forwarding a packet to a link of a trunk. The packet forwarding apparatus includes sections for learning the source address and a receiving path of the packet by learning information for identifying a port through which to receive the packet, a trunk of a link connected to the port, and a change in the receiving path of a packet received by the trunk, packet forwarding processing unit for, when a receiving path of packets having the same source address is changed in the trunk, selecting one link from among a plurality of links of the trunk in order to forward a packet having a destination address that is the same as the source address and forwarding the packet, and a learning requesting unit for requesting the performance of learning for the packet inside the packet forwarding apparatus.

Description:
BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a method and apparatus for forwarding a packet and, in particular, to a method and apparatus for forwarding a packet and having a link aggregation capability that can increase a communication bandwidth by combining a plurality of links into one virtual logical link and prohibiting a packet received by a link having the link aggregation attribute from being forwarded to all of the links connected to the apparatus. 
   2. Description of the Related Art 
   A packet forwarding apparatus for forwarding a packet in accordance with a destination address of the packet has a capability of learning a forwarding path. The packet forwarding apparatus learns receiving positional information corresponding to a source address of the received packet. In general, the packet forwarding apparatus learns a number that identifies a port (i.e., a port number) as the receiving positional information. That is, the packet forwarding apparatus registers the latest receiving positional information for the source address of the received packet in a learning table. The packet forwarding apparatus determines the forwarding address on the basis of a destination address of the received packet using the learning table. If the packet forwarding apparatus searches the learning table and cannot find the corresponding information in the learning table, the packet forwarding apparatus forwards the packet to all of ports thereof, that is, all the ports other than the port from which the packet was received. This operation is referred to as “flooding”. 
   The packet forwarding apparatus learns by receiving a packet generated by flooding and resisters the learned information in the learning table. However, in general, only some of the information items in the learning table registered by flooding are used. Most of the information items in the learning table are not used, and therefore, the resource of the learning table is wasted. Accordingly, the packet forwarding apparatus deletes the information items in the learning table that are not relearned in a predetermined time period. This operation is referred to as “aging”. 
   For a packet forwarding apparatus having the link aggregation capability, even when one of the links packed using the link aggregation capability malfunctions, the packet forwarding apparatus can continue the operation thereof by using the other normal links. A link packed into one by the link aggregation function is referred to as a “trunk”. When the packet forwarding apparatus receives a packet via a trunk, the learning information about the receiving position of the packet is not a port number, but a number that identifies the trunk (i.e., a trunk number). Accordingly, the packet forwarding apparatus generates a link aggregation table identifying ports packed into the trunk. That is, the link aggregation table including a correspondence between a port number and a trunk number is generated. Subsequently, if the receiving positional information corresponding to the destination address in the received packet indicates a trunk on the basis of the learning result, the packet forwarding apparatus selects one port from among the ports of the link aggregation on the basis of a predetermined computation and forwards the packet to that port. The port selected from among a plurality of links, i.e., a plurality of ports of the link aggregation for forwarding depends on the predetermined computation of the packet forwarding apparatus. Accordingly, when a terminal station communicates a packet with a server via a target packet forwarding apparatus having a link aggregation attribute, the forwarding path from the terminal station to the server may be different from the forwarding path from the server to the terminal station. 
   In a stackable packet forwarding apparatus in which line units are stackable to allow installation of additional ports, a learning table is provided for each line unit. 
   When the stackable packet forwarding apparatus forwards a packet via a link included in a trunk and the link used for transmitting a packet to the target apparatus is the same as the link used for receiving a packet from the target apparatus, the occurrence of flooding can be prevented. However, when the links for receiving and transmitting a packet are different and the link aggregation is set for a plurality of line units, a line unit of the packet forwarding apparatus that has received a packet destined for a server from a terminal station may not have received a packet destined for the terminal station from the server (i.e., another line unit has received that packet). In such a case, since a learning table is provided for each line unit and the information learned by each line unit is not relearned, aging occurs. Therefore, when the packet forwarding apparatus forwards the packet via a link of the trunk again, the flooding occurs. 
   When forwarding a packet via a link of a trunk and the link used for transmitting a packet to the target apparatus is the same as the link for receiving a packet from the target apparatus, the occurrence of flooding can be prevented. However, if the same link is used in the trunk as the selected link for receiving and transmitting and the link malfunctions, the packet forwarding apparatus cannot continue the communication by using another normal link. 
   To prevent the occurrence of flooding, Japanese Unexamined Patent Application Publication No. 2005-86668, for example, describes a technique in which information in learning tables of all of line units including links of link aggregation are set to be the same. More specifically, a learning packet is sent to the line units at predetermined timings. However, sending the learning packet disadvantageously reduces a use communication bandwidth of the packet forwarding apparatus. Furthermore, the configuration of the packet forwarding apparatus is complicated in order to generate the learning packet. 
   SUMMARY OF THE INVENTION 
   As noted above, in a packet forwarding apparatus having a link aggregation capability, when a packet is forwarded between two users (e.g., a terminal station and a server) via a link of a trunk, a link in a direction from the terminal station to the server may be different from a link in a direction from the server to the terminal station. In addition, different learning tables may be used for the links. For example, a line unit containing a link in a direction from the terminal station to the server may be different from a line unit containing a link in a direction from the server to the terminal station. In addition, these line units may manage different learning tables. In such a case, some of learning data items obtained by flooding is subject to aging since these data items are not relearned. Accordingly, when a packet is forwarded using the link of the trunk again, the flooding occurs. 
   Accordingly, it is an object of the present invention to provide a packet forwarding apparatus having a link aggregation capability and a packet forwarding method thereof capable of preventing the occurrence of flooding when forwarding a packet. 
   According to a first aspect of the present invention, a packet forwarding apparatus for forwarding a packet in accordance with a destination address using a trunk is provided in which the trunk combines a plurality of links connected to the packet forwarding apparatus into one link. The packet forwarding apparatus includes learning means for learning the source address and a receiving path of the packet by learning information for identifying a port through which to receive the packet, information for identifying a trunk of a link connected to the port, and information for identifying that the receiving path of a packet received by the trunk has been changed; packet forwarding means for, when a receiving path of packets having the same source address is changed in the trunk, selecting one link from among a plurality of links of the trunk in order to forward a packet having a destination address that is the same as the source address and forwarding the packet via the selected link; and learning requesting means for, when a receiving path of packets having the same source address is changed in the trunk, requesting the performance of learning for the packet having a destination address that is the same as the source address inside the packet forwarding apparatus. 
   In general, in packet forwarding apparatuses having a link aggregation capability, a link used for forwarding a packet may be changed to another link in the trunk. In this case, flooding may occur. According to the first aspect of the present invention, the packet forwarding apparatus can request performance of learning inside the packet forwarding apparatus so as to prevent the occurrence of flooding. 
   The packet forwarding apparatus can employ an intra-device frame header containing the information for identifying the trunk and information for requesting the performance of learning for the packet inside the packet forwarding apparatus and wherein the intra-device frame header is transferred together with the received packet transferred inside the packet forwarding apparatus. 
   In general, in packet forwarding apparatuses having a link aggregation capability, a link used for forwarding a packet may be changed to another link in the trunk. In this case, flooding may occur. According to the first aspect of the present invention, the packet forwarding apparatus can request performance of learning to another line unit in the packet forwarding apparatus so as to prevent the occurrence of flooding. 
   The packet forwarding apparatus can further include learning means for learning information for indicating whether the receiving path of packets having the same source address has been changed in the trunk a predetermined number of times or more, packet forwarding means for, when the receiving path has been changed the predetermined number of times or more, selecting one link from among a plurality of links of the trunk and forwarding the packet via the selected link, and learning requesting means for, when the receiving path has been changed the predetermined number of times or more, requesting the performance of learning for the packet inside the packet forwarding apparatus. 
   In general, in packet forwarding apparatuses having a link aggregation capability, a link used for forwarding a packet may be changed to another link in the trunk a predetermined number of times or more. In this case, flooding may occur. According to the first aspect of the present invention, the packet forwarding apparatus can request performance of learning inside the packet forwarding apparatus so as to prevent the occurrence of flooding. 
   According to a second aspect of the present invention, a packet forwarding apparatus for forwarding a packet in accordance with a destination address using a trunk is provided in which the trunk combines a plurality of links connected to the packet forwarding apparatus into one link. The packet forwarding apparatus includes at least one line unit including a plurality of ports, a learning table, a packet forwarding processing unit, and a learning request unit; a setting control unit for controlling the at least one line unit to set a plurality of links forming the trunk; and an intra-device interface unit for connecting the line unit with the setting control unit. Learning results of information for identifying the port through which to receive the packet, information for identifying a trunk of a link connected to the port, and information for identifying that the receiving path of a packet received by the trunk has been changed are registered in the learning table. When the receiving path of packets having the same source address is changed in the trunk, the packet forwarding processing unit selects one link from among a plurality of links of the trunk in order to forward a packet having a destination address that is the same as the source address and forwards the packet via the selected link, and, when a receiving path of packets having the same source address is changed in the trunk, the learning requesting unit requests the performance of learning for the packet having a destination address that is the same as the source address inside the packet forwarding apparatus. 
   In general, in packet forwarding apparatuses having a link aggregation capability and including expandable line units, a link used for forwarding a packet may be changed to another link in the trunk. In this case, flooding may occur. According to the second aspect of the present invention, the packet forwarding apparatus can request performance of learning to a learning table of each of the relevant line units so as to prevent the occurrence of flooding. 
   According to a third aspect of the present invention, a method for forwarding a packet in accordance with a destination address using a trunk is provided in which the trunk combines a plurality of links connected to a packet forwarding apparatus into one link. The method includes the steps of learning the source address and a receiving path of the packet by learning information for identifying a port through which to receive the packet, information for identifying a trunk of a link connected to the port, and information for identifying that the receiving path of a packet received by the trunk has been changed; when a receiving path of packets having the same source address is changed in the trunk, selecting one link from among a plurality of links of the trunk in order to forward a packet having a destination address that is the same as the source address and forwarding the packet via the selected link; and, when a receiving path of packets having the same source address is changed in the trunk, requesting the performance of learning for the packet having a destination address that is the same as the source address inside the packet forwarding apparatus. 
   In general, in packet forwarding methods having a link aggregation capability, a link used for forwarding a packet may be changed to another link in the trunk. In this case, flooding may occur. According to the third aspect of the present invention, the packet forwarding method can request performance of learning to the packet forwarding apparatus so as to prevent the occurrence of flooding. 
   According to the present invention, in a packet forwarding apparatus having a link aggregation capability and expandable line units each including a learning table, when a link used for forwarding a packet is changed to another link in a trunk, the packet forwarding apparatus can request the performance of learning to the relevant line units so as to prevent the occurrence of flooding. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a diagram illustrating an L 2  switch; 
       FIG. 2  illustrates an exemplary structure of the L 2  switch; 
       FIG. 3  illustrates an exemplary configuration of a line unit; 
       FIG. 4  illustrates a MAC frame format; 
       FIG. 5  illustrates a first exemplary structure of an intra-device frame header; 
       FIG. 6  illustrates a first exemplary structure of a learning table; 
       FIG. 7  illustrates an exemplary structure of a trunk number table; 
       FIG. 8  illustrates an exemplary structure of a flooding setting table; 
       FIG. 9  illustrates an exemplary structure of a port state table; 
       FIG. 10  illustrates an exemplary structure of a trunk member table; 
       FIG. 11  is a flow chart illustrating an exemplary procedure of a process of receiving a frame performed by the L 2  switch; 
       FIG. 12  is a flow chart illustrating an exemplary procedure of a process of transmitting a frame performed by the L 2  switch; 
       FIG. 13  is a flow chart illustrating an exemplary procedure of a learning process performed by the L 2  switch; 
       FIG. 14  illustrates a second exemplary structure of the intra-device frame header; and 
       FIG. 15  illustrates a second exemplary structure of the learning table. 
   

   DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Embodiments of the present invention are described in detail below with reference to the accompanying drawings. Note that, in the drawings, the same components or similar components are designated using the same reference numerals. Additionally, the following embodiments are described with reference to a case where a packet forwarding apparatus according to the present invention is applied to a layer- 2  switch (an L 2  switch) of a stackable type. However, the packet forwarding apparatus according to the present invention is not limited to an L 2  switch of a stackable type. 
   First Embodiment 
     FIG. 1  is a diagram illustrating an L 2  switch  1 . A terminal  2  is connected to a port p 1  of a line unit  13  of the L 2  switch  1 . A terminal  3  is connected to a port p 2  of a line unit  11  of the L 2  switch  1 . A server  4  is connected to a port p 4  of the line unit  11  and a port p 4  of a line unit  12  of the L 2  switch  1 . These two ports p 4  are included in a trunk having a trunk number  50 . That is, the port p 4  of the line unit  11  and the port p 4  of the line unit  12  are part of the same link aggregation. 
     FIG. 2  illustrates an exemplary structure of the L 2  switch  1 . The L 2  switch  1  includes a setting control unit  1 C, line units  11 ,  12 , and  13 , and a back wiring board (back board)  1 B. The term “line units  10 ” collectively refers to the line units  11 ,  12 , and  13 . The back board  1 B connects the line units  10  to the setting control unit  1 C. The setting control unit  1 C includes a central processing unit (CPU) section  1001 , a memory section  1002 , and a maintenance interface section  1003 . The setting control unit  1 C sets maintenance data in the line units  10 , monitors status data of the line units  10 , and controls the state transition of the line units  10  using a signal via the back board  1 B. The CPU section  1001  sets data for maintenance received from an external apparatus (not shown) in the line units  10 . In addition, the CPU section  1001  controls processing of data for monitoring the states of the line units  10 . The memory section  1002  stores control software used by the CPU section  1001 , data to be set in the line units  10 , and monitored data of the line units  10 . The maintenance interface section  1003  includes an interface with an external apparatus for maintenance so as to transmit and receive a variety of setting data and monitored data. The CPU section  1001 , the memory section  1002 , and the maintenance interface section  1003  are connected to each other in the setting control unit  1 C. The setting control unit  1 C is connected to the line units  10  via the back board  1 B. Each of the line units  10  transmits and receives a forwarding packet to and from the other line units  10  via the back board  1 B. The line units  10  are expandable in accordance with the number of links and the transfer speeds of the links contained in the L 2  switch  1 . 
     FIG. 3  illustrates an exemplary configuration of the line unit  10 . The line unit  10  includes the four Ethernet (trade name) ports p 1 , p 2 , p 3 , and p 4 . The line unit  10  further includes an input monitor section  111 , an input frame forwarding section  112 , an intra-device signal transmitting section  113 , an intra-device signal receiving section  114 , and an output frame forwarding section  115 . The input monitor section  111  monitors the validity of a received frame. The input frame forwarding section  112  determines the forwarding destination of the frame and adds an intra-device frame header, which is information transferred between the line units  10 , to the frame. The intra-device signal transmitting section  113  transmits the frame having the intra-device frame header to another line unit. The intra-device signal receiving section  114  receives a frame having the intra-device frame header and determines whether the frame is destined for its own line unit. The output frame forwarding section  115  performs a learning operation on the received frame and forwards the frame to the corresponding port. The line unit  10  further includes a learning table  201 , a trunk number table  204 , a flooding setting table  205 , a port state table  202 , and a trunk member table  203 , which are illustrated in  FIGS. 6 to 10 , respectively. The line unit  10  is described in detail below after descriptions referring to  FIGS. 6 to 10  are completed. 
     FIG. 4  illustrates a MAC frame format. The MAC frame (hereinafter simply referred to as a “frame”) includes a destination MAC address (hereinafter simply referred to as a “destination address”) for specifying the destination, a source MAC address (hereinafter simply referred to as a “source address”) for specifying the source, a payload which is a field containing transmission data, and a frame check sequence (FCS) used for detecting a frame transmission error. 
   In the present embodiment, the address of the terminal  2  is referred to as an “0A”, the address of the terminal  3  is referred to as an “0B”, and the address of the server  4  is referred to as an “0E”. 
     FIG. 5  illustrates a first exemplary structure of the intra-device frame header. The intra-device frame header includes a destination unit bitmap used as a unit identifier of the destination, a trunk learning request unit bitmap used as a unit identifier of a target of trunk learning, a destination port number used as an output port identifier, a receiving unit number used as a receiving unit identifier, a receiving port number used as a receiving port identifier, and a receiving trunk number used as a receiving trunk identifier. The destination unit bitmap and the trunk learning request unit bitmap are configured in accordance with the line unit  10  included in the L 2  switch  1 . That is, the destination unit bitmap indicates, using the bit position, a destination line unit of the frame to which the intra-device frame header is attached. The trunk learning request unit bitmap indicates, using the bit position, a line unit to which a learning operation is requested in terms of the frame to which the intra-device frame header is attached. For the two bitmaps, by setting a bit corresponding to the target line unit  10  to a logical level “1”, the desired line unit is selected. The destination port number indicates a destination port of the frame. When flooding is performed for all of the ports, the destination port number is set to “00”. Examples F 51  to F 56  indicate particular information in the intra-device frame header in accordance with a transmission sequence of a frame performed between the terminal  2  and the server  4 . The examples F 51  to F 56  will be described in more detail below. 
     FIG. 6  illustrates a first exemplary structure of the learning table  201 . Each of the line units  10  has the learning table  201 . The learning table  201  includes a source address, a unit number, a port number, a trunk number, and a change flag. That is, for each source address of the received frame, the learning table  201  includes the unit number for identifying the line unit  10  that has received the frame and the port number for identifying a port that has received the frame. If the port is part of an link aggregation, the learning table  201  further includes the trunk number for identifying the link aggregation and the change flag. The change flag is set to a logical level “1” when the number of changes in a port for receiving frames having the same source address exceeds a threshold value. Examples F 61  to F 64  indicate the information in the learning table  201  in accordance with a transmission sequence of a frame performed between the terminal  2  and the server  4 . 
     FIG. 7  illustrates an exemplary structure of the trunk number table  204 . Each of the line units  10  has the trunk number table  204 . The trunk number table  204  is used for the input monitor section  111  to determine whether the port that has received a frame is part of a link aggregation. If the port that has received a frame is part of a link aggregation, the trunk number table  204  is used for referencing a trunk number for identifying the link aggregation. More specifically, the trunk number table  204  indicates, for each of the ports of the line unit  10 , the information about whether the port is part of a link aggregation. If the port is part of a link aggregation, the trunk number table  204  further includes the trunk number of the link aggregation. In the present embodiment shown in  FIG. 1 , since the port p 4  of the line unit  11  and the port p 4  of the line unit  12  are part of the same link aggregation having a trunk number “50”, the trunk number “50” is set for the port p 4  of the line unit  11  and the port p 4  of the line unit  12  whereas a value “0” for indicating that the port is not part of the link aggregation is set for the other ports. 
     FIG. 8  illustrates an exemplary structure of the flooding setting table  205 . Each of the line units  10  has the flooding setting table  205 . The flooding setting table  205  is used for the output frame forwarding section  115  to determine a port to be subject to flooding. The ports other than the ports that are part of a link aggregation are set to a logical level “1”. A port having a logical level “1” is a port used for forwarding a frame when flooding is performed. Additionally, the ports that are part of the link aggregation are set to a logical level “0”. The ports having a logical level “0” are not used for forwarding a frame when flooding is performed. By referencing this table and determining which ports are used for flooding, the frame to be flooded is forwarded from only one port. In this way, the destination apparatus does not receive multiple frames to be flooded. In the present embodiment, the port p 4  of the line unit  11  and the port p 4  of the line unit  12  are part of a link aggregation having the same trunk number. As shown in  FIG. 8 , a frame to be flooded is transmitted to the port p 4  of the line unit  11 , but not to the port p 4  of the line unit  12 . 
     FIG. 9  illustrates an exemplary structure of the port state table  202 . Each of the line units  10  has the port state table  202 . The port state table  202  is used for the input frame forwarding section  112  to determine whether a port to which a frame is forwarded is part of a link aggregation. If the port to which a frame is forwarded is part of a link aggregation, the port state table  202  is used for determining whether a link connected to the port to which a frame is forwarded is available. The port state table  202  includes a unit number, a port number, and a port state. In the port state table  202 , a port that is not part of a link aggregation is set to “1111”, a port that is part of a link aggregation and has a normal port state is set to “0000”, and a port that is part of a link aggregation and has an abnormal port state is set to “0001”. In the present embodiment, the port p 4  of the line unit  11  (U 1 ) and the port p 4  of the line unit  12  (U 2 ) are part of the same link aggregation and the both ports have a normal port state. 
     FIG. 10  illustrates an exemplary structure of the trunk member table  203 . Each of the line units  10  has the trunk member table  203 . When the port for the destination of a frame is part of a link aggregation, the input frame forwarding section  112  references the trunk member table  203  to identify the ports of the link aggregation. The trunk member table  203  includes, for a trunk number serving as an identifier of the link aggregation, the number of ports in the link aggregation and a pair consisting of the positional information (i.e., the unit number) about a port in the link aggregation and the port number. In the present embodiment, the number of ports is two for the trunk having the trunk number “50”. The port p 4  of the line unit  11  (U 1   p   4 ) and the port p 4  of the line unit  12  (U 2   p   4 ) form the link aggregation. 
   The line unit  10  shown in  FIG. 3  is described in detail next. 
   A frame received by, for example, the port p 1  is terminated by the input monitor section  111 . The input monitor section  111  monitors whether the frame is normal. If the received frame is not normal, the input monitor section  111  discards the frame. If the received frame is normal, the input monitor section  111  performs the following processing. 
   The input monitor section  111  references the trunk number table  204  shown in  FIG. 7 , that is, a value in the table corresponding to the target port. If the value is “0”, the target port is not part of a link aggregation. Therefore, the input monitor section  111  sets a value “00” in the receiving trunk number field in the intra-device frame header shown in  FIG. 5 . However, if the value in the trunk number table  204  is a value other than “0”, the target port is part of a link aggregation. Since the number in the trunk number table  204  represents the trunk number, the input monitor section  111  sets that trunk number in the receiving trunk number field in the intra-device frame header. 
   The input monitor section  111  sets a unit number that identifies its own line unit, a port number that identifies a port that has received the frame, and the above-described trunk number in the receiving unit number, a receiving port number, and receiving trunk number in the intra-device frame header, respectively. Subsequently, the input monitor section  111  attaches this intra-device frame header to the received frame. The input monitor section  111  then transmits the frame to the input frame forwarding section  112 . 
   Upon receiving the frame having the intra-device frame header, the input frame forwarding section  112  searches for the source address in the learning table  201  shown in  FIG. 6  on the basis of a media access control (MAC) address (a destination address) so as to obtain the corresponding unit number, port number, trunk number, and change flag. Thereafter, in the port state table  202  shown in  FIG. 9 , the input frame forwarding section  112  references the port state of a port indicated by the unit number and the port number in the added intra-device frame header. Subsequently, in the trunk member table  203  shown in  FIG. 10 , the input frame forwarding section  112  references the port number that is part of the trunk indicated by the trunk number referenced in the learning table  201 . 
   The input frame forwarding section  112  sets the unit number and the port number referenced in the learning table  201  to the destination unit bitmap and the destination port number in the intra-device frame header attached to the received frame, respectively. Thereafter, the input frame forwarding section  112  transmits the frame to the intra-device signal transmitting section  113 . 
   The intra-device signal transmitting section  113  receives the frame having the attached intra-device frame header from the input frame forwarding section  112  and transmits the frame to the back board  1 B. 
   The intra-device signal receiving section  114  receives the frame having the attached intra-device frame header from the back board  1 B. The intra-device signal receiving section  114  then references the destination unit bitmap in the received intra-device frame header. If the bit for the line unit number corresponding to the its own line unit has a logical level “1”, the intra-device signal receiving section  114  transfers the frame to the output frame forwarding section  115 . However, if this bit has a logical level “0”, the intra-device signal receiving section  114  discards the frame. 
   The output frame forwarding section  115  searches the learning table  201  using the source address contained in the transferred frame so as to start learning. As used herein, the term “learning” refers to memorizing a correspondence between the source address contained in the frame and a port that has received the frame. The output frame forwarding section  115  searches the learning table  201  using the destination address in the transferred frame as a search key. As a result of the search operation, if flooding is required, the output frame forwarding section  115  transfers the frame to the corresponding ports on the basis of the flooding setting table  205 . 
   Note that the port state table  202 , the trunk member table  203 , the trunk number table  204 , and the flooding setting table  205  are controlled by the setting control unit  1 C described with reference to  FIG. 2 . The setting control unit  1 C sets information in these tables. 
     FIG. 11  is a flow chart illustrating the procedure of a reception process of a frame performed by the L 2  switch  1 . The process of receiving a frame via a port performed by the L 2  switch  1 , the processing of the frame performed by the input monitor section  111  and the input frame forwarding section  112 , and the process of transferring the frame having an attached intra-device frame header to the back board  1 B via the intra-device signal transmitting section  113  are illustrated in  FIG. 11 . 
   At step S 10 , it is determined whether the received frame is normal. If the received frame is abnormal, the process proceeds to step S 101 . However, if the received frame is normal, the process proceeds to step S 1 . 
   At step S 101 , the received frame is discarded. 
   At step S 11 , an intra-device frame header is generated in which the unit number of a unit having a port that has received the frame and the port number of the port are set in the receiving unit number field and the receiving port number field, respectively. To determine whether the port is part of a link aggregation, the input monitor section  111  references the trunk number table  204  of its own line unit. If the port is part of a link aggregation, that is, if the corresponding port in the trunk number table  204  contains a trunk number other than a value “0”, the trunk number of the port is set in the receiving trunk number field. However, if the port is not part of a link aggregation, that is, if the corresponding port in the trunk number table  204  contains a trunk number “0”, the value of, for example, “00” is set in the receiving trunk number field as an identifier. In this way, the intra-device frame header is generated. 
   At step S 12 , the input frame forwarding section  112  references the destination address in the received frame to determine whether a unicast data transfer in which data is transferred to one destination address or a multicast/broadcast data transfer in which data is transferred to a plurality of destination addresses is required. 
   At step S 13 , the input frame forwarding section  112  searches the learning table  201  using the destination address in the received frame as a search key. 
   If, at step S 12 , it is determined that a multicast/broadcast data transfer is required for the received frame or if, at step S 13 , the destination address of the received frame is not found in the learning table  201 , the process proceeds to step S 14 . 
   At step S 14 , the input frame forwarding section  112  sets information indicating that the received frame is subjected to flooding in the intra-device frame header. That is, the input frame forwarding section  112  sets the destination port number in the intra-device frame header to “00”. In addition, the input frame forwarding section  112  sets the bits in the destination unit bitmap corresponding to all of the line units  10  installed in the L 2  switch  1  to “1”s. 
   If, at step S 13 , the destination address in the received frame is found in the learning table  201 , the process proceeds to step S 15 . 
   At step S 15 , the input frame forwarding section  112  references the trunk number registered in the learning table  201  to determine whether the trunk number is learned in the learning table  201 . That is, the input frame forwarding section  112  determines whether the trunk number is registered or a value “00” that indicates no registration is registered. 
   If, at step S 15 , the trunk number is registered in the learning table  201 , that is, if a port used for forwarding the received frame is part of a link aggregation, the process proceeds to step S 16 . 
   At step S 16 , the input frame forwarding section  112  references the change flag registered in the learning table  201  to determine whether the port for the destination address is changed to another one among the ports of the link aggregation as a port for receiving a frame from the same source address, that is, whether the change flag is set to a logical level “1”. 
   If, at step S 16 , it is determined that the change flag of the trunk is set to a logical level “0”, that is, if the port for the destination address is not changed to another one among the ports of the link aggregation, the process proceeds to step S 17 . 
   At step S 17 , the input frame forwarding section  112  references the port state table  202  to determine whether the port for the destination address is a target of the link aggregation. If the port for the destination address is a target of the link aggregation, the input frame forwarding section  112  determines whether the port is available. 
   If, at step S 15 , it is determined that the port for the destination address of the received frame is not part of a link aggregation or if, at step S 17 , the port for the destination address is available, the process proceeds to step S 18 . That is, if the port for the destination address of the received frame is not part of a link aggregation or if the port is part of a link aggregation and the port is available, the process proceeds to step S 18 . 
   At step S 18 , in addition to the intra-device frame header generated at step S 11 , the input frame forwarding section  112  sets the unit number and the port number referenced in the learning table  201  to the destination unit bitmap and the destination port number in the intra-device frame header to generate an intra-device frame header. The input frame forwarding section  112  then attaches the generated intra-device frame header to the frame. The frame is transmitted to the back board  1 B. 
   If, at step S 16 , it is determined that the change flag of the trunk is set to a logical level “1”, or if, at step S 17 , the port for the destination address is unavailable, that is, if the port is part of the link aggregation, but the port is unavailable, the process proceeds to step S 19 . 
   At step S 19 , the input frame forwarding section  112  references the trunk member table  203  and selects a port for the destination address from among the ports of a link aggregation corresponding to the trunk number referenced at step S 15 . Subsequently, the input frame forwarding section  112  generates an intra-device frame header in which the unit number and the port number of the port are set in the destination, unit bitmap and the destination port number, respectively. 
   At step S 20 , the input frame forwarding section  112  references the trunk member table  203  to find all of the line units  10  having the ports of the link aggregation corresponding to the trunk. Thereafter, the input frame forwarding section  112  generates an intra-device frame header in which the corresponding bits in the trunk learning request unit bitmap are set to a logical level “1”. The input frame forwarding section  112  then adds the destination unit bitmap and the destination port number generated at step S 19  and the trunk learning request unit bitmap generated at this step (S 20 ) to the intra-device frame header generated at step S 11 . The input frame forwarding section  112  attaches this intra-device frame header to the frame and the intra-device signal transmitting section  113  transmits the frame having the intra-device frame header to the back board  1 B. 
     FIG. 12  is a flow chart illustrating the procedure of transmitting the frame performed by the L 2  switch  1 . The process of receiving the frame having the intra-device frame header from the back board  1 B performed by the intra-device signal receiving section  114  of the L 2  switch  1  and the process of the frame performed by the output frame forwarding section  115 , and the process of forwarding the frame via a port performed by the output frame forwarding section  115  are illustrated in  FIG. 12 . 
   At step S 30 , the intra-device signal receiving section  114  receives the frame having the intra-device frame header from the back board  1 B. 
   At step S 31 , the output frame forwarding section  115  determines whether the received frame is destined for its own line unit  10  on the basis of the destination unit bitmap in the intra-device frame header of the received frame. Alternatively, the output frame forwarding section  115  determines whether the received frame indicates a learning request to its own line unit  10  on the basis of the trunk learning request unit bitmap in the intra-device frame header of the received frame. 
   If, at step S 31 , it is determined that the received frame is not destined to its own line unit  10 , the process proceeds to step S 32 . 
   At step S 32 , the output frame forwarding section  115  discards the received frame having the intra-device frame header. 
   If, at step S 31 , it is determined that the received frame is destined for its own line unit  10 , the process proceeds to step S 33 . 
   At step S 33 , a learning process is performed. The learning process is described in detail below with reference to  FIG. 13 . 
   If, at step S 31 , it is determined, on the basis of the destination unit bitmap in the intra-device frame header of the received frame, that the received frame is destined for its own line unit  10  and the learning process is performed at step S 33 , the process proceeds to step S 34 . 
   At step S 34 , the output frame forwarding section  115  determines whether the destination port number in the intra-device frame header indicates a specific port number, that is, whether the destination port number is “00” or a port number corresponding to a specific port. 
   If, at step S 34 , it is determined that the destination port number in the intra-device frame header indicates a specific port number, the process proceeds to step S 35 . 
   At step S 35 , the output frame forwarding section  115  forwards the frame to only a port of the destination port number. 
   If, at step S 34 , it is determined that the destination port number in the intra-device frame header is “00”, that is, if it is determined that a flooding transmission is instructed, the process proceeds to step S 36 . 
   At step S 36 , the output frame forwarding section  115  forwards the frame to all of the ports having the corresponding bits in the flooding setting table  205  set to a logical level “1”. 
   Note that the intra-device frame header attached to the frame is removed and discarded at step S 35  or S 36 . 
     FIG. 13  is a flow chart illustrating the procedure of the learning process performed by the L 2  switch  1 . The learning process contained in the frame transmitting process shown in  FIG. 12  is illustrated in  FIG. 13 . 
   At step S 331 , the L 2  switch  1  searches the learning table  201  using the source address of the frame as a search key. 
   If, at step S 331 , a learning process for the source address of the frame has not been performed, that is, if data for the source address of the frame is not found in the learning table  201 , the process proceeds to step S 332 . 
   At step S 332 , the L 2  switch  1  registers the receiving unit number, the receiving port number, and the receiving trunk number contained in the intra-device frame header in the unit number, the port number, and the trunk number of the learning table  201 , respectively. 
   If, at step S 331 , a learning process for the source address of the frame has been performed, the process proceeds to step S 334 . 
   At step S 334 , the L 2  switch  1  reads out the learning data on the basis of the source address to determine whether the trunk number registered in the learning table  201  is equal to the receiving trunk number contained in the intra-device frame header. 
   If, at step S 334 , it is determined that the trunk number registered in the learning table  201  is different from the receiving trunk number contained in the intra-device frame header or if it is determined that the trunk number is “00” which indicates that the trunk number is not registered, the process proceeds to step S 335 . 
   At step S 335 , the L 2  switch  1  updates the data (i.e., the unit number, the port number, and the trunk number) registered in the learning table  201  to the receiving unit number, the receiving port number, and the receiving trunk number contained in the intra-device frame header, respectively, on the basis of the source address. That is, the L 2  switch  1  performs relearning. 
   If, at step S 334 , it is determined that the trunk number registered in the learning table  201  is equal to the receiving trunk number contained in the intra-device frame header, the process proceeds to step S 336 . 
   At step S 336 , it is determined whether the unit number and the port number registered in the learning table  201  are equal to the receiving unit number and the receiving port number contained in the intra-device frame header, respectively, on the basis of the source address. 
   If, at step S 336 , it is determined that the unit number and the port number registered in the learning table  201  are equal to the receiving unit number and the receiving port number contained in the intra-device frame header, respectively, the process proceeds to step S 337 . 
   At step S 337 , the L 2  switch  1  overwrites the receiving unit number, the receiving port number, and the receiving trunk number contained in the intra-device frame header to the data (i.e., the unit number, the port number, and the trunk number) registered in the learning table  201 , respectively, on the basis of the source address. That is, the L 2  switch  1  performs relearning. 
   If, at step S 336 , it is determined that either the unit number registered in the learning table  201  is different from the receiving unit number or the port number registered in the learning table  201  is different from the receiving port number contained in the intra-device frame header, the process proceeds to step S 338 . 
   At step S 338 , the L 2  switch  1  updates the data (i.e., the unit number, the port number, and the trunk number) registered in the learning table  201  to the receiving unit number, the receiving port number, and the receiving trunk number contained in the intra-device frame header, respectively, on the basis of the source address. 
   Since it is determined at step S 336  that either the unit number registered in the learning table  201  is different from the receiving unit number contained in the intra-device frame header or the port number registered in the learning table  201  is different from the receiving port number contained in the intra-device frame header, the port that receives a frame from the same source address has been changed to another port in the same trunk. Therefore, at step S 339 , the L 2  switch  1  increments a monitoring counter (not shown), which is provided for each trunk number, by one. If the count value in the monitoring counter exceeds a predetermined value, the L 2  switch  1  sets the corresponding change flag in the learning table  201  to a logical level “1” on the basis of the source address. 
   That is, the L 2  switch  1  performs relearning at steps S 338  and S 339 . 
   The frame forwarding operation between the terminal  2  and the server  4  is described next with reference to  FIGS. 11 to 13 . 
   Description is made in accordance with the following operations: 
   (1) Operation of receiving a frame having a broadcast destination address from the terminal  2  performed by the L 2  switch  1 ; 
   (2) Operation of transmitting the frame received in operation (1) performed by the L 2  switch  1 ; 
   (3) Operation of receiving a frame having a unicast destination address from the server  4  to the terminal  2  performed by the L 2  switch  1 ; 
   (4) Operation of transmitting the frame received in operation (3) to the terminal  2  performed by the L 2  switch  1 ; 
   (5) Operation of receiving a frame having a unicast destination address from the terminal  2  to the server  4  performed by the L 2  switch  1 ; 
   (6) Operation of transmitting the frame received in operation (5) to the server  4  performed by the L 2  switch  1 ; 
   (7) Operation of re-receiving a frame having a unicast destination address from the server  4  to the terminal  2  using a link different from the link used in operation (3) performed by the L 2  switch  1 ; 
   (8) Operation of transmitting the frame received in operation (7) to the terminal  2  performed by the L 2  switch  1  (in this operation, an operation (Example 1) in the case where data about the source address is registered in operations (1) and (2) and an operation (Example 2) in the case where data about the source address is subject to aging are described); 
   (9) Operation of re-receiving a frame having a unicast destination address from the terminal  2  to the server  4  performed by the L 2  switch  1 ; and 
   (10) Operation of the L 2  switch  1  when transmitting the frame received in operation (9) to the server  4 . 
   Operation (1) 
   An operation of receiving a normal frame having a broadcast destination address from the terminal  2  via the port p 1  of the line unit  13  performed by the L 2  switch  1  is described with reference to  FIG. 11 . 
   At step S 10 , the input monitor section  111  of the line unit  13  determines whether a received frame is normal. 
   At step S 11 , the input monitor section  111  references the trunk number table  204  of its own line unit (i.e., the line unit  13 ) to determine whether the port (p 1 ) that has received the normal frame is part of a link aggregation. Since the port p 1  is not part of a link aggregation, the input monitor section  111  generates an intra-device frame header in which the receiving unit number, the receiving port number, and the receiving trunk number are set to “U 3 ”, “p 1 ”, and “00”, respectively. 
   This intra-device frame header is attached to the received frame. Subsequently, the frame is transferred to the input frame forwarding section  112 . 
   At step S 12 , the input frame forwarding section  112  references the destination address of the frame. In this description, since the destination address is a broadcast address, the process proceeds to step S 14 . 
   At step S 14 , the input frame forwarding section  112  sets information indicating that the frame is to be subject to flooding in the intra-device frame header. That is, the input frame forwarding section  112  sets a value “00” that indicates the necessity of flooding in the destination port number in the intra-device frame header. In addition, the input frame forwarding section  112  sets bits in the destination unit bitmap corresponding to all the line units  10  installed in the L 2  switch  1  to a logical level “1”. Additionally, since no units to be set in the trunk learning request unit bitmap are present, that is, since the trunk learning request unit bitmap in the intra-device frame header to be transmitted from the port to the back board is not used, all the bits of the trunk learning request unit bitmap are set to a logical level “0”. In  FIG. 5 , a symbol “−” indicates the logical level “0”. In addition, the L 2  switch  1  has an installation table (not shown) including information about the installed line units  10 . The input frame forwarding section  112  references the installation table. As noted above, the input frame forwarding section  112  updates the intra-device frame header generated by the input monitor section  111  and attached to the frame. Subsequently, the input frame forwarding section  112  transfers the intra-device frame header to the back board  1 B via the intra-device signal transmitting section  113 . For example, the input frame forwarding section  112  appends the frame to the intra-device frame header and transfers the frame. 
   The frame having the intra-device frame header and transferred to the back board  1 B is transmitted to all the installed line units  10 . The intra-device frame header in such a case is shown by the example F 51  in  FIG. 5 . 
   Operation (2) 
   An operation of transmitting the frame having a broadcast destination address and received from the terminal  2  in operation (1) performed by the L 2  switch  1  is described with reference to  FIGS. 12 and 13 . 
   In this description, since the frame is a broadcast frame, all the installed line units  10  perform the following operation. 
   Note that it is assumed that, in the learning tables  201  of the corresponding line units  10 , a learning operation is not performed using the source address of the terminal  2 . 
   At step S 30 , the intra-device signal receiving section  114  of the line unit  10  receives the frame having the intra-device frame header (shown by the example F 51  in  FIG. 5 ). 
   At step S 31 , the intra-device signal receiving section  114  determines whether the received frame is destined for its own line unit on the basis of the destination unit bitmap in the intra-device frame header. Note that the intra-device signal receiving section  114  recognizes the unit number of its own line unit on the basis of the installation table (not shown) in advance. In this description, since the bits of the destination unit bitmap in the intra-device frame header corresponding to all the installed line units  10  are set to a logical level “1”, the process proceeds to step S 33 . 
   At step S 33 , the output frame forwarding section  115  registers the source address of the frame and the receiving unit number and the receiving port number contained in the intra-device frame header in the learning table  201 . This operation is referred to as “learning”. 
   The process at step S 33  is described in detail next with reference to  FIG. 13 . 
   At step S 331 , the output frame forwarding section  115  searches the learning table  201  on the basis of the source address of the frame. 
   If, at step S 331 , it is determined that learning for the source address of the frame has not been performed, the process proceeds to step S 332 . At step S 332 , the output frame forwarding section  115  registers a source address “0A” of the frame, and a receiving unit number “U 3 ”, a receiving port number “p 1 ”, and a receiving trunk number “00” contained in the intra-device frame header in the source address, the unit number, the port number, and the trunk number in the learning table  201 , respectively. The information in the learning table  201  is shown by the example F 61  in  FIG. 6 . The information in this learning table  201  is registered in the learning tables  201  of all the installed line units  10 . 
   Referring back to  FIG. 12 , the following description is made. 
   At step S 34 , the output frame forwarding section  115  references the destination port number in the intra-device frame header to determine whether the destination port number is “00” which indicates the necessity of flooding or a specific port number other than a value “00”. In this description, the destination port number has been set to “00” (i.e., the necessity of flooding) at step S 14  shown in  FIG. 11 . 
   At step S 36 , the output frame forwarding section  115  references the flooding setting table  205  and transmits the frame to the ports having the value “1” in the flooding setting table  205 . In this description, the frame is forwarded from all the ports except for the port p 4  of the line unit  12 . The frame destined for the server  4  is forwarded from the port p 4  of the line unit  11 . 
   Operation (3) 
   The operation of receiving a frame having a unicast destination address from the server  4  to the terminal  2  performed by the L 2  switch  1  is described next with reference to  FIG. 11 . In this description, the source address of the frame is “0E” and the destination address of the frame is “0A”. 
   The server  4  is configured to have two network interface cards (NICs) providing link redundancy of two physical ports. The server  4  determines which one of the two NICs is used for transmitting a frame to the terminal  2 . In this description, the server  4  transmits a frame to the port p 4  of the line unit  12 . 
   At step S 10 , the input monitor section  111  of the line unit  12  determines whether the received frame is normal. 
   At step S 11 , the input monitor section  111  references the trunk number table  204  of its own line unit (i.e., the line unit  12 ) to determine whether the port (p 4 ) that has received the normal frame is part of a link aggregation. Since the port p 4  is part of a link aggregation having a trunk number “50”, the input monitor section  111  generates an intra-device frame header in which the receiving unit number, the receiving port number, and the receiving trunk number are set to “U 2 ”, “p 4 ”, and “50”, respectively. 
   At step S 12 , the input frame forwarding section  112  references the destination address of the frame. In this description, since the destination address is “0A” which is a unicast address, the process proceeds to step S 13 . 
   At step S 13 , the input frame forwarding section  112  searches the learning table  201  on the basis of the destination address “0A” of the frame. In this description, data relating to the destination address “0A” of the frame has been registered in the learning tables  201  of all the installed line units  10  in step S 332  of Operation (1). Accordingly, the process proceeds to step S 15 . Note that the found data when the input frame forwarding section  112  searches the learning table  201  is shown by the example F 61  in  FIG. 6 . 
   At step S 15 , the input frame forwarding section  112  references the trunk number registered in the learning data to determine whether the trunk number has a specific trunk number. That is, if the trunk number is “00”, the learning data is not registered in the learning table. However, if the trunk number is a specific value other than “00”, the learning data is registered. In this description, the information registered in the learning table  201  is indicated by the example F 61  in  FIG. 6  and a specific trunk number is not registered. Therefore, the process proceeds to step S 18 . 
   At step S 18 , the input frame forwarding section  112  attaches this frame to the intra-device frame header generated on the basis of the data (F 61  in  FIG. 6 ) found at step S 13 . Subsequently, the input frame forwarding section  112  transfers the frame to the back board  1 B via the intra-device signal transmitting section  113 . In this description, the intra-device frame header is generated on the basis of the unit number “U 3 ” and the port number “p 1 ” in the learning data found for the source address “0A” at step S 13 . As shown by the example F 52  in  FIG. 5 , the bit for the line unit  13  in the destination unit bitmap is set to a logical level “1”, the destination port number is set to “p 1 ”, the receiving unit number is set to “U 2 ”, the receiving port number is set to “p 4 ”, and the receiving trunk number is set to “50”. 
   Operation (4) 
   The operation performed when the L 2  switch  1  transmits the unicast frame destined for the terminal  2  received from the server  4  in operation (3) to the terminal  2  is described next with reference to  FIGS. 12 and 13 . 
   Here, it is assumed that, in the learning table  201  of the corresponding the line unit  13 , a learning operation has not been performed using the source address of the server  4 . 
   At step S 30 , the intra-device signal receiving section  114  of the line unit  13  receives the frame having the intra-device frame header (shown by the example F 52  in  FIG. 5 ) generated at step S 18  of Operation (3). 
   At step S 31 , the intra-device signal receiving section  114  determines whether the received frame is destined for its own line unit on the basis of the destination unit bitmap in the intra-device frame header. In this description, since the destination line unit is the line unit  13 , this frame is discarded in the line units  11  and  12  by the process at step S 32 . 
   In this description, the line unit  13  performs the following processes. 
   At step S 33 , the output frame forwarding section  115  registers the source address of the frame, and the receiving unit number, the receiving port number, and receiving trunk number contained in the intra-device frame header in the learning table  201 . 
   The process at step S 33  is described in detail next with reference to  FIG. 13 . 
   At step S 331 , the output frame forwarding section  115  of the line unit  13  searches the learning table  201  on the basis of the source address of the received frame. 
   If, at step S 331 , it is determined that learning for the source address of the frame has not been performed, the process proceeds to step S 332 . At step S 332 , the output frame forwarding section  115  registers a source address “0E” of the frame, a receiving unit number “U 2 ”, a receiving port number “p 4 ”, and a receiving trunk number “50” contained in the intra-device frame header in the source address, the unit number, the port number, and the trunk number in the learning table  201 , respectively. The information in the learning table  201  is shown by the example F 62  in  FIG. 6 . The learning process is performed only by the line unit  13 . 
   Referring back to  FIG. 12 , the following description is made. 
   At step S 34 , the output frame forwarding section  115  of the line unit  13  references the destination port number in the intra-device frame header to determine whether the destination port number is “00” which indicates the necessity of flooding or a specific port number other than a value “00”. In this description, the destination port number has been set to a specific port number “p 1 ” at step S 18  of Operation (3). 
   At step S 35 , the output frame forwarding section  115  forwards this frame via the port p 1 . As a result, this frame reaches the terminal  2 . When the frame is transmitted from the port, the intra-device frame header attached to the frame is removed. 
   Operation (5) 
   The operation performed when the L 2  switch  1  receives a frame having a unicast destination address destined for the server  4  from the terminal  2  is described next with reference to  FIG. 11 . In this case, the source address of the frame is “0A” and the destination address of the frame is “0E”. 
   Note that, in this stage, the learning table  201  of the line unit  11  contains information indicated by the example F 61  in  FIG. 6 . The learning table  201  of the line unit  12  contains information indicated by the example F 61  in  FIG. 6 . The learning table  201  of the line unit  13  contains information indicated by the examples F 61  and F 62  in  FIG. 6 . 
   At step S 10 , the input monitor section  111  of the line unit  13  determines whether the received frame is normal. 
   At step S 11 , the input monitor section  111  references the trunk number table  204  of its own line unit (i.e., the line unit  13 ) to determine whether the port (p 1 ) that has received the normal frame is part of a link aggregation. Since the port p 1  is not part of a link aggregation, the input monitor section  111  generates an intra-device frame header in which the receiving unit number, the receiving port number, and the receiving trunk number are set to “U 3 ”, “p 1 ”, and “00”, respectively. 
   This intra-device frame header is attached to the received frame. Subsequently, the frame is transferred to the input frame forwarding section  112 . 
   At step S 12 , the input frame forwarding section  112  references the destination address of the frame. In this description, since the destination address is “0E” which is a unicast address, the process proceeds to step S 13 . 
   At step S 13 , the input frame forwarding section  112  searches the learning table  201  on the basis of the destination address “0E” of the frame. In this description, data relating to the destination address “0E” of the frame has been registered in the learning table  201  of the line unit  13  in step S 332  of Operation (4). The registered data is data shown by the examples F 61  and F 62  in  FIG. 6 . 
   At step S 15 , the input frame forwarding section  112  references the registered learning data (i.e., data indicated by the example F 62 ) to determine whether the trunk number has been registered. That is, if the trunk number is “00”, the learning data is not registered in the learning table. However, if the trunk number is a specific value other than “00”, the learning data is registered. In this description, since the information registered in the learning table  201  is indicated by the example F 62  in  FIG. 6 , the learning data is registered in the learning table of its own line unit, and therefore, a specific trunk number is registered. 
   At step S 16 , the input frame forwarding section  112  references the registered learning data to determine whether the change flag indicates a change in the receiving port of the link aggregation, that is, whether the change flag is set to a logical level “1”. In this description, the registered data in the learning table  201  is data indicated by the example F 62  in  FIG. 6 . Since the change flag is set to a logical level “0”, the registered data indicates that the receiving port of the link aggregation has not been changed. 
   At step S 17 , the input frame forwarding section  112  references the port state table  202  to determine the state of the port (the unit number U 1  and the port number p 4 ) which is searched for at step S 13 . As noted above, the port state table  202  indicates whether a port is a target of a link aggregation and, if the port is a target of a link aggregation, whether the port is available. In this description, since the port p 4  is part of a link aggregation and the state of the port p 4  is normal, the process proceeds to step S 18 . 
   In this description, at step S 18 , the input frame forwarding section  112  determines that a link from the server  4  to the L 2  switch  1  is the link to the port p 4  of the line unit  12  on the basis of the data found at step S 13  (i.e., data indicated by the example F 62  in  FIG. 6 ). Subsequently, to improve the reliability of the system using the L 2  switch  1 , the input frame forwarding section  112  determines that it forwards the frame to the server  4  using a different link in the same trunk. Thus, the input frame forwarding section  112  references the trunk member table  203  shown in  FIG. 10  and selects a different port “U 1   p   4 ” (the port p 4  of the line unit  11 ). The input frame forwarding section  112  then generates an intra-device frame header on the basis of that port and attaches the frame to the generated intra-device frame header. Thereafter, the input frame forwarding section  112  transfers the frame to the back board  1 B via the intra-device signal transmitting section  113 . In this description, as shown by the example F 53  in  FIG. 5 , the bit of the destination unit bitmap corresponding to the line unit  11  is set to a logical level “1”, the destination port number is set to “p 4 ”, the receiving unit number is set to “U 3 ”, the receiving port number is set to “p 1 ”, and the receiving trunk number is set to “00”. 
   Operation (6) 
   The operation performed when the L 2  switch  1  transmits the frame having a unicast destination address and received in Operation (5) to the server  4  is described next with reference to  FIGS. 12 and 13 . 
   At step S 30 , the intra-device signal receiving section  114  of the line unit  11  receives the frame having the intra-device frame header generated at step S 18  of Operation (5). 
   At step S 31 , the intra-device signal receiving section  114  determines whether the received frame is destined for its own line unit on the basis of the destination unit bitmap in the intra-device frame header. In this description, since the destination line unit is the line unit  11 , this frame is discarded in the line units  12  and  13  by the process at step S 32 . 
   In this description, the line unit  11  performs the following processes. 
   At step S 33 , the output frame forwarding section  115  registers the source address of the frame, the receiving unit number, the receiving port number, and receiving trunk number contained in the intra-device frame header in the learning table  201 . 
   The process at step S 33  is described in detail next with reference to  FIG. 13 . 
   At step S 331 , the output frame forwarding section  115  of the line unit  11  searches the learning table  201  on the basis of the source address of the received frame. In this description, since the source address of the received frame is “0A” and the registered data indicated by the sample F 61  in  FIG. 6  is present in the learning table  201 , the data that was being searching for is found. 
   At step S 334 , the output frame forwarding section  115  determines whether the trunk number registered in the learning table  201  (the trunk number “00” in the example F 61  of  FIG. 6 ) is equal to the trunk number contained in the intra-device frame header received at step S 30  (the receiving trunk number “00” in the example F 53  of  FIG. 5 ). In this description, since the trunk number in the learning table  201  is “00” (not registered) and a link aggregation is not formed, the process proceeds to step S 335 . 
   At step S 335 , the output frame forwarding section  115  overwrites the unit number, the port number, and the trunk number in the learning table  201  corresponding to the source address. Accordingly, relearning is performs so that the registered data in the learning table  201  corresponding to the source address “0A” remains unchanged (i.e., data indicated by the sample F 61  in  FIG. 6 ). 
   Referring back to  FIG. 12 , the following description is made. 
   At step S 34 , the output frame forwarding section  115  of the line unit  11  references the destination port number in the intra-device frame header to determine whether the destination port number is “00” which indicates the necessity of flooding or a specific port number other than a value “00”. In this description, the destination port number has been set to “p 4 ” (i.e., a specific port) at step S 18  of Operation (5). 
   The output frame forwarding section  115  forwards this frame to the server  4  via the port p 4 . 
   Operation (7) 
   The operation performed when the L 2  switch  1  re-receives a frame having a unicast destination address from the server  4  to the terminal  2  is described with reference to  FIG. 11 . In this description, the source address of the frame is “0E” and the destination address of the frame is “0A”. 
   The server  4  is configured to have two network interface cards (NICs) providing link redundancy of two physical ports. The server  4  determines which one of the two NICs is used for transmitting a frame to the terminal  2 . Unlike the previous example (described with reference to Operation (3)), in this description, the server  4  transmits a frame to the port p 4  of the line unit  11 . 
   At step S 10 , the input monitor section  111  of the line unit  11  determines whether the received frame is normal. 
   At step S 11 , the input monitor section  111  references the trunk number table  204  of its own line unit (i.e., the line unit  11 ) to determine whether the port (p 4 ) that has received the normal frame is part of a link aggregation. Since the port p 4  is part of a link aggregation having a trunk number “50”, the input monitor section  111  generates an intra-device frame header in which the receiving unit number, the receiving port number, and the receiving trunk number are set to “U 1 ”, “p 4 ”, and “50”, respectively. 
   At step S 12 , the input frame forwarding section  112  references the destination address of the frame. In this description, since the destination address is “0A” which is a unicast address, the process proceeds to step S 13 . 
   At step S 13 , the input frame forwarding section  112  searches the learning table  201  on the basis of the destination address “0A” of the frame. 
   In this description, the subsequent steps are described with reference to the following two examples (first and second examples). 
   In the first example, data relating to the destination address “0A” of this frame has been registered in the learning table  201  of the line unit  11  by the process in step S 332  of Operation (2). 
   In the second example, data relating to the destination address “0A” of this frame has been subjected to aging in the learning table  201  of the line unit  11 . 
   FIRST EXAMPLE 
   If, at step S 13 , it is determined that a search target is registered as a result of the search operation of the learning table  201 , the process proceeds to step S 15 . At step S 15 , the input frame forwarding section  112  references a trunk number in the corresponding entry of the learning table  201  to determine whether the trunk number is registered. In this description, since the registered data in the learning table  201  is data indicated by the sample F 61  in  FIG. 6  and the trunk number is “00”, the trunk number is not registered. 
   At step S 18 , the input frame forwarding section  112  attaches this frame to the intra-device frame header generated on the basis of the data (see the example F 61  in  FIG. 6 ) found at step S 13 . Subsequently, the input frame forwarding section  112  transfers the intra-device frame header to the back board  1 B via the intra-device signal transmitting section  113 . In the intra-device frame header of this description, as shown by the example F 54  in  FIG. 5 , the bit corresponding to the line unit  13  in the destination unit bitmap is set to a logical level “1”, the destination port number is set to “p 1 ”, the receiving unit number is set to “U 1 ”, the receiving port number is set to “p 4 ”, and the receiving trunk number is set to “50”. 
   SECOND EXAMPLE 
   If, at step S 13 , it is determined that data relating to the destination address “0A” is not registered in the learning table  201 , the process proceeds to step S 14 . At step S 14 , the input frame forwarding section  112  sets information indicating that the frame is to be subject to flooding in the intra-device frame header. That is, the input frame forwarding section  112  sets the destination port number in the intra-device frame header to “00”. In addition, the input frame forwarding section  112  sets the bits corresponding to all the line units  10  installed in the L 2  switch  1  in the destination unit bitmap to a logical level “1”. Additionally, since no units to be set in the trunk learning request unit bitmap are present, that is, since the trunk learning request unit bitmap in the intra-device frame header to be transmitted from the port to the back board is not used, all the bits of the trunk learning request unit bitmap are set to a logical level “0”. In  FIG. 5 , a symbol “−” indicates the logical level “0”. In addition, the L 2  switch  1  has an installation table (not shown) including information about the installed line units  10 . The input frame forwarding section  112  references the installation table. As noted above, the input frame forwarding section  112  updates the intra-device frame header generated by the input monitor section  111  and attached to the frame. Subsequently, the input frame forwarding section  112  transfers the frame having the intra-device frame header to the back board  1 B via the intra-device signal transmitting section  113 . 
   The frame having the intra-device frame header and transferred to the back board  1 B is transmitted to all the installed line units  10 . The intra-device frame header in such a case is shown by the example F 55  in  FIG. 5 . The bits of the destination unit bitmap corresponding to the line units  11 ,  12 , and  13  are set to a logical level “1”, the destination port number is set to “00”, the receiving unit number is set to “U 1 ”, the receiving port number is set to “p 4 ”, and the receiving trunk number is set to “50”. 
   Operation (8) 
   The operation performed when the L 2  switch  1  transmits the unicast frame received from the server  4  in operation (7) to the terminal  2  is described next with reference to  FIGS. 12 and 13 . 
   The following description is made with reference to the cases corresponding to the first and second examples in Operation (7). 
   FIRST EXAMPLE 
   In this example, the source address of the frame is “0E”. The destination address of the frame is a unicast address of “0A”. The intra-device frame header is indicated by the example F 54  in  FIG. 5 . 
   At step S 30 , the intra-device signal receiving section  114  of the line unit  13  receives the frame having the intra-device frame header processed at step S 18  of Operation (7). 
   At step S 31 , the intra-device signal receiving section  114  determines whether the received frame is destined for its own line unit on the basis of the destination unit bitmap in the intra-device frame header. In this description, since the destination line unit is the line unit  13 , this frame is discarded in the line units  11  and  12  by the process at step S 32 . 
   In this description, the line unit  13  performs the following processes. 
   At step S 33 , the output frame forwarding section  115  registers the source address of the frame, the receiving unit number, the receiving port number, and receiving trunk number contained in the intra-device frame header in the learning table  201 . 
   The process at step S 33  is described in detail next with reference to  FIG. 13 . 
   At step S 331 , the output frame forwarding section  115  of the line unit  13  searches the learning table  201  on the basis of the source address of the received frame. In this description, the source address is “0E”. The learning table  201  contains information indicated by the examples F 61  and F 62  shown in  FIG. 6 . 
   At step S 334 , the output frame forwarding section  115  determines whether the trunk number registered in the learning table  201  is equal to the trunk number contained in the intra-device frame header received at step S 30 . If, at step S 334 , it is determined that these trunk numbers are not equal, the corresponding link is part of a different link aggregation. Accordingly, at step S 335 , the output frame forwarding section  115  updates the unit number, the port number, and the trunk number in the learning table  201  corresponding to the source address to the receiving unit number, the receiving port number, and the receiving trunk number contained in the intra-device frame header, respectively. In this description, since the trunk number in the learning table (the example F 62  in  FIG. 6 ) has the same value “50” as the trunk number in the intra-device frame header (the example F 54  in  FIG. 5 ), the process proceeds to step S 336 . 
   At step S 336 , the output frame forwarding section  115  determines whether the unit number “U 2 ” and the port number “p 4 ” (“F 62 ” in  FIG. 6 ) registered in the learning table  201  are equal to the receiving unit number “U 1 ” and the receiving port number “p 4 ” (the example F 54  in  FIG. 5 ) contained in the intra-device frame header, respectively. In this description, the unit number in the example “F 62 ” of  FIG. 6  is different from the unit number in the example “F 54 ”, of  FIG. 5 . 
   If, at step S 336 , it is determined that either the unit number registered in the learning table  201  is different from the receiving unit number contained in the intra-device frame header or the port number registered in the learning table  201  is different from the receiving port number contained in the intra-device frame header or if it is determined that both the unit number registered in the learning table  201  is different from the receiving unit number contained in the intra-device frame header and the port number registered in the learning table  201  is different from the receiving port number contained in the intra-device frame header, the process proceeds to step S 338 . That is, if frames having the same source address are received by different ports having the same trunk number regardless of whether the ports belong to different units or the same unit, the process proceeds to step S 338 . 
   At step S 338 , the output frame forwarding section  115  updates the unit number “U 2 ”, the port number “p 4 ”, and the trunk number “50” registered in the learning table  201  corresponding to the source address (see the example F 62  in  FIG. 6 ) to the receiving unit number “U 1 ”, the receiving port number “p 4 ”, and the receiving trunk number “50” contained in the intra-device frame header (see the example F 54  in  FIG. 5 ), respectively. Accordingly, relearning is performed so that entry data corresponding to the source address “0E” in the learning table  201  are updated to the data indicated by the example F 63  shown in  FIG. 6 . 
   Since the port that receives a frame from the same source address has been changed to another port in the same trunk at step S 338 , the output frame forwarding section  115 , at step S 339 , increments a monitoring counter (not shown) for counting the number of changes for each trunk by one. If the count value in the monitoring counter exceeds a predetermined threshold value, the output frame forwarding section  115  sets the change flag in the corresponding entry of the learning table  201  to a logical level “1”. In this description, it is assumed that the count value exceeds the predetermined value, and therefore, the change flag is set to a logical level “1”. Accordingly, the information registered in the learning table  201  of the line unit  13  is shown by the example F 63  of  FIG. 6 . 
   Referring back to  FIG. 12 , the following description is made. 
   At step S 34 , the output frame forwarding section  115  references the destination port number in the intra-device frame header to determine whether the destination port number is “00” which indicates the necessity of flooding or a specific port number other than a value “00”. In this description, since the destination port number has been set to “p 1 ” in step S 18  of Operation (7), the process proceeds to step S 35 . 
   At step S 35 , the output frame forwarding section  115  forwards this frame to the terminal  2  via the port p 1 . 
   SECOND EXAMPLE 
   In this example, the source address of the frame is “0E”. The frame has an intra-device frame header including information indicating the necessity of flooding. The information in the intra-device frame header is indicated by the example F 55  in  FIG. 5 . 
   At step S 30 , the intra-device signal receiving section  114  of the line unit  10  receives the frame having the intra-device frame header. 
   At step S 31 , the intra-device signal receiving section  114  determines whether the received frame is destined for its own line unit on the basis of the destination unit bitmap in the intra-device frame header. Note that the intra-device signal receiving section  114  recognizes the line unit number of its own line unit on the basis of the above-described installation table (not shown). 
   If, at step S 31 , it is determined that the received frame is transmitted to its own line unit, the process proceeds to step S 33 . In this description, all of the installed line units  10  perform the following processing. 
   The output frame forwarding section  115  registers the source address of the frame and the receiving unit number and the receiving port number contained in the intra-device frame header in the learning table  201 . 
   The process at step S 33  is described in detail next with reference to  FIG. 13 . 
   At step S 331 , the output frame forwarding section  115  of each of the installed line units  10  searches the learning table  201  on the basis of the source address of the received frame. 
   If, in this description, it is determined that the learning data is deleted by aging, the process proceeds to step S 332 . 
   At step S 332 , the output frame forwarding section  115  registers a source address “0E” of the frame, a receiving unit number “U 1 ”, a receiving port number “p 4 ”, and a receiving trunk number “50” contained in the intra-device frame header in the learning table  201 . The information in the learning table  201  is shown by the example F 64  in  FIG. 6 . This information is registered in the learning tables  201  of all the installed line units  10 . 
   Referring back to  FIG. 12 , the following description is made. 
   At step S 34 , the output frame forwarding section  115  references the destination port number in the intra-device frame header to determine whether the destination port number is “00” which indicates the necessity of flooding or a specific port number other than a value “00”. In this description, the destination port number has been set to “00” (i.e., flooding) in step S 14  of Operation (7). 
   At step S 36 , the output frame forwarding section  115  references the flooding setting table  205  and transmits the frame to the ports having the value “1” in the flooding setting table  205 . In this description, the frame is forwarded from all the ports except for the port p 4  of the line unit  12 . 
   Operation (9) 
   The operation performed when the L 2  switch  1  re-receives a frame having a unicast destination address of the server  4  from the terminal  2  is described next with reference to  FIG. 11 . In this case, the source address of the frame is “0A” and the destination address of the frame is “0E”. 
   Note that, in this description, it is assumed that the first example of the operation (8) or (9) has been performed. In addition, it is assumed that, in operation (7), a link from the server  4  to the terminal  2  is changed from the port p 4  of the line unit  12  to the port p 4  of the line unit  11  in the L 2  switch  1 , and aging is carried out on the learning data indicated by the example F 62  in  FIG. 6  in the learning table  201  of the line unit  12 . 
   That is, in this stage, the learning table  201  of the line unit  11  contains information indicated by an example F 61  in  FIG. 6 . The learning table  201  of the line unit  12  contains no data due to the aging. The learning table  201  of the line unit  13  contains information indicated by the example F 63  shown in  FIG. 6 . 
   At step S 10 , the input monitor section  111  of the line unit  13  determines whether the received frame is normal. 
   At step S 11 , the input monitor section  111  references the trunk number table  204  of its own line unit (i.e., the line unit  13 ) to determine whether the port (p 1 ) that has received the normal frame is part of a link aggregation. Since the port p 1  is not part of a link aggregation, the input monitor section  111  generates an intra-device frame header in which the receiving unit number, the receiving port number, and the receiving trunk number are set to “U 3 ”, “p 1 ”, and “00”, respectively. 
   This intra-device frame header is attached to the received frame. Subsequently, the frame is transferred to the input frame forwarding section  112 . 
   At step S 12 , the input frame forwarding section  112  references the destination address of the frame. In this case, since the destination address is “0E” which is a unicast address, the process proceeds to step S 13 . 
   At step S 13 , the input frame forwarding section  112  searches the learning table  201  on the basis of the destination address “0E” of the frame. In this description, data relating to the destination address “0E” of the frame has been registered in the learning table  201  of the line unit  13  in step S 338  of the first example of Operation (8). The registered data is data shown by the example F 63  in  FIG. 6 . 
   At step S 15 , the input frame forwarding section  112  references the registered learning data to determine whether the trunk number has been registered. In this description, the information registered in the learning table  201  is indicated by the example F 63  in  FIG. 6 . That is, a trunk number “50” is registered. 
   At step S 16 , the input frame forwarding section  112  references the registered learning data to determine whether the change flag is set to a logical level “1”. In this description, the registered data in the learning table  201  is data indicated by the example F 63  in  FIG. 6 . That is, the change flag is set to a logical level “1”. 
   At step S 19 , the input frame forwarding section  112  references the trunk member table  203  that indicates ports of the trunk, and selects one port from among the ports of the trunk on the basis of a predetermined computation. Subsequently, the input frame forwarding section  112  sets the destination unit bitmap and the destination port number in the intra-device frame header. 
   At step S 20 , the input frame forwarding section  112  references the trunk member table  203  to set the trunk learning request bit map in the intra-device frame header. That is, the input frame forwarding section  112  sets the bits in the trunk learning request unit bitmap corresponding to all the line units  10  forming the corresponding trunk to a logical level “1”. Note that the trunk learning request unit bitmap is used for requesting the trunk learning. In this description, the bits corresponding to the line units  11  and  12  are set to a logical level “1”. That is, the information indicated by the example F 56  shown in  FIG. 5  is set in the intra-device frame header. 
   Operation (10) 
   The operation performed when the L 2  switch  1  transmits the frame having a unicast address and re-received from the terminal  2  in operation (9) is described next with reference to  FIGS. 12 and 13 . 
   At step S 30 , the intra-device signal receiving section  114  of the line unit  10  receives the frame having the intra-device frame header processed at step S 20  of Operation (9). 
   At step S 31 , the intra-device signal receiving section  114  determines whether the received frame is destined for its own line unit  10  on the basis of the destination unit bitmap or the trunk learning request bitmap in the intra-device frame header. In this description, since the destination line unit is the line unit  11  and the trunk learning request units are the line units  11  and  12 , this frame is discarded in the line unit  13  by the process at step S 32 . 
   In this description, the line units  11  and  12  perform the following processes. 
   At step S 33 , the output frame forwarding section  115  of each of the line units  11  and  12  registers the source address of the frame, the receiving unit number, the receiving port number, and receiving trunk number contained in the intra-device frame header in the learning table  201 . 
   The process at step S 33  is described in detail next with reference to  FIG. 13 . 
   The learning process of the line unit  11 , which is a destination unit, is as follows. 
   At step S 331 , the output frame forwarding section  115  of the line unit  11  searches the learning table  201  on the basis of the source address of the received frame. In this description, the learning table  201  contains information indicated by the example F 61  shown in  FIG. 6 . 
   At step S 334 , the output frame forwarding section  115  determines whether the trunk number registered in the learning table  201  is equal to the trunk number contained in the intra-device frame header received at step S 30 . In this description, since the trunk number in the learning table is “00”, that is, since a trunk number is not set, a trunk number is not registered. 
   At step S 335 , the output frame forwarding section  115  updates the unit number, the port number, and the trunk number in the learning table  201  corresponding to the source address to the receiving unit number, the receiving port number, and the receiving trunk number contained in the intra-device frame header, respectively. In this description, relearning is performed so that data corresponding to the source address “0A” in the learning table  201  remain unchanged to be data indicated by the example F 61  shown in  FIG. 6 . 
   Referring back to  FIG. 12 , the following description is made. 
   At step S 34 , the output frame forwarding section  115  references the destination port number in the intra-device frame header to determine whether the destination port number is “00” which indicates the necessity of flooding or a specific port number other than a value “00”. In this description, since the destination port number has been set to a specific port “p 4 ” in step S 19  of Operation (9). 
   At step S 35 , the output frame forwarding section  115  forwards this frame to the server  4  via the port p 4 . 
   The learning process in the line unit  12  specified in the trunk learning request unit is as follows. 
   At step S 331 , the output frame forwarding section  115  of the line unit  12  searches the learning table  201  on the basis of the source address of the received frame. In this description, when, as described in Operation (2), the frame having a broadcast destination address is transmitted from the terminal  2 , the learning process is performed at step S 332 . However, this learning data has been deleted by aging. 
   At step S 332 , the output frame forwarding section  115  registers, in the learning table  201 , a source address “0A” of the frame, a receiving unit number “U 3 ”, and a receiving port number “p 1 ” contained in the intra-device frame header. In this case, the learning data is the same as that shown by the example F 61  in  FIG. 6 . 
   As noted above, aging is carried out on data in the learning table  201  that is not updated and relearned. In general, if the data registered in the learning table  201  is not relearned within five minutes, aging is carried out on that data. Accordingly, aging is carried out on the learning data registered by the broadcast operation performed in Operation (2). For example, as noted in the description of searching the learning table at step S 13  of Operation (7), aging is carried out on the learning data of the source address “0A” in the learning table  201  of the line unit  11  or  12  unless relearning is performed on the learning data. Therefore, when the line unit  11  or  12  in which aging has been carried out receives a frame destined for the terminal  2  from the server  4  and searches the learning table  201 , the corresponding data is not found, resulting in the occurrence of flooding. 
   In the present embodiment, if the port that receives a frame is changed to another port in the trunk, the L 2  switch  1  sends a request for performing a learning process to all the line units  10  containing the ports of the trunk. Upon receiving the request for learning, the line units  10  perform a learning process. Therefore, frame forwarding without flooding caused by aging can be realized. 
   In addition, among ports of a link aggregation, a port used for transmitting a frame may be changed to another one due to malfunction of the port. Accordingly, it is desirable that the threshold value for setting the change flag in Operation (8) is more than or equal to two. The threshold value can be set by the setting control unit  1 C in accordance with the setting information provided by a maintenance personnel. 
   Second Embodiment 
     FIG. 14  illustrates a second exemplary structure of the intra-device frame header. In the second exemplary structure, a receiving VLAN-ID (virtual local area network-identification) is added to the first intra-device frame header. The receiving VLAN-ID is used for identifying a virtual LAN of a receiving port of a frame identified by a receiving unit number and a receiving port number. 
     FIG. 15  illustrates a second exemplary structure of the learning table  201 . A VLAN-ID is added to the first structure of the learning table  201  shown in  FIG. 6 . The VLAN-ID is used for identifying a virtual LAN to which an apparatus, such as a terminal, having the corresponding source address belongs. 
   By using the second intra-device frame header shown in  FIG. 14  and the second learning table shown in  FIG. 15  in place of the first intra-device frame header shown in  FIG. 5  and the first learning table shown in  FIG. 6 , respectively, an L 2  switch having VLAN capability can forward a frame using ports having the same VLAN-ID as a group. 
   Consequently, like the first embodiment, according to the second embodiment, when a port for receiving a frame is changed to another port in the same trunk, learning is performed by all of the line units  10  that contain the ports of the trunk. As a result, frame forwarding without the occurrence of flooding can be realized.