Patent Publication Number: US-2006007917-A1

Title: Frame transfer method and edge switch

Description:
This application claims the benefit of a Japanese Patent Application No. 2004-200684 filed Jul. 7, 2004, in the Japanese Patent Office, the disclosure of which is hereby incorporated by reference.  
     BACKGROUND OF THE INVENTION  
      1. Field of the Invention  
      The present invention generally relates to frame transfer methods and edge switches, and more particularly to a frame transfer method for transferring frames in a service provider network that connects to a user network in a broadband network in which a plurality of user networks are connected via the service provider network, and to an edge switch for transferring frames using such a frame transfer method.  
      2. Description of the Related Art  
      The extended tag virtual local area network (VLAN) is popularly used as a technique for realizing a broadband network. The extended tag VLAN is simple and corresponding switches can be made at a low cost. However, in a broadband network which uses the extended tag VLAN, a core switch must learn an extremely large number of MAC addresses.  
      Accordingly, when a learning table for the MAC address is exhausted and a rewriting frequently occurs, the performance of the core switch deteriorates. But if the learning table is made large in order to prevent exhaustion, the cost of the core switch becomes high. In addition, the extent to which the learning table is to be enlarged depends on the user network, and there is a problem in that it is difficult to determine a suitable table size.  
      A system called “Ethernet-over-Ethernet” (Ethernet is a registered trademark) has been proposed to overcome the problem described above, and has been applied to broadband network services. According to the Ethernet-over-Ethernet, with respect to a user frame that is input from the user network side, an edge switch adds a VLAN tag, an MAC address allocated to a port thereof, and an MAC address allocated to a port of a destination edge switch, and sends the user frame with the added VLAN tag and MAC addresses to the core network side. The core switch makes a frame transfer based on the VLAN tag and the MAC address of the edge switch. The destination edge switch eliminates the added VLAN tag, the MAC address allocated to the port of the source edge switch and the MAC address allocated to the port of the destination edge switch to restore the original user frame, and sends the user frame to the user network side.  
      The MAC address allocated to the port and added at the edge switch is used for the MAC address learning at the core switch. The number of MAC addresses to be learned is at the maximum the number of ports of the edge switches within the network, and the number of MAC addresses to be learned can be suppressed compared to the extended tag VLAN.  
      A Japanese Laid-Open Patent Application No. 2002-164937 proposes forming a core network of a virtual private network (VPN) by a label switch network, forming an access network with respect to the core network by the VLAN, and providing an apparatus for carrying out an interface function between the label switch network and the VLAN at an edge (or end) of the label switch network.  
      A Japanese Laid-Open Patent Application No. 2002-247083 proposes a VLAN tunneling ring system which includes an input edge node for adding VLAN encapsulation information to the packet even when an output port operates as an untagged port.  
      A Japanese Laid-Open Patent Application No. 2002-344476 proposes encapsulating an Ethernet packet (EP) generated in a first network within another EP, and allocating a source address of the encapsulated EP as an address of a port that receives the EP.  
      According to the conventional systems which use the MAC address allocated to the output port that connects to the user network of the destination edge switch as the destination MAC address, and use the MAC address allocated to the input port that connects to the user network of the source edge switch as the source MAC address, when performing the frame encapsulation, the core switches within the service provider network carry out a transfer process using the MAC address of the port added in the edge switch.  
      When actually carrying out the transfer process between the core switches, it is sufficient if the destination edge switch can be recognized, and the use of the MAC address of the port is not an effective way of utilizing the learning table from the point of view of efficiently utilizing the limited capacity of the learning table. In addition, when carrying out the transfer process within the edge switch, the destination MAC address within the header of the frame (that is, the MAC address allocated to the port of the edge switch) or the destination MAC address within the user frame is used. However, an upper limit of the number of destinations of the frame becomes the number of ports within the edge switch, and there was a problem in that the process of determining the transfer destination within the edge switch using the MAC address as a retrieval (or search) key is inefficient. This is because the number of combinations that can be represented by the MAC address is considerably large compared to the number of ports within the edge switch.  
     SUMMARY OF THE INVENTION  
      Accordingly, it is a general object of the present invention to provide a novel and useful frame transfer method and edge switch, in which the problems described above are suppressed.  
      Another and more specific object of the present invention is to provide a frame transfer method and an edge switch, which can reduce the capacity of a learning table within an edge switch and improve the efficiency of a process that determines a transfer destination within the edge switch.  
      Still another object of the present invention is to provide a frame transfer method for transferring frames in a service provider network that couples to a user network in a broadband network in which a plurality of user networks are coupled via the service provider network, comprising receiving a user frame that is input from a user network by a first edge switch within the service provider network that is coupled to the user network; and adding header information to the user frame in the first edge switch and outputting the frame that is added with the header information from the first edge switch to a core switch within the service provider network, wherein the header information includes a first address that is unique within the service provider network and is allocated to the first edge switch, a second address that is unique within the service provider network and is allocated to a second edge switch coupled to a user network that becomes a destination, a virtual local area network (VLAN) value allocated to a network within the service provider network, a first port identifier identifying a port of the first edge switch that receives the user frame, and a second port identifier identifying a port of the second edge switch. According to the frame transfer method of the present invention, the transfer process in the edge switch can be carried out using the first and second port identifiers within the header information, and the transfer process in the core switch can be carried out using the first and second addresses within the header information. By using the first and second port identifiers, it becomes possible to reduce the capacity of the learning table within the edge switch, to thereby improve the efficiency of the process which determines the transfer destination within the edge switch.  
      A further object of the present invention is to provide an edge switch for transferring frames in a service provider network that couples to a user network in a broadband network in which a plurality of user networks are coupled via the service provider network, comprising a port configured to receive a user frame that is input from a user network, within the service provider network that is coupled to the user network; and a header information adding part configured to add header information to the user frame and outputting the frame that is added with the header information to a core switch within the service provider network, wherein the header information includes a first address that is unique within the service provider network and is allocated to the edge switch, a second address that is unique within the service provider network and is allocated to another edge switch coupled to a user network that becomes a destination, a virtual local area network (VLAN) value allocated to a network within the service provider network, a first port identifier identifying the port that receives the user frame, and a second port identifier identifying a port of the other edge switch. According to the edge switch of the present invention, the transfer process in the edge switch can be carried out using the first and second port identifiers within the header information, and the transfer process in the core switch can be carried out using the first and second addresses within the header information. By using the first and second port identifiers, it becomes possible to reduce the capacity of the learning table within the edge switch, to thereby improve the efficiency of the process which determines the transfer destination within the edge switch.  
      Another object of the present invention is to provide an edge switch for transferring frames in a service provider network that couples to a user network in a broadband network in which a plurality of user networks are coupled via the service provider network, comprising a first port configured to receive a frame that is added with header information and is input from the service provider network; a second port configured to receive a frame that is input from a user network that is coupled to the service provider network; and a learning part configured to learn the header information, wherein the header information includes an address that is unique within the service provider network and is allocated to another edge switch within the service provider network that is coupled to the user network, a virtual local area network (VLAN) value allocated to a network within the service provider network, a first port identifier identifying the second port that receives the frame, a sending source address within a user frame in the frame that is input from the service provider network, a VLAN value within a VLAN tag that is allocated within the user network, and a second port identifier identifying the first port of the edge switch that receives the frame from the service provider network. According to the edge switch of the present invention, the transfer process in the edge switch can be carried out using the first and second port identifiers within the header information, and the transfer process in the core switch can be carried out using the first and second addresses within the header information. By using the first and second port identifiers, it becomes possible to reduce the capacity of the learning table within the edge switch, to thereby improve the efficiency of the process which determines the transfer destination within the edge switch.  
      Still another object of the present invention is to provide an edge switch for transferring frames in a service provider network that couples to a user network in a broadband network in which a plurality of user networks are coupled via the service provider network, comprising at least one port configured to receive a user frame that is input from a user network, within the service provider network that is coupled to the user network; at least one port configured to receive a frame that is added with header information and is input from the service provider network; a header information adding part configured to add header information to the user frame and outputting the frame that is added with the header information to a core switch within the service provider network; and a learning part configured to learn the header information, wherein the header information includes a first address that is unique within the service provider network and is allocated to the edge switch, a second address that is unique within the service provider network and is allocated to another edge switch coupled to a user network that becomes a destination, a virtual local area network (VLAN) value within a VLAN tag that is allocated to a network within the service provider network, a first port identifier identifying the port that receives the user frame from the user network, a second port identifier identifying a port of the other edge switch, a third port identifier identifying the port that receives the frame from the service provider network, and a sending source address within a user frame in the frame that is input from the service provider network. According to the edge switch of the present invention, the transfer process in the edge switch can be carried out using the first and second port identifiers within the header information, and the transfer process in the core switch can be carried out using the first and second addresses within the header information. By using the first and second port identifiers, it becomes possible to reduce the capacity of the learning table within the edge switch, to thereby improve the efficiency of the process which determines the transfer destination within the edge switch.  
      Other objects and further features of the present invention will be apparent from the following detailed description when read in conjunction with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a system block diagram showing an embodiment of a frame sending part within an edge switch according to the present invention;  
       FIGS. 2A through 2D  are diagrams showing frame formats at various parts of the frame sending part shown in  FIG. 1 ;  
       FIG. 3  is a system block diagram showing an embodiment of a frame receiving part within the edge switch according to the present invention;  
       FIGS. 4A through 4D  are diagrams showing frame formats at various parts of the frame receiving part shown in  FIG. 3 ;  
       FIG. 5  is a system block diagram showing an embodiment of the edge switch according to the present invention that is formed by the frame sending part shown in  FIG. 1  and the frame receiving part shown in  FIG. 3 ;  
       FIG. 6  is a diagram showing an embodiment of a network;  
       FIGS. 7A through 7D  are diagrams showing set values in tables that are stored in edge switches;  
       FIG. 8  is a diagram for explaining a transfer process of an unlearned unicast frame;  
       FIGS. 9A through 9D  are diagrams showing contents of tables within the edge switches;  
       FIG. 10  is a diagram for explaining a transfer process of a learned unicast frame;  
       FIGS. 11A through 11D  are diagrams showing contents of tables within the edge switches;  
       FIG. 12  is a diagram for explaining a transfer process of a multicast frame;  
       FIGS. 13A through 13D  are diagrams showing contents of tables within the edge switches; and  
       FIG. 14  is a diagram for explaining a transfer process of a unicast frame for a case where learned contents of core switches are erased. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS  
      The present invention is characterized in that, when encapsulating a frame in a source edge switch that transmits the frame in a service provider network, a modified format is employed such that input port information related to an input port connecting to a user network of the source edge switch and output port information related to an output port connecting to a user network of a destination edge switch are added to a generally employed format.  
      By employing this modified format, a transfer process within the source and destination edge switches is carried out using a port identifier that is embedded within the frame, and a transfer process between core switches within a service provider network and within the core switches is carried out using an MAC address of the edge switch that is added in the edge switch.  
       FIG. 1  is a system block diagram showing an embodiment of a frame sending part within an edge switch according to the present invention. A frame sending part  10  shown in  FIG. 1  includes input ports in1 through in4 for receiving user frames from user networks, and output ports out5 through out8 for outputting the frames to a core switch side. The number of input ports and the number of output ports are respectively not limited to 4.  
      The user frames received by the input ports in1 through in4 are supplied to a processor  12 . It is assumed for the sake of convenience that the received user frames have the format of a VLAN frame shown in  FIG. 2A .  FIGS. 2A through 2D  are diagrams showing frame formats at various parts of the frame sending part  10  shown in  FIG. 1 . The VLAN frame shown in  FIG. 2A  includes a payload, a VLAN value (vlan) within a VLAN tag that is allocated within the user network, a sending source MAC address (sa), and a destination MAC address (da).  
      The processor makes access to a content-addressable memory (CAM)  16  via a memory interface  14 . The CAM  16  includes as constituent elements thereof a physical port number, a port identifier, and a VLAN value allocated to the network within the service provider network.  
      The memory interface  14  receives from the processor  12  the physical port number (in) that receives the frame, and makes access to the CAM  16  using the physical port number (in) as the retrieval key, so as to obtain a port identifier (p_in) and the VLAN value (VLAN) allocated to the network within the service provider network, and to send the port identifier (p_in) and the VLAN value (VLAN) to the processor  12 . Necessary data are written in advance within the CAM  16  via an external interface  18 . The processor  12  transfers to a processor  20  the port identifier (p_in) and the VLAN value (VLAN) allocated to the network within the service provider network, together with the received frame. As shown in  FIG. 2B , the data transferred to the processor  20  has the port identifier (p_in) and the VLAN value (VLAN) added to the frame shown in  FIG. 2A .  
      The processor  20  makes access to the CAM  16  using the destination MAC address (da) within the user frame, the VLAN value (vlan) within the VLAN tag that is allocated within the user network, and the VLAN value (VLAN) that is allocated to the network within the service provider network that is added by the processor  12  as retrieval keys, so as to obtain an MAC address (DA) allocated to an edge switch connected to the user network that becomes the destination, a port identifier (p_out) of the edge switch connected to the user network that becomes the destination, and a port identifier (ps_out) on the core switch side of the source edge switch shown in  FIG. 1  that outputs the frame. Of course, the MAC address (DA) and the port identifier (ps_out) on the core switch side of the source edge switch may be obtained from another CAM.  
      In addition, the processor  20  makes access to a memory  24  and obtains an MAC address (SA) that is allocated to the source edge switch. The MAC address of the source edge switch is set in advance in the memory  24  via the external interface  18 . The processor  20  assembles a frame which is added with the port identifier (p_out), the MAC address (SA), the MAC address (DA) and the port identifier (ps_out) as shown in  FIG. 2C , based on the information read from a CAM  22  and the memory  24 , and transfers the assembled frame to a processor  26 .  
      In a case where the user frame is a multicast frame, the processor  20  adds to the frame that is input to the processor  26  a multicast address, as the destination address of the output frame. In addition, the processor  20  specifies the multicasting to the output port within the source edge switch shown in  FIG. 1  to which the processor  20  belongs, and regards the multicasting as being specified to the output port of the destination edge switch. The multicast address that is used as the destination address is recognizable in both the core switch and the edge switch. The specifying of the multicasting to the output port is made by a format that is recognizable within the edge switch.  
      In a case where the user frame is a unicast frame and no corresponding information is stored in the CAM  22 , the processor  20  carries out a process similar to that carried out when the multicast frame is received.  
      The processor  26  makes access to the CAM  16  via the memory interface  14  when the MAC address (DA) allocated to the edge switch connected to the user network that becomes the destination, in the received frame, is not the multicast address.  
      The memory interface  14  receives from the processor  26  the port identifier (ps_out) on the core switch side of the source edge switch that outputs the frame, and makes access to the CAM  16  using the port identifier (ps_out) as the retrieval key, so as to obtain and send to the processor  26  the physical port number (out). The processor  26  eliminates the port identifier (ps_out) on the core switch side of the source edge switch that outputs the frame, within the received frame, and outputs the frame to the port on the core switch side that is obtained by the retrieval. The frame format in this state becomes as shown in  FIG. 2D .  
      In a case where the DA of the frame received by the processor  26  is the multicast address, the frame is output to all ports accommodating the VLAN value (VLAN) allocated to the network within the service provider network within the frame.  
      In general, in order to determine the port for outputting the frame, a CAM which stores the VLAN value as the retrieval key and the bit-map of the output ports as the contents, is used. When using this CAM in common as the CAM  16  so as to function as both, the retrieval key becomes the VLAN value (VLAN) allocated to the network within the service provider network, and all port information matching the retrieval key is read to the processor  26  via the memory interface  14 . The processor  26  sends with respect to all ports corresponding to the all port information the frame that is obtained by eliminating, the port identifier (ps_out) on the core switch side of the source edge switch that outputs the frame, within the received frame.  
       FIG. 3  is a system block diagram showing an embodiment of a frame receiving part within the edge switch according to the present invention. A frame receiving part  30  shown in  FIG. 3  includes input ports in5 through in8 for receiving the frames from the core switch within the service provider network, and output ports out1 through out4 for outputting the frames to the user network side. The number of input ports and the number of output ports are respectively not limited to 4.  
      The frames received by the input ports in5 through in8 are supplied to a processor  32 . It is assumed for the sake of convenience that the received frames have the format of a VLAN frame shown in  FIG. 4A .  FIGS. 4A through 4D  are diagrams showing frame formats at various parts of the frame receiving part shown in  FIG. 3 . The processor  32  makes access to a CAM  36  via a memory interface  34 . The CAM  36  includes as constituent elements thereof a physical port number, a port identifier, and a VLAN value allocated to the network within the service provider network.  
      The memory interface  14  receives from the processor  32  the physical port number (in) that receives the frame, and makes access to the CAM  36  using the physical port number (in) as the retrieval key, so as to obtain a port identifier (p_in), and to send the port identifier (p_in) to the processor  32 . Data are written in advance within the CAM  36  via an external interface  38 . The processor  32  transfers to a processor  40  the port identifier (p_in), together with the received frame. As shown in  FIG. 4A , the data transferred to the processor  40  has the port identifier (p_in) added to the received frame.  
      The processor  40  compares the MAC address of the destination edge switch shown in  FIG. 3  stored in a memory  42  and the MAC address (DA) within the received frame, and discards (or annuls) the received frame if the compared MAC addresses do not match. The compared MAC addresses do not match if the frame is flooded in a state where a learning table to be used is in an unlearned state when transferring the frame by the core switch within the service provider network. If the edge switch transfers the frame that is not intended therefor to the user network side, the traffic within the user network is unnecessarily increased thereby, and this is the reason why the received frame is discarded when the compared MAC addresses do not match. On the other hand, when the compared MAC addresses match, the received frame is sent as it is to a processor  44 . However, in a case where the MAC address (DA) is a multicast address, the received frame is sent to the processor  44  without comparing the MAC addresses.  
      The processor  44  writes the sending source MAC address (sa) within the user frame of the received frame, the VLAN value (vlan) within the VLAN tag that is allocated within the user network, the VLAN value (VLAN) allocated to the network within the service provider network, the MAC address (SA) allocated to the source edge switch within the received frame, the port identifier (p_in) of the port connected to the user network by the source edge switch, and port identifier (pd_in) of the port that receives the frame from the core switch side, into a CAM  48 , so as to carry out a learning process.  
      In this embodiment, the CAM  48  shown in  FIG. 3  is used in common as the CAM  22  shown in  FIG. 1  to function as both. However, the CAM  48  and the CAM  22  may be provided independently, and the data write with respect to the CAM  48  and the CAM  22  may be carried out independently.  
      The data write (or learning) with respect to the CAM  48  is not limited to the dynamic writing responsive to the received frame. An external memory interface  46  is provided in order to enable setting of the data write (or learning) with respect to the CAM  48  from the external interface  38 .  
      The processor  44  transfers the received frame with respect to a processor  50 , simultaneously as carrying out the data write with respect to the CAM  48 . In this case, the received frame that is transferred with respect to the processor  50  has a frame format shown in  FIG. 4C . Of course, the frame transfer with respect to the processor  50  may be made with a frame format shown in  FIG. 4B , and a frame format shown in  FIG. 4D  may be used within the processor  50 . Accordingly, the learning of the frame and the determination of the transfer destination of the frame within the edge switch can be carried out in parallel.  
      The processor  50  makes access to the CAM  38  via the memory interface  34  in a case where the port identifier (p_out) of the port of the edge switch connected to the user network which becomes the destination, of the received frame, is not a multicast address. The memory interface  34  receives from the processor  50  the port identifier (p_out) of the port of the edge switch connected to the user network which becomes the destination, makes access to the CAM  36  using the port identifier (p_out) as the retrieval key, so as to obtain and send to the processor  50  the physical port number (out).  
      The processor  50  eliminates from the received frame the port identifier (p_out) of the port of the edge switch connected to the user network which becomes the destination and the VLAN value (VLAN) allocated to the network within the service provider network, and outputs the frame to the port on the service provider network side obtained by the retrieval. The frame output to the port on the service provider network side has a frame format shown in  FIG. 4D .  
      In a case where the port identifier (p_out) of the port of the edge switch connected to the user network which becomes the destination, of the received frame, is a multicast address, the frame is output to all user ports accommodating the VLAN value (VLAN) allocated to the network within the service provider network within the frame.  
      In general, in order to determine the port for outputting the frame, a CAM which stores the VLAN value as the retrieval key and the bit-map of the output ports as the contents, is used. When using this CAM in common as the CAM  36  so as to function as both, the retrieval key becomes the VLAN value (VLAN) allocated to the network within the service provider network, and all port information matching the retrieval key is read to the processor  50  via the memory interface  34 . The processor  50  sends with respect to all ports corresponding to the all port information the frame that is obtained by eliminating, the port identifier (p_out) of the edge switch connected to the user network that becomes the destination, and the VLAN value (VLAN) allocated to the network within the service provider network, within the received frame.  
       FIG. 5  is a system block diagram showing an embodiment of the edge switch according to the present invention that is formed by the frame sending part shown in  FIG. 1  and the frame receiving part shown in  FIG. 3 . In  FIG. 5 , those parts which are the same as those corresponding parts in  FIGS. 1 and 3  are designated by the same reference numerals, and a description thereof will be omitted. In this embodiment shown in  FIG. 5 , a common CAM  52  is used to function as both the CAM  22  shown in  FIG. 1  and the CAM  48  shown in  FIG. 3 . In addition, a common external interface  54  is used to function as both the external interface  18  shown in  FIG. 1  and the external interface  38  shown in  FIG. 3 .  
       FIG. 6  is a diagram showing an embodiment of a network applied with the present invention. In  FIG. 6 , a service provider network  66  provides a relay between user-A networks  61 ,  63  and  64  and user-B networks  62  and  65 . An edge switch  71  connects to the networks  61  and  62 , an edge switch  72  connects to the network  63 , an edge switch  73  connects to the network  64 , and an edge switch  74  connects to the network  65 . Core switches  81  through  84  are provided in the service provider network  66  and transfer the general VLAN frames.  
      MAC addresses E1 through E4 are respectively set to the edge switches  71  through  74  of the present invention. With respect to each port of all of the switches within the service provider network  66 , a VLAN value 0x100 is set to a port which is indicated by leftwardly descending hatching for relaying the frame of the user-A network, and a VLAN value 0x200 is set to a port which is indicated by a rightwardly descending hatching for relating the frame of the user-B network. VLAN values 0x100, 0x200 are set to a port which is indicated by cross-hatching for relating the frame of the user-A network and the frame of the user-B network. Port identifiers 0x0001 through 0x0003 are respectively set to ports port1 through port3 of the edge switches  71  through  74 .  
       FIGS. 7A through 7D  are diagrams showing set values in tables that are stored in the memories  24  and  42  and the CAMs  16 ,  36  and  52  of each of the edge switches  71  through  74 , in an unlearned state. In the case of the edge switch  71 , for example, the MAC address E1 of the edge switch  71  is stored in the memories  24  and  42  of the edge switch  71  as shown in  FIG. 7A . The port identifier 0x0001 and the accommodating VLAN value 0x100 with respect to the physical port number port1, the port identifier 0x0002 and the accommodating VLAN value 0x200 with respect to the physical port number port2, and the port identifier 0x0003 and the accommodating VLAN values 0x100, 0x200 with respect to the physical port number port3 are stored in the CAMs  16  and  36 . No information is set in the CAM  52  since it is still in the unlearned state. Similarly, the values set in the edge switches  72 ,  73  and  74  are respectively shown in  FIGS. 7B, 7C  and  7 D.  
      A description will now be given of a transfer process for a case (unlearned unicast frame) where the destination of the frame input from the user-A network is not yet learned within the edge switch.  
       FIG. 8  is a diagram for explaining the transfer process of the unlearned unicast frame. In  FIG. 8 , those parts which are the same as those corresponding parts in  FIG. 6  are designated by the same reference numerals, and a description thereof will be omitted. Suppose that the frame is sent from an apparatus having an MAC address #1 within the user-A network  61  with respect to an apparatus having an MAC address #3 within the user-A network  63 , as shown in  FIG. 8 . It is assumed that the an identifier VLAN-ID=0x123 is allocated to the VLAN within the service provider network  66  which relays the frame from the user-A network  61 . The edge switch  71  which receives the frame from the user-A network  61  by the port port1 thereof, makes access to the tables within the CAMs  16  and  36  of the edge switch  71 , and reads the port identifier 0x0001 and the accommodating VLAN value 0x100 allocated to the port port1.  
      Next, a retrieval is made from the tables in the CAM  52  using, as the retrieval key, the identifier VLAN-ID=0x123 and the destination MAC address #3 within the user frame and the accommodating VLAN value 0x100 read from the table described above. Since it is found as a result of the retrieval that no registration exists with respect to the retrieval key, a multicast frame is sent with respect to all ports (excluding the port that receives the frame) accommodating the VLAN value 0x100 of the ports on the core switch side.  
      The frame that is sent to the core switch  81  is added with the following data with respect to the user frame.  
      Destination MAC address DA: ff:ff:ff:ff:ff:ff (all “1”: multicast address)  
      Sending Source MAC address SA: E1  
      VLAN value (VLAN) within VLAN tag: 0x100  
      Port identifier p_in of the switch that receives the frame from the user network: 0x0001  
      Port identifier p_out connected to the user network of the edge switch that becomes the destination: 0xffff  
      The transfer process within the service provider network  66  is carried out using the destination MAC address (ff:ff:ff:ff:ff:ff) and the VLAN value (0x100), and the frame is transferred in the order of the core switch  81 , the core switch  82  and the edge switch  72 , and in the order of the core switch  81 , the core switch  84  and the edge switch  74 .  
      When the edge switch  72  receives the frame, the edge switch  72  writes the following entries in the tables within the CAM  52 .  
      Sending source MAC address SA: E1  
      Port identifier p_in of the switch that receives the frame from the user network: 0x0001  
      Port identifier of the edge switch  72  that receives the frame from the core switch side: 0x0002  
      VLAN value (VLAN) within the service provider network  66 : 0x100  
      VLAN value (vlan) within the user frame: 0x123  
      Sending Source MAC address (sa) within the user frame: #1  
      Similarly, when the edge switch  74  receives the frame, the edge switch  74  writes the following entries in the tables within the CAM  52 .  
      Sending source MAC address SA: E1  
      Port identifier p_in of the switch that receives the frame from the user network: 0x0001  
      Port identifier of the edge switch  74  that receives the frame from the core switch side: 0x0001  
      VLAN value (VLAN) within the service provider network  66 : 0x100  
      VLAN value (vlan) within the user frame: 0x123  
      Sending Source MAC address (sa) within the user frame: #1  
      Since the frame received by the edge switches  72  and  74  is the multicast frame, the edge switches  72  and  74  eliminate the information added in the sending source edge switch  71  from the frame and send the frame with respect to all ports on the user network side of the user-A network accommodating the VLAN value 0x100 within the service provider network  66 .  FIGS. 9A through 9D  are diagrams showing contents of the tables within each of the edge switches  71  through  74  after relaying the frame. In  FIGS. 9A through 9D , the VLAN value is indicated as VLAN-ID.  
      Next, a description will be given of a transfer process for a case (learned unicast frame) where the destination of the frame input from the user-A network has been learned within the edge switch.  
       FIG. 10  is a diagram for explaining the transfer process of the learned unicast frame. In  FIG. 10 , those parts which are the same as those corresponding parts in  FIG. 8  are designated by the same reference numerals, and a description thereof will be omitted. It is assumed that, in a state where the tables within the edge switches  71  through  74  are in the states shown in  FIGS. 9A through 9D , the frame is sent from an apparatus having an MAC address #3 within the user-A network  63  with respect to an apparatus having an MAC address #1 within the user-A network  61 , as shown in  FIG. 10 .  
      The edge switch  72  which receives the frame from the user-A network  63  by the port port1 thereof, makes access to the tables within the CAMs  16  and  36  of the edge switch  72 , and reads the port identifier 0x0001 and the accommodating VLAN value 0x100 allocated to the port port1.  
      Next, a retrieval is made from the tables in the CAM  52  using, as the retrieval key, the VLAN value (vlan)=0x123 and the destination MAC address #1 within the user frame and the accommodating VLAN value 0x100 read from the table described above. Since it is found as a result of the retrieval that a registration exists with respect to the retrieval key, the frame is output to the port port2 on the core switch side based on the port identifier 0x0002 of the edge switch  72  stored within the table of the CAM  52 .  
      The frame that is sent to the core switch  81  is added with the following data with respect to the user frame.  
      Destination MAC address DA: E1  
      Sending Source MAC address SA: E2  
      VLAN value (VLAN) within VLAN tag: 0x100  
      Port identifier p_in of the switch that receives the frame from the user network: 0x0001  
      Port identifier p_out connected to the user network of the edge switch that becomes the destination: 0x0001  
      The transfer process within the service provider network  66  is carried out using the destination MAC address (E1) and the VLAN value (0x100), and the frame is transferred in the order of the core switch  82 , the core switch  81  and the edge switch  71 .  
      When the edge switch  71  receives the frame, the edge switch  71  writes the following entries in the tables within the CAM  52 .  
      Sending source MAC address SA: E2  
      Port identifier p_in of the switch that receives the frame from the user network: 0x0001  
      Port identifier of the edge switch  71  that receives the frame from the core switch side: 0x0003  
      VLAN value (VLAN) within the service provider network  66 : 0x100  
      VLAN value (vlan) within the user frame: 0x123  
      Sending Source MAC address (sa) within the user frame: #3  
      Since the frame received by the edge switch  71  is the unicast frame intended for the edge switch  71 , the edge switch  71  eliminates the information added in the sending source edge switch  72  from the frame and sends the frame with respect to the port port1 by judging this port port1 from the destination port identifier 0x0001.  FIGS. 11A through 11D  are diagrams showing contents of the tables within each of the edge switches  71  through  74  after relaying the frame. In  FIGS. 11A through 11D , the VLAN value is indicated as VLAN-ID.  
      Next, a description will be given of a transfer process for a case where the frame input from the user-B network is a multicast frame.  
       FIG. 12  is a diagram for explaining the transfer process of the multicast frame. In  FIG. 12 , those parts which are the same as those corresponding parts in  FIG. 8  are designated by the same reference numerals, and a description thereof will be omitted. It is assumed that, in a state where the tables within the edge switches  71  through  74  are in the states shown in  FIGS. 11A through 11D , the multicast frame is sent from an apparatus having an MAC address #2 within the user-B network  62  with respect to each apparatus within the user-B networks, via the service provider network  66 , as shown in  FIG. 12 . It is assumed that an identifier 0x456 is allocated to the VLAN value (vlan) within the user frame of the user-B network  62 .  
      The edge switch  71  which receives the multicast frame from the user-B network  62  by the port port2 thereof, makes access to the tables within the CAMs  16  and  36  of the edge switch  71  and reads the port identifier 0x0002 and the accommodating VLAN value 0x200 allocated to the port port2.  
      Since the received frame is the multicast frame, the edge switch  71  sends the multicast frame with respect to all ports (excluding the port that receives the multicast frame) accommodating the VLAN value 0x200 of the ports on the core switch side, without making a retrieval with respect to the tables.  
      The frame that is sent to the core switch  81  is added with the following data with respect to the user frame.  
      Destination MAC address DA: ff:ff:ff:ff:ff:ff (all “1”: multicast address)  
      Sending Source MAC address SA: E1  
      VLAN value (VLAN) within VLAN tag: 0x200  
      Port identifier p_in of the switch that receives the frame from the user network: 0x0002  
      Port identifier p_out connected to the user network of the edge switch that becomes the destination: 0xffff  
      The transfer process within the service provider network  66  is carried out using the destination MAC address (ff:ff:ff:ff:ff:ff) and the VLAN value (0x200), and the frame is transferred in the order of the core switch  81 , the core switch  84 , the core switch  83  and the edge switch  73 .  
      When the edge switch  73  receives the frame, the edge switch  73  writes the following entries in the tables within the CAM  52 .  
      Sending source MAC address SA: E1  
      Port identifier p_in of the switch that receives the frame from the user network: 0x0002  
      Port identifier of the edge switch  73  that receives the frame from the core switch side: 0x0001  
      VLAN value (VLAN) within the service provider network  66 : 0x200  
      VLAN value (vlan) within the user frame: 0x456  
      Sending Source MAC address (sa) within the user frame: #2  
      Since the frame received by the edge switch  73  is the multicast frame, the edge switch  73  eliminates the information added in the sending source edge switch from the frame and sends the frame with respect to all ports on the user network side accommodating the VLAN value 0x200 within the service provider network  66 .  FIGS. 13A through 13D  are diagrams showing contents of the tables within each of the edge switches  71  through  74  after relaying the frame. In  FIGS. 13A through 13D , the VLAN value is indicated as VLAN-ID.  
      Next, a description will be given of a transfer process for a case where the frame input from the user-A network is learned in the edge switch  71  but is not learned in the core switch.  
       FIG. 14  is a diagram for explaining the transfer process of the unicast frame for the case where learned contents of the core switches are erased. After the learned contents of the core switches  81  through  84  are erased, it is assumed that the frame is sent from the apparatus having the MAC address #1 within the user-A network  61  with respect to the apparatus having the MAC address #3 within the user-A network  63 , as shown in  FIG. 14 .  
      The edge switch  71  which receives the frame from the user-A network  61  by the port port1 thereof, makes access to the tables within the CAMs  16  and  36  of the edge switch  71 , and reads the port identifier 0x0001 and the accommodating VLAN value 0x100 allocated to the port port1.  
      Next, a retrieval is made from the tables in the CAM  52  using, as the retrieval key, the VLAN-ID=0x123 and the destination MAC address #3 within the user frame and the accommodating VLAN value 0x100 read from the table described above. Since it is found as a result of the retrieval that a registration exists with respect to the retrieval key, the frame is output to the port port3 on the core switch side based on the port identifier 0x0003 of the edge switch  71  stored within the table of the CAM  52 .  
      The frame that is sent to the core switch  81  is added with the following data with respect to the user frame.  
      Destination MAC address DA: E2  
      Sending Source MAC address SA: E1  
      VLAN value (VLAN) within VLAN tag: 0x100  
      Port identifier p_in of the switch that receives the frame from the user network: 0x0001  
      Port identifier p_out connected to the user network of the edge switch that becomes the destination: 0x0001  
      The transfer process within the service provider network  66  is carried out using the destination MAC address (E2) and the VLAN value (0x100), and in the case where the learning table of the core switch  81  is in the unlearned state, the frame is transferred in the order of the core switch  81 , the core switch  82  and the edge switch  72 , and in the order of the core switch  81 , the core switch  84  and the edge switch  74 .  
      When the edge switch  72  receives the frame, the edge switch  72  writes the following entries in the tables within the CAM  52 .  
      Sending source MAC address SA: E1  
      Port identifier p_in of the switch that receives the frame from the user network: 0x0001  
      Port identifier of the edge switch  72  that receives the frame from the core switch side: 0x0002  
      VLAN value (VLAN) within the service provider network  66 : 0x100  
      VLAN value (vlan) within the user frame: 0x123  
      Sending Source MAC address (sa) within the user frame: #1  
      Since the destination MAC address is E2, the edge switch  74  judges that the received frame is not intended for the edge switch  74 , and discards the received frame.  
      Since the frame received by the edge switch  72  is the unicast frame intended for the edge switch  72 , the edge switch  72  eliminates the information added in the sending source edge switch from the frame and sends the frame with respect to the port port1 by judging this port port1 from the destination port identifier 0x0001.  
      Therefore, by using the edge switch of the present invention in the broadband network, it becomes possible to use a generally used switch which processes the VLAN frame as the core switch. In addition, since the MAC address which is the target of the learning in the core switch becomes the MAC address allocated to the edge switch, it becomes possible to suppress the number of learning processes to be carried out compared to the conventional techniques. As a result, it is possible to avoid the learning table for the MAC address from becoming exhausted, and to simultaneously improve the performance, that is, the transfer delay, of the core switch. Moreover, because the judging of the transfer destination within the edge switch and the frame encapsulation can be carried out in parallel, it becomes possible to improve the performance of the edge switch.  
      The CAMs  16  and  22  and the processors  12  and  20  form a header information adding part or means. The CAM  22  and the processor  20  form second and third identifier retrieving parts or means. The CAM  48  and the processor  44  form a learning part or means. The CAM  36  and the processor  50  form a user network output port retrieving part or means.  
      Further, the present invention is not limited to these embodiments, but various variations and modifications may be made without departing from the scope of the present invention.