Abstract:
Disclosed herewith is a network that can connect as many VPNs as possible to itself. In order to realize such a network, in the frame transfer method of the present invention, which enables a transfer path to be formed between nodes in the network so as to transfer frames, a terminal for transmitting frames or node located outside the transfer path, upon transmitting a frame through the transfer path, writes the transfer path selection information related to the transfer path and the output line information related to the output line from the terminating node of the transfer path in the frame according to the destination information set in the frame and transmits the frame to the destination. The starting node of the transfer path decides the transfer path used for transferring the frame according to the transfer path selection information set in the frame to transmit the frame while the terminating node of the transfer path decides the output line used to output the frame therefrom according to the output line set in the frame, thereby transmitting the frame to the output line.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    1. Field of the Invention  
           [0002]    The present invention relates to a frame transfer method, and more particularly to a frame transfer method employed for VPN services for realizing virtual private networks (VPN).  
           [0003]    2. Description of Related Art  
           [0004]    There is a VPN service proposed for forming a virtual private network (VPN) in an enterprise by connecting a plurality of the enterprise sites separated physically away from one another. In recent years, another VPN service has started. The new VPN service transfers frames according to MAC addresses, which are of the Ethernet (trademark). Each of this type networks is a comparatively small in scale and formed, for example, within an urban community and referred to as a MAN (Metropolitan Area Network).  
           [0005]    On the other hand, there is a technique for realizing a wide ranged large scale network configured by a plurality of such the MANs. This technique is an application of the MPLS (Multi Protocol Label Switching) proposed, for example, in the IETF Draft “Encapsulation Methods for Transport of Layer 2 Frames Over IP and MPLS Networks”, draft-martini-12 circuit-encap-mpls-04.txt referred to as the conventional technique  1  and in the IETF Draft “Transport of Layer 2 Frames Over MPLS”, draft-martini-12 circuit-encap-mpls-08.txt referred to as the conventional technique  2 . In those conventional techniques  1  and  2 , a path referred to as a tunnel LSP (Label Switching Path) is formed in a backbone network connected to a plurality of MANs and a plurality of paths referred to as VC LSPs are formed in this path (tunnel LSP). A node located at the inlet of the back-born network that connects the MANs adds a tunnel label and a VC label to each received frame. Both of the tunnel and VC labels are identifiers of those LSPs. And, the nodes in the back-born network transfer those frames while the node located at the outlet of the back-born network processes the frames according to their VC labels.  
           [0006]    [0006]FIG. 2 shows a block diagram of a network to which such a conventional frame transfer method applies.  
           [0007]    Hereunder, the conventional techniques  1  and  2  will be described with reference to the block diagram of the network shown in FIG. 2. In the network shown in FIG. 2, the sites LAN-A1 and LAN-A2 of an enterprise A are connected to each other through MAN-1, MAN-3, and a backbone network that connects those MAN-1 and MAN3 respectively. The backbone network is configured by PEs (PE: Provider Edge Node) 1 to 3 located on the edge thereof and PCs (PC: Provider Core Node) 1 to 3. In the backbone network, tunnel LSPs (T-LSP2 and T-LSP4) are formed. The T-LSP2 transfers frames in the direction of PE1-&gt;PC2-&gt;PC3-&gt;PE3 while the T-LSP4 transfers frames in the opposite direction. In the T-LSP2, a VC-LSP-A1 is formed so as to transfer frames from the LAN-A1 to the LAN-A2. In the T-LSP-4, a VC-LSP-A2 is formed so as to transfer frames from the LAN-A2 to the LAN-A1. In addition, another LSP used for communications between each site of an enterprise B and each site of an enterprise C is formed in the backbone network. The LSP illustration is omitted in FIG. 2, however.  
           [0008]    PE1 of the backbone network, when receiving a frame from the LAN-A1, adds a tunnel label that is the T-LSP2 identifier and a VC label that is the VC-LSP-A1 identifier to the frame, then transfer the frame to the PC2. The PC2, as well as the PC3 refer to the tunnel label to transfer the frame to the PE3. The PE3 then refers to the VC label to transfer the frame to a line connected to the MAN-3. Consequently, the MAN-1 and is connected to the MAN-3, thereby the VPN service of the enterprise A is realized.  
           [0009]    Next, the problems of the conventional techniques  1  and  2  will be described with reference to the network shown in FIG. 1, which is the same as the network shown in FIG. 2. In FIG. 1 are shown only the LSPs formed among the sites of the enterprises A and B.  
           [0010]    In the network shown in FIG. 1, the sites LAN-B1 to B4 of the enterprise B are connected to one another through the MAN-1 to MAN-4, as well as the backbone network that connects those MANs. In the backbone network, tunnel LSPs (T-LSP1 and T-LSP3) are formed. The T-LSP1 transfers frames in the direction of PE1-&gt;PC1-&gt;PE2 and the T-LSP3 transfers frames in the opposite direction. In the backbone network, other tunnels LSP (T-LSP2) and LSP (T-LSP4) are also formed. The LSP (T-LSP2) transfers frames PE1-&gt;PC2-&gt;PC3-&gt;PE3 and the LSP (T-LSP4) transfers frames in the opposite direction. In the T-LSP1, a VC-LSP-B1 is formed so as to transfer frames from the LAN-B1 to the LAN-B2. In the T-LSP3, a VC-LSP-B3 is formed so as to transfer frames in the opposite direction. In the T-LSP2, a VC-LSP-B2 is formed so as to transfer frames from the LAN-B1 to the LAN-B3 and B4. In the T-LSP4, a VC-LSP-B4 is formed so as to transfer frames in the opposite direction. In the backbone network are also formed still other LSPs; an LSP used for the communications among the sites of the enterprise A, an LSP used for communications among the sites of the enterprise C, and an LSP used for the communications between PE2 and PE3, although those LSPs are not shown in FIG. 1.  
           [0011]    In a network configured as described above, the PE1, when receiving a frame from the LAN-B1, cannot decide to which of LAN-B2, B3, and B4 the frame should be transmitted. In other words, the PE1 cannot decide which of the tunnels (VC-LSP-B1 in the T-LSP1 and T-LSP-B2 in the T-LSP2) should be used to transfer the frame through the VC-LSP. This is also the same for the PE3, which cannot decide which of the lines connected to MAN-3 andMAN-4 should be used to transfer the frame. Consequently, the conventional techniques  1  and  2  described above cannot connect any site over three or more MANs.  
           [0012]    On the other hand, there is a technique for connecting a site over three or more MANs. This technique enables the subject PE to learn an output line number, a tunnel LSP, and a VC-LSP in accordance with the MAC address set in each frame. Such the technique is known well as the conventional technique  3  (TETF Draft “Virtual Private Switched Network Services over an MPLS Network”, draft-vkompella-ppvpn-mpls-00.txt) and the conventional technique  4  (IETF Draft “Transparent VLAN Services over MPLS”, draft-lasserre-vkopella-ppvpn-tis-00.txt). A PE, when receiving a frame from a PC belonging to the backbone network, stores transfer information consisting of the line number of the line to which the frame is inputted, the tunnel LSP, and the VC-LSP therein in accordance with the source MAC address set in the received frame. And, the PE, when receiving a frame from a MAN node, stores transfer information consisting of the line number of the line to which the frame is inputted therein corresponding to the source MAC address set in the frame. When receiving a frame addressed to the stored MAC address, the PE transfers the frame according to the transfer information corresponding to the MAC address.  
           [0013]    Next, the conventional technique  3  will be described in detail with reference to FIG. 1. The PE1, when receiving a frame from the terminal T7 belonging to the LAN-B3, stores the line number of the line connected to the PC2, the VC-LSP-B2, and the T-LSP2 therein in correspondence with the MAC address of the terminal T7. When the PE1 receives a frame addressed to the terminal T7 from the MAN-1, the PE1 transfers the frame according to the line number, the VC-LSP-B2, and the T-LSP2 stored therein as described above. The PE3, when receiving a frame from a terminal T7, stores the line number of the line to which the frame is inputted and the MAC address of the terminal T7 so that the line number and the MAC address are corresponded to each other. And, the PE3, when receiving a frame addressed to the terminal T7 from the PC3, transfers the frame to the line corresponding to the line number stored therein.  
           [0014]    As described above, according to any of the conventional techniques  3  and  4 , when a frame is received from a MAN, it is possible to decide to which of the remaining two or more MANs the frame should be transmitted. This is why a site can be connected over three or more MANs, thereby the problems of the conventional techniques  1  and  2  are solved.  
           [0015]    The conventional techniques  1  to  4  described above, however, are often confronted with the following problem that will arise in construction of a large scale network that comes to include many enterprises (contractors) connected over a plurality of MANs. The conventional techniques  1  and  2  also come to be confronted with another problem that a site connected over three or more MANs as described above cannot be connected to a network that employs any of the conventional techniques  1  and  2 .  
           [0016]    Furthermore, a network that employs any of the conventional techniques  3  and  4  causes another problem to arise; the capacity of a table provided in each node to store transfer information often becomes insufficient. In other words, every PE that employs any of the conventional techniques  3  and  4  is required to learn such transfer information as output line numbers, tunnel LSPs, VC LSPs in correspondence with the MAC addressesofall the enterprises stored in the PE. For example, the PE1 shown in FIG. 1 is required to learn such the transfer information so as to make it correspond to the MAC addresses of all the terminals T1 to T11 of the enterprises A to C. And, the table provided in such a PE so as to store such transfer information is limited in capacity and due to the limited capacity of the table, the networks that employ any of the conventional techniques  3  and  4  come to be disabled to store information of many enterprises (contractors).  
           [0017]    Under such circumstances, it is an object of the present invention to provide a network that can hold information of many more enterprises (contractors) than any conventional networks by forming nodes on the edge of the subject network, which are used to store frame transfer information corresponding to the destination address of each frame.  
         SUMMARY OF THE INVENTION  
         [0018]    In order to solve the above conventional problems, the frame transfer method of the present invention, employed for a network configured by a plurality of nodes and a plurality of terminals connected to one another through a line, enables a transfer path for transferring frames to be formed between nodes in the network, so that a node, upon receiving of a frame from a different node, writes a transfer path identifier used to identify a target transfer path in the frame, thereby the frame is transferred to the destination. A terminal for transferring a received frame or node located outside the transfer path, when transmitting a frame through the transfer path, writes transfer path selection information related to a target transfer path and output line selection information related to an output line of the terminating node of the transfer path in the frame according to the destination information set in the frame, then transmits the frame to the destination. The starting node of the transfer path decides a target transfer path according to the transfer path selection information set in the frame to transmit the frame to the transfer path while the terminating node of the transfer path decides a target output line according to the output line selection information set in the frame, thereby transmitting the frame to the output line.  
           [0019]    In another aspect, the frame transfer method of the present invention forms a virtual line (e.g., a VC path) in the transfer path for each logical network formed in the network, so that a terminal for transmitting a frame or node, which is located outside the transfer path, when transmitting the frame, writes a logical network identifier used to identify a target logical network in the frame, then transmits the frame. On the other hand, the starting node of the transfer path decides a target virtual line (a virtual line and an output line connected to the virtual line) according to the logical network identifier and the transfer path selection information set in the frame while the terminating node of the transfer path decides a target output line to output the frame therefrom according to the logical network identifier and the transfer path selection information set in the frame, thereby transmitting the frame to the output line.  
           [0020]    In still another aspect, the frame transfer method of the present invention forms a virtual line (e.g., a VC path) in the transfer path for each logical network formed in the network, so that a terminal for transmitting a frame or node, which is located outside the transfer path, when transmitting the frame, writes a logical network identifier used to identify a target logical network in the frame to transmit the frame. On the other hand, the starting node of the transfer path decides a target virtual line (a virtual line and an output line connected to the virtual line) according to the logical network identifier and the transfer path selection information set in the frame, then writes the virtual line identifier information (e.g., a VC label) used to identify the virtual line in the frame to be transferred while the terminating node of the transfer path decides a target output line to output the frame therefrom according to the virtual line identifier information and the output line selection information set in the frame, thereby enabling transmission of the frame to the output line.  
           [0021]    Further, the frame transfer device of the present invention is provided with a plurality of input lines and a plurality of output lines and used to transfer frames inputted to the plurality of input lines to the plurality of output lines. The frame transfer device also includes a frame information transmission unit for transmitting frame information consisting of one or both of the destination address information and the network identifier set in the header of the frame, a transfer table for storing a plurality of transfer entries, each storing destination information consisting of at least one or both of the path selection information and the output line selection information used by a frame transfer device located in the downstream of the frame transfer device in a path connected to a terminal identified by the destination address information to decide the destination of the frame, a header process unit for referring to the transfer table upon receiving the frame information to decide an output line number and destination information corresponding to the destination address information set in the header of the frame, and destination information writing means for writing the destination information in the frame.  
           [0022]    In another aspect, the frame transfer device of the present invention, provided with a plurality of input lines and a plurality of output lines and used to output frames inputted from the plurality of input lines to the plurality of output lines, further includes a frame information transmission unit for transmitting frame information to a destination, the frame information consisting of path selection information and network identifier corresponding to the destination address information written by a frame transfer device located in the upstream of the above frame transfer device, a transfer table for storing a plurality of transfer entries, each consisting of an output line number and path information corresponding to the frame information, a header process unit for referring to the transfer table upon receiving of the frame information to decide both of the output line number and the path information, a frame switch for transferring the frame to the output line corresponding to the output line number, and destination information writing means for writing the path information in the frame.  
           [0023]    In still another aspect, the frame transfer device of the present invention, provided with a plurality of input lines and a plurality of output lines and used to output frames inputted from the plurality of input lines to the plurality of output lines, further includes a frame information transmission unit for transmitting frame information consisting of one or more of a network identifier, an input line number that is an identifier of the input line to which the frame is inputted and path information that is an identifier of the path through which the frame is transferred, as well as output line selection information corresponding to the destination address information written by a different frame transfer device located in the upstream of the above frame transfer device; a transfer table for storing a plurality of transfer entries, each consisting of an output line number corresponding to the frame information; a header process unit for referring to the transfer table upon receiving of the frame information to decide a target output line number; and a frame switch for transferring the frame to the line corresponding to the output line number.  
           [0024]    In order to solve the above conventional problems, the frame transfer method of the present invention, which is employed for a network consisting of a plurality of nodes and terminals connected to one another through a line, enables a transfer path for transferring frames to be formed between nodes in the network, so that one of the nodes, upon receiving a frame from a different node, writes a path identifier for identifying a target transfer path in the frame and transmits the frame to the destination. A terminal for transmitting a frame or node, which is located outside the transfer path, when transmitting a frame through the transfer path, writes the transfer path information related to the transfer path and the output line information related to an output line from the terminating node of the transfer path in the frame to be transmitted while the starting node of the transfer path decides a target transfer path used for transferring the frame according to the transfer path selection information set in the frame, then transmits the frame to the transfer path according to the destination information set in the frame. And, the terminating node of the transfer path also decides a target output line for outputting the frame therefrom according to the output line information set in the frame, thereby transmitting the frame to the output line.  
           [0025]    Concretely, a network that employs the frame transfer method of the present invention controls as follows so as to construct a large scale network that can connect many enterprises (contractors). Concretely, each PE of the backbone network does not learn any such transfer information as an output line number, a tunnel LSP, a VC LSP, etc. corresponding to each MAC address. Instead, a node located in the upstream of the PE adds information equivalent to such the transfer information to transmit each frame. This added information includes a line to which the frame is transferred by a PE located at the inlet of the backbone network, a tunnel SP, VC LSP information, a line to which the frame is transferred by a PE located at the outlet of the backbone network. Each PE transfers frames according to this information.  
           [0026]    For example, if a frame is to be transferred from the LAN-B1 terminal T2 to the LAN-B2 terminal T7, the frame will be transferred as follows. The ME located in the upstream of both PE1 and PE3, when receiving a frame, writes two pieces of information in the frame; the information for specifying that the PE1 uses both T-LSP2 and VC-LSP2 in a line connected to the PC2 and the information for specifying that the PE3 uses a line connected to the MAN-3 for transferring the frame. Then, the ME transmits the frame to the destination.  
           [0027]    A node required to store information corresponding to each MAC address cannot have so many enterprises (contractors), since it comes to be located at the side of the network edge. This is why such a node is just required to store only the information corresponding to the MAC address of each terminal of those less enterprises. In the above example, the ME2 stores information related to only the terminals of the enterprise B, so that the ME2 comes to store information corresponding to the MAC addresses of the terminals (T2, T5 to T8, and T11). Consequently, the capacity of the table for storing such the information does not prevent the increase of the number of contracted enterprises; the network that employs the present invention will thus be able to cope with many more enterprises (contractors) than any other conventional networks.  
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]    [0028]FIG. 1 is a block diagram of a network to which the frame transfer method of the present invention applies;  
         [0029]    [0029]FIG. 2 is a block diagram of a network to which a conventional frame transfer method applies;  
         [0030]    [0030]FIG. 3 is a format of the DIX Ethernet frames transmitted by a terminal T2;  
         [0031]    [0031]FIG. 4 is a chart for describing a table  1500  formed in a node ME2;  
         [0032]    [0032]FIG. 5 is a chart for describing a table  1000  formed in a node ME2;  
         [0033]    [0033]FIG. 6 is a format of frames handled in a MAN;  
         [0034]    [0034]FIG. 7 is a chart for describing a table  1100  formed in a node MC;  
         [0035]    [0035]FIG. 8 is a chart for describing a table  1200  formed in a node PE1;  
         [0036]    [0036]FIG. 9 is a format of frames handled in a backbone network;  
         [0037]    [0037]FIG. 10 is a format of a tunnel shim header  446 ;  
         [0038]    [0038]FIG. 11 is a format of a VC shim header  447 ;  
         [0039]    [0039]FIG. 12 is a chart for describing a table  2400  formed in a node PE3;  
         [0040]    [0040]FIG. 13 is a chart for describing a table  1300  formed in each of nodes MC and ME2;  
         [0041]    [0041]FIG. 14 is a format of frames handled in a MAN;  
         [0042]    [0042]FIG. 15 is a format of frames handled in the backbone network;  
         [0043]    [0043]FIG. 16 is a block diagram of a major portion of the node ME2;  
         [0044]    [0044]FIG. 17 is a block diagram of a header process unit  1700  provided in the ME2;  
         [0045]    [0045]FIG. 18 is a format of frames handled in the node ME2;  
         [0046]    [0046]FIG. 19 is another format of frames handled in the node ME2;  
         [0047]    [0047]FIG. 20 is a block diagram of a major portion of each of the nodes PE1 and PE3;  
         [0048]    [0048]FIG. 21 is a block diagram of a header process unit provided in the node PE1;  
         [0049]    [0049]FIG. 22 is a format of frames handled in each of the nodes PE1 and PE3; and  
         [0050]    [0050]FIG. 23 is another format of frames handled in each of the nodes PE1 and PE3. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0051]    Next, an preferred embodiment of the present invention will be described with reference to the accompanying drawings.  
         [0052]    [0052]FIG. 1 shows a block diagram of a network to which the frame transfer method of the present invention can apply.  
         [0053]    The network shown in FIG. 1 realizes VPN-A to C (VPN: (Virtual Private Network, A to C: enterprises A to C) in the VPN service. The VPN-A to C are connected to one another through a backbone network and a plurality of MANs (Metropolitan Area Network) 1 to 6. The VPN-A is configured by site LANs (Local Area Network) A1 and A2, the VPN-B is configured by site LANs B1 to B4, and the VPN-C is configured by site LANs C1 and C2 respectively. Each of the LANs is configured by a CE (Customer Edge Node) used to connect the LAN to a MAN and one or more terminals T (T: Terminal). A MAN used to transfer frames between each LAN and the backbone network is configured by an ME (MAN Edge Node) located at the edge and an MC (MAN Core Node) located at the core of the network. The backbone network connected to the MAN is configured by PEs (Provider Edge Nodes) 1 to 3 and PCs (Provider Core Nodes) 1 to 3 located at the core.  
         [0054]    In the backbone network are formed a plurality of tunnel LSPs (LSP: Label Switching Path). In each of those tunnel LSPs, a T-LSP1 is formed so as to transfer frames in the direction of PE1-&gt;PC1-&gt;and PE2 while a T-LSP3 is formed so as to transfer frames in the opposite direction. In addition, a T-LSP2 is formed so as to transfer frames in the direction of PE1-&gt;PC2-&gt;PC3-&gt;PE3 and a T-LSP4 is formed so as to transfer frames in the opposite direction. In the T-LSP1 is formed a VC-LSP-B1, which is used to transfer frames from the LAN-B1 to the LAN-B2, as well as a VC-LSP-B3 used to transfer frames in the opposite direction. And, in the T-LSP2 are formed a VC-LSP-B2 used to transfer frames from the LAN-B1 to the LAN-B3 and B4, as well as a VC-LSP-B4 used to transfer frames in the opposite direction. In the tunnel LSP is also formed some other LSPs used for communications among the sites of the enterprise A, among the sites of the enterprise C, and between PE2 and PE3, although they are not shown here.  
         [0055]    When any of the conventional techniques  3  and  4  described above is employed for the backbone network, the PE1 is required to store line numbers, tunnel labels, and VC labels corresponding to the MAC addresses of the terminals T4 to T11, as well as line numbers corresponding to the MAC addresses of the terminals T1 to T3. Concretely, the PE1 of the backbone network is required to learn and store such transfer information as tunnel labels, VC labels, or line numbers corresponding to the MAC addresses of the terminals T1 to T11 of all the contracted enterprises. However, the table provided in the PE to store such the transfer information is limited in capacity. The table thus becomes a bottleneck sometimes in each network that employs any of the conventional techniques  3  and  4 , so that it might be impossible to store many contracted enterprises in the table.  
         [0056]    On the other hand, in any network that employs the frame transfer method of the present invention, the PE of the backbone network is not required to learn such transfer information as output line numbers, tunnel LSPs, VC LSPs corresponding to the MAC addresses. A node located in the upstream of the PE adds information equivalent to such the transfer information to each frame to be transmitted. This added information consists of such information as line, tunnel LSP, and VC LSP used by the PE located at the inlet of the backbone network, as well as the subject frame that stores information of the line number to which the frame is to be transferred by the PE located at the outlet of the backbone network. Each PE transfers each frame according to this information.  
         [0057]    In the frame transfer method of the present invention, each node that stores information corresponding to the MAC address set in each frame is located on the edge of the network. Therefore it does not need to store so many contracted enterprises. Because such the node is just required to store information corresponding to the MAC addresses of not so many terminals of each contracted enterprise, the capacity of the table for storing such the information will thus not prevent the number of contracted enterprises from increasing.  
         [0058]    Concretely, when the ME2 transfers a frame to the terminal T7 of the LAN-B3, the ME2 instructs the PE1 to specify lines connected to the PC2, the LSP-B2, and the T-LSP2. The ME2 also instructs the PE3 to specify a line connected to the MAN-3. At this time, the ME2 is just required to store the LSP selection information and the output line selection information as transfer information related to the terminals (T2, T5, T6 to T8, and T11) of the enterprise B; the ME2 is not required to store any transfer information related to the terminals of the enterprises A and C.  
         [0059]    Next, a description will be made for the operation of each node when the terminal T2 of LAN-B1 transfers frames addressed to the terminal T7 of LAN-B3 with use of the frame transfer method of the present invention.  
         [0060]    [0060]FIG. 3 shows a format of DIX Ethernet II frames transmitted by the terminal T2.  
         [0061]    The DIX Ethernet II frame format consists of a header part  410 , a data part  420 , and an FCS part  430 .  
         [0062]    The header part consists of fields of preamble  411 , SFD (Start of Frame Delimiter)  412 , source MAC address (SMAC: Source MAC)  413 , destination MAC address (DMAC: Destination MAC)  414 , and type  415 . The preamble field  411  includes information for enabling a frame receiving device to find the start of a frame and the SFD field includes information for denoting the start of the frame. In those fields, hexadecimal values “01010101” and “AB” are set respectively. The SMAC field  413  sets the source address of the frame while the DMAC field  414  sets the destination address of the frame. The type  415  denotes a protocol of the network layer stored in the data part  420 . For example, “0800” (HEX) denotes that the received frame is a Novell NetWare frame. The data part  420  consists of fields of data  421  and padding  422 . The padding  422  fills the space of the frame so that the frame becomes at least 64 bytes in full data length. The FCS  430  part has an FCS field  431 . A device, when receiving a frame, checks this FCS field  431  to decide the validity/invalidity of the frame.  
         [0063]    The ME2, when receiving a frame addressed to the terminal T7 from the terminal T2, identifies that the frame belongs to the enterprise B according to the line number of the line (hereinafter, referred to as the input line number), through which the frame is received. This enterprise identification by the ME2 is realized by referring to a table  1500  (FIG. 4) provided in the ME2 to read the VLAN ID  1501 - i  set in each entry therein according to the input line number written in the frame. The table  1500  stores the VLAN ID, which is an enterprise identifier set for each input line number.  
         [0064]    The ME2 then decides a target output line (hereinafter, to be referred to as an output line number) from which the frame is to be output and the destination site information according to the DMAC  414 . This decision of the output line number and the destination site information is realized by referring to a table  1000  (FIG. 5) that stores both output line number and destination site information in correspondence with the MAC address of each terminal.  
         [0065]    Concretely, the ME2 reads a plurality of entries  1010 - i  one by one from the table  1000  and compares the DMAC  414  set in the header part  410  of the frame with the MAC address  1002 - i  set in each entry to decide the line number  1001 - i  and the destination site information  1003 - i  set in the “matching” entry  1010 - i  as both target line number and destination site information. This destination site information (two bits) consists of single-bit LSP selection information  1013 - i  used to decide a target LSP at the inlet PE1 of the backbone network and single-bit output line selection information  1023 - i  used to decide an output line at the outlet PE3 of the backbone network.  
         [0066]    The ME2 then adds a header to the frame and transmits the frame to the MC (MAN Core). The added header includes the destination site information bit for denoting whether or not the destination site information  1003 - i  is valid. The destination site information  1003 - i  consists of determined enterprise information (VLAN ID) and destination site information  1003 - i . This header may be a VLAN Tag described in the IEEE 802.1Q.  
         [0067]    [0067]FIG. 6 shows a format of frames transmitted from the ME2 and handled in the MAN-1 after a VLAN Tag is added to each of the frames. In the frame format shown in FIG. 6, a VLAN Tag  416  is inserted between the SMAC  413  and the type  415  in the header part in the frame format shown in FIG. 3.  
         [0068]    The TPID (Tag Protocol Identifier)  501  set in the VLAN Tag  416  is used for the Token Ring, FDDI, etc. When it is used by the Ethernet (trademark), it is represented as “8100” in hexadecimal. The CFI (Canonical Format Indicator)  503  is single-bit information used for the Token Ring communication. The UP (User Priority)  502  is 3-bit information denoting a transfer priority level. In this embodiment, this UP  502  is used as LSP selection information  505  (1 bit) for storing LSP selection information, the output line selection information  506  (1 bit) for storing output line selection information, and the destination site information bit  507  for denoting valid/invalid of both of the LSP selection information  505  and the output line selection information  506  (1 bit). The VLAN ID  504  is an identifier of a VLAN (Virtual LAN). In this embodiment, it is used as an enterprise (VPN) identifier. The PE1 writes the LSP selection information  1013 - i , the output line selection information  1023 - i , and “1” (valid) in the LSP selection information  505 , the output line selection information  506 , and the destination site information bit  507  of the UP  502  respectively and writes the VLAN ID  1501  corresponding to the enterprise B in the VLAN ID  504 .  
         [0069]    The terminals T2 or CE2 may be configured so that the information of the enterprise B is written in the VLAN ID  504  of the VLAN Tag  416  in each frame to be transmitted. In this connection, the ME2 adds none of the enterprise identifier and the VLAN Tag  416  to the frame.  
         [0070]    The MC in the MAN-1, when receiving such a frame, decides a target output line number according to the DMAC  414  set in the frame and transfers the frame to the output line. The ME3 transfers frames similarly. Such the output line decision by the MC or ME3 is realized by referring to a table  1100  (FIG. 7) that stores a plurality of entries  1100 - i , each storing a line number  1101 - i  and a MAC address  1102 - i . The MC or ME3 reads those entries  1110 - i  one by one from the table  1100  and compares the MAC address  1102 - i  in each of the entries  1110 - i  with the DMAC  414  set in the header part  510  to decide the line number  1101 - i  in the “matching” entry  1110 - i  as the target output line number.  
         [0071]    The PE1, when receiving a frame through the MC or ME3, identifies the enterprise to which the frame belongs according to the VLAN ID  504  set in the header part  510  in the frame to decide that it is the enterprise B. Then, the PE1 decides one or more sets, each consisting-of an output line number, a VC LSP, and a tunnel LSP. The PE1 also selects one of those sets according to the LSP selection information  505  set in the UP  502  of the header part  510 . In this embodiment, the PE1 selects the set 1 consisting of the line numbers of the lines to the PC2, a VC-LSP-B2, and the T-LSP2, as well as the set 2 consisting of line numbers of the lines to the PC1, the VC-LSP-B1, and the T-LSP1 according to the VLAN ID  504 , then decides the set 1 according to the LSP selection information  505  as the information used for transferring the frame.  
         [0072]    This decision is realized by, for example, referring to a table  1200  (FIG. 8) that stores a plurality of entries  1210 - i . The PE1 reads those entries  1210 - i  one by one from the table  1200  and compares the information written in the frame with that set in each entry so that the VLAN ID  504  set in the header part  510  of the frame is compared with the VLAN ID  1201 - i  set in each entry  1210 - i  and the LSP selection information  505  set in the header part  510  of the frame is compared with the LSP selection information  1202 - i  set in each entry respectively. The PET then decides the line number  1204 - i  as the target output line number, the tunnel label  1205 - i  as the target tunnel label and the VC label  1206 - i  as the target VC label, set in the “matching” entry  1210 - i  respectively.  
         [0073]    The PE1 then adds the values of both tunnel label  1205 - i  and VC label  1206 - i  to the frame to be transmitted to the backbone network.  
         [0074]    [0074]FIG. 9 shows a format of the frames handled in the backbone network, transmitted by the PE1 after the header information related to both tunnel label and VC label are added to each of the frames.  
         [0075]    In the frame format shown in FIG. 9, a capsule header part  740  is added to the frame and the fields of the preamble  411  and the SFD  412  are deleted from the header part  510  of the frame format shown in FIG. 6, thereby forming the new header part  710 . The capsule header part  740  consists of the same fields  441  to  445  as those of the header part  510  (FIG. 6), as well as a tunnel shim header  446 , and a VC shim header  447 .  
         [0076]    [0076]FIG. 10 shows the tunnel shim header  446  formatted as described in the RFC  3032  and FIG. 11 shows the VC shim header  447  formatted as described in the RFC  3032 .  
         [0077]    The tunnel shim header  446  consists of fields of tunnel label  801 , experimental tunnel EXP  802 , tunnel S bit  803 , and tunnel TTL (Time to Live)  804 .  
         [0078]    Similarly, the VC shim header  446  consists of fields of VC label  901 , 3-bit VC EXP  902 , VC S bit  903 , and VC TTL  904 . In this embodiment, the lower one bit of the VC EXP  902  is used for the output line selection information  905  and the upper second bit is used for the VC EXP information bit  906  to be set for denoting valid/invalid of the output line selection information  905 . The MSB  907  is not used. The PET stores the information of the tunnel label  1205 - i  and the VC label  1206 - i  decided above in the tunnel label  801  and in the VC label  901  respectively.  
         [0079]    Finally, the PET writes the value of the output line selection information  506  (one bit) of the UP  502  in the output line selection information  905  of the VC EXP  902  so as to notify the PE3 of the output line selection information, then writes “1” (valid) in the VC EXP information bit  906 . After this, the PET transmits the frame to the line corresponding to the line number  1204 - i.    
         [0080]    The PC2 transfers the frame to the PC3 according to the tunnel label  801 , then updates the tunnel label  801 . Similarly, the PC3 transfers the frame to the PC3 according to the tunnel label  801 . The PC3 may delete the tunnel shim header  446  at this time. When the header  446  is deleted, transmission of unnecessary information is prevented, thereby the network band can be used more efficiently.  
         [0081]    The PE3, when receiving this frame, identifies the enterprise to which the frame belongs according to both the input line number and the VC label  901  to decide one or more target line numbers (a line to MAN-3 and a line to MAN-4 in this embodiment). The PE3 also decides the line number of the line to MAN-3 as the target output line number according to the output line selection information  905  set in the VC EXP  902 .  
         [0082]    The output line decision by the PE3 is realized by referring to a table  2400  (FIG. 12) that stores a plurality of entries  2410 - i , each storing an input line number  2401 - i , a VC label  2402 - i , a VC EXP  2403 - i , and an output line number  2404 - i . Concretely, the PE3 reads those entries  2410 - i  one by one from the table  2400  and compares the information written in the frame with that set in each entry  2410 - i  so that the input line number in the frame is compared with the input line number set in each read entry  2410 - i  and the VC label  901  set in the capsule header part  740  of the frame with the VC label  2402 - i  set in each entry, the output line selection information  905  set in the VC EXP  902  of the frame is compared with the output line selection information  2406 - i  set in the VC EXP  3403 - i  in each entry  2410 - i  to decide the output line number  2404 - i  in the “matching” entry as the target output line number.  
         [0083]    The 3-bit VC EXP  2403 - i  consists of the output line selection information  2406 - i  (1 bit), the VC EXP information bit  2407 - i  (1 bit) denoting valid/invalid of the VC EXP  2403 - i , a non-used bit  2408 - i  (1 bit). The value in this VC EXP information bit  2407 - i  is fixed at “1”.  
         [0084]    After this, the PE3 deletes the capsule header part  740  (FIG. 9) from the frame and adds the preamble  411  and the SFD  412  to the header part of the frame, thereby the frame is formatted as shown in FIG. 6 and the frame is transmitted to the line corresponding to the output line number  2404 - i.    
         [0085]    Each node in the MAN-3 decides the target output line number according to the DMAC  414  set in the header part  510  to transfer the frame to the LAN-B3 similarly to the MC in the MAN-1.  
         [0086]    As described above, because both PE1 and PE3 are not required to store information corresponding to the MAC address of each terminal, the table for storing such the information will not prevent the network from expanding in scale.  
         [0087]    The information corresponding to the MAC address of each terminal may be set in the tables  1000  and  1100  from the administration terminal connected to each node. When there are many terminals T and such terminals T are often added/deleted to/from the network, such the information should be set in the tables  1000  and  1100  automatically. This auto setting of such the information is realized by making each node perform flooding, notifying, and learning operations. Hereinafter, these three operations will be described.  
         [0088]    [Flooding] 
         [0089]    If no entry  1010 - i  is set in the table  1000  (FIG. 5) formed in the ME2 nor in the table  1100  (FIG. 7) formed in the MC in correspondence with the DMAC  414  set in a frame transmitted from the T2 to the ME2, each node in the network transmits the frame to all the terminals T of the same contractor (which, in the present embodiment, refers to an enterprise to which same VLAN ID is assigned).  
         [0090]    Each node in a MAN decides one or more output line numbers to which the frame is to be transmitted according to the VLAN ID. Here, the MC in the MAN-1 is picked up as an example. Because only the LAN-A1 and the LAN-B1 are connected to the MAN-1, the MC is just required to transmit the frames of enterprises A and B; it is not required to transmit the frames of the enterprise C. To transfer a frame of the enterprise A, therefore, the MC sets a line number connected to the ME1 for transferring the frame to the LAN-A1 and a line number connected to the ME3 for transferring the frame to the LAN-A2 according to the VLAN-A2 of the enterprise A respectively. Similarly, to transfer a frame of the enterprise B, the MC sets a line number connected to the ME2 for transferring the frame to the LAN-B1 and a line number connected to the ME3 for transferring the frame to the LAN-B2 and LAN-B3 according to the VLAN ID of the enterprise B respectively. And, to realize such the operations, the MC refers to a table  1300  (FIG. 13). The table  1300  is used for flooding operation and provided with a bit map  1310 - i  prepared for each VLAN ID. Frame output YES/NO information is set in the output line VLDj field  130   j - i  located in the bit map  1310 - i  with respect to each output line j.  
         [0091]    At first, the flooding operation of the ME2 will be described. The ME2, when receiving a frame from the terminal T2, refer to the above table  1500  ((FIG. 4) that stores a VLAN ID, which is an enterprise identifier, in correspondence with each input line number) to decide the VLAN ID. Then, the ME2 refer to the table  1000  ((FIG. 5) that stores both output line number and destination site information in correspondence with each MAC address). When the table  1000  includes no entry  1010 - i  corresponding to the DMAC  414  set in the frame, the ME2 reads the bit map  1310 - i  from the table  1300 , corresponding to the VLAN ID of the enterprise B so as to perform a flooding operation. This bit map  1310 - i  stores data set so as to output the frame to a line connected to the MC and a line to the CE2 according to the VLAN ID of the enterprise B respectively. However, because there is no need to transmit the frame to the input line at this time, the ME2 decides that only the line to the MC is the target output line. And, because the ME2 cannot obtain no destination site information at this time, the ME2 writes “0” (invalid) in the destination site information bit  502 , then transmits the frame to the MC.  
         [0092]    Next, the flooding operation by the MC will be described. The MC, when receiving a frame from the terminal T2, refer to the table  1100  ((FIG. 7) that stores a MAC address set in correspondence with each line number) similarly to the ME2. When the table  1100  includes no entry  1110  corresponding to the DMAC  414 , the MC reads the bit map  1310 - i  from the table  1100 , corresponding to the VLAN ID  504  of the enterprise so as to perform the flooding operation. Because no terminal of the enterprise B is connected to any of the ME1 and the ME4, this bit map  1310 - i  stores data needed to output the frame just to a line to the ME2 and a line to the ME3 according to the VLAN ID of the enterprise B. However, because there is no need to transmit the frame to the input line here, the MC decides that only the line to the ME3 is the target output line and transmits the frame to the ME3.  
         [0093]    The ME3, when receiving a frame from the terminal T2, also performs the flooding operation similarly.  
         [0094]    Next, the flooding operation by the PE1 will be described. The PE1, when receiving a frame from the terminal T2, identifies “0” (invalid) set in the destination site information bit  507  of the UP  502 , thereby the PE1 performs a flooding operation. In this flooding operation, the PE1 transfers a copy of the frame to each of the output lines and LSPs connected to the sites of the target enterprise (enterprise B in this example). This decision of all the output lines and LSPs by the PE1 is realized by, for example, masking the LSP selection information  1202 - i  (regardless whether or not the “matching” is detected with respect to LSP selection information  1202 - i ) and referring to a table  1200  ((FIG. 8) that stores a plurality of entries, each storing a line number, a tunnel label, and a VC label). Concretely, the PE1 reads those entries  1210 - i  one by one from the table  1200  and compares the information written in the frame with that set in each entry so that the VLAN ID  504  set in the header part  510  of the frame is compared with the VLAN ID  1201 - i  set in each entry. The PE1 decides so that the frame is transmitted to the output line and the LSP specified by a set of a line number  1204 - i , a tunnel label  1205 - i , and a VC label  1206 - i  set in every VLAN-ID-matching entry  1210 - i , thereby transferring the frame to the decided output line. At this time, the PE1 writes “0” (invalid) in the VC EXP information bit  906  of the VC EXP  902 .  
         [0095]    Next, the flooding operation by the PE3 will be described. The PE3, when receiving a frame in which the VC EXP information bit  906  “0” is set in the VC EXP field  902 , begins a flooding operation. In this flooding operation, the PE3 identifies the enterprise to which the frame belongs according to the input line number and the VC label  901  set in the frame and decides one or more target output line numbers, then transmits a copy of the frame to all the lines corresponding to those output line numbers.  
         [0096]    For example, this decision of the target output line numbers is realized by referring to the table  2400  ((FIG. 12) that stores a plurality of entries, each storing an output line number) by masking the VC EXP  2403 - i  (regardless whether or not “matching” is detected with respect to the VC EXP  2403 - i ). Concretely, the PE3 reads those entries  2410 - i  one by one from the table  2400  to compare the information written in the frame with that set in each entry  2410 - i  so that the input line number written in the frame is compared with the input line number  2401 - i  in each entry and the VC label  901  set in the capsule header part  740  of the frame is compared with the VC label  2402 - i  set in each entry. The PE3 then decides the output line numbers  2404 - i  set in all the VC-label-“matching” entries  2401 - i  (line numbers of the lines to MAN-3 and MAN-4 in this embodiment) as the target output line numbers and transfer the frame to all the decided lines.  
         [0097]    [Notifying Operation] 
         [0098]    Next, the notifying operation for notifying the object of destination site information will be described.  
         [0099]    The PE3, when transferring a frame addressed to the terminal T7 to the terminal T2, writes the output line selection information used to transfer the frame to the terminal T7 in the frame. The ME2 stores this output line selection information corresponding to the MAC address of the terminal TV through a learning operation to be described later.  
         [0100]    For example, the decision of this output line selection information is realized by referring to the table  2400  ((FIG. 12) that stores a plurality of entries  2410 - i , each storing an output line number). Concretely, the PE3 reads those entries  2410 - i  one by one from the table  2400  to compare the information written in the frame with that set in each entry  2410 - i  so that the input line number written in the frame is compared with the input line number  2401 - i  set in each entry, the VC label corresponding to the VC-LSP-B2 used for the frame transfer in the opposite direction of the VC-LSP-B4 is compared with the VC label  2402 - i  set in each entry, and the output line number used for the frame transfer is compared with the input line number  2401 - i  set in each entry to write the output line selection information  2406 - i  obtained from the “matching” entry  2410 - i  in the output line selection information field  506  of the UP  502  of the frame.  
         [0101]    On the other hand, the PE1, when transferring a frame addressed to the terminal T7 to the terminal T2, writes the LSP selection information used for the frame transfer (LSP selection information corresponding to the line number of a line connected to PC2, T-LSP2 and VC-LSP-B2) in the frame to be transferred to the terminal T2 through the terminal TV. The ME2 stores this LSP selection information in correspondence with the MAC address of the terminal T7 through a learning operation to be described later.  
         [0102]    The decision of this LSP selection information is realized, for example, by referring to the table  2400  (FIG. 12). Concretely, the PE1 reads those entries  2410 - i  one by one from the table  2400  to compare the information written in the frame with that set in each entry  2410 - i  so that the input line number written in the frame is compared with the output line number  2404 - i  set in each entry and the VC label corresponding to the VC-LSP-B2 is compared with the VC label  2402 - i  set in each entry, then writes the LSP selection information  2405 - i  (1 bit) obtained from the “matching” entry in the LSP selection information  506  field of the frame.  
         [0103]    [Learning Operation] 
         [0104]    It should be avoided to always perform a flooding operation. Otherwise, the line bandwidth cannot be used efficiently. The MC thus performs a learning operation so as to store an input line number corresponding to the source MAC address set in each inputted frame. On the other hand, the ME performs a learning operation so as to store destination site information notified by the above notifying operation.  
         [0105]    The MC, when receiving a frame, reads the entries  1110 - i  one by one from the table  1100  (FIG. 7)) that stores a MAC address in correspondence with each line number) to compare the information written in the frame with that set in each entry  1110 - i  so that the input line number written in the frame is compared with the line number  1101 - i  set in each entry and the SMAC  413  written in the frame is compared with the MAC address  1102 - i  set in each entry. When there is no “matching” entry  1110 - i  found in the comparison, the MC registers the input line number and the SMAC  414  written in the frame as new items  1101 - i  and  1102 - i  in an entry  1110 - i  to be set in the table  1100 .  
         [0106]    Similarly, the ME2, when receiving a frame from the MC, reads the entries  1010 - i  one by one from the table  1000  ((FIG. 5)) that stores both output line number and destination site information in correspondence with each MAC address) to compare the information written in the frame with that set in each entry  1010 - i  so that the input line number in the frame is compared with the line number  1001 - i  set in each entry, the SMAC  413  written in the frame is compared with the MAC address  1002 - i  set in each entry, the LSP selection information  505  written by the PE1 and output line selection information  506  written by the PE3 in the frame are compared with LSP selection information  1013 - i  and output line selection information  1023 - i  in the destination site information  1003 - i  set in each entry. And, when there is no “matching” entry  1010 - i  found in the comparison, the ME2 writes the items input line number of the frame,  413 ,  506 , and  505  specified in the frame as a line number  1001 - i , a MAC address  1002 - i , output line selection information  1023 - i , and LSP selection information  1013 - i  that are all set in an entry  1010 - i  to be registered in the table  1000 . The PE in the backbone network is not required to transfer any frame according to the DMAC  414 , so that it does not perform such the learning operation.  
         [0107]    While a description has been made for a case in which the ME2 maps destination site information in the UP  502  and the PE1 maps output line selection information in the VC EXP  902 , the fields of the UP  502  and VC EXP  902  might come to be too small in capacity to map destination site information and output line selection information as described above when the subject enterprise has many sites connected over many MANs. This is because the UP  502  and the VC EXP  902  are as small as 3 bits in length. In such a case, the ME2 can add one more VLAN Tag and write destination site information (LSP selection information and output line selection information) in this VLAN ID  604  (12 bits). FIG. 14 shows such a format of the frames to be transmitted from the ME2. Unlike the frame format shown in FIG. 6, the frame format shown in FIG. 14 has a plurality of VLAN Tags  416  and  417 . In FIG. 14, the VLAN Tag  417  is a new field added as described above.  
         [0108]    Similarly, the PE1 can add one more shim header to the frame so as to write output line selection information therein. FIG. 15 shows such a format of the frames to be transmitted from the PET. Unlike the frame format shown in FIG. 9, the frame format shown in FIG. 15 has three shim headers. In other words, an extension shim header  448  is newly added to the frame format.  
         [0109]    Each node in the network operates in correspondence with such the header configuration.  
         [0110]    [Node: ME] 
         [0111]    Next, a description will be made for the operation by the ME used in a network of the present invention with reference to FIGS. 16 and 17. FIG. 16 shows a block diagram of a major portion of the ME2. FIG. 17 shows a block diagram of a header process unit  1700 .  
         [0112]    In the embodiment to be described below, the LAN-B1 terminal T2 transfers frames to the LAN-B3 terminal T7 and performs the flooding operation.  
         [0113]    As shown in FIG. 16, the ME2 is configured by a received frame process unit  1602 - j  provided to cope with a plurality of input lines  1601 - j  (j=1 to M) to which frames are inputted, a transmit frame process unit  1604 - j  provided to cope with a plurality of output lines  1605 - j  (j=1 to M) from which frames are output, a header process unit  1700  used to process the header part of each inputted frame, and a frame switch  1603  used to switch frames among output lines. This header process unit  1700  analyzes the header of each frame to decide the frame input enterprise (VLAN ID), the output line number, and the destination site information. The frame switch  1603  switches frames among output lines according to the output line number decided by the header process unit  1700 .  
         [0114]    [Transfer and Flooding Operations by ME2] 
         [0115]    At first, a description will be made for a case in which the ME2 receives a frame from the LAN-B1 CE2, then transmits the frame to the MC.  
         [0116]    [0116]FIG. 18 shows a format of the frames handled in the ME2 in this connection. Unlike the frame format shown in FIG. 3, the frame format shown in FIG. 18 has an internal header part  1840  added newly thereto and both of the preamble  411  and the SFD  412  are deleted therefrom, thereby forming the new header part  1810 . This internal header part  1840  consists of fields of input line number  1841 , output line number  1842 , destination site information  1843  (consisting of fields of LSP selection information  1846  and output line selection information  1847 ), destination site information bit  1845  describing valid/invalid of the field  1843 , and VLAN ID  1844 .  
         [0117]    The received frame process unit  1602 - j , when receiving a frame through an input line  1601 - j , deletes both preamble  411  and SFD  412  from the frame and adds the internal header part  1840  to the frame, then writes the identifier “j” of the frame input line  1601 - j  in the input line number field  1841 . Then, the received frame process unit  1602 - j  stores the frame once therein and transmits the frame header information FH-j consisting of the internal header part  1840  and the header part  1810  to the header process unit  1700 . The values of the output line number  1842 , the destination site information  1843 , the destination site information bit  1845 , and the VLAN ID  1844  set in the frame header information FH-j transmitted to the header part process unit  1700  are all meaningless.  
         [0118]    The header process unit  1700  decides the enterprise (VLAN ID) that has transmitted the frame, the output line number, and the destination site information (2 bits of LSP selection information and output line selection information) with reference to the tables  1500  and  1000  (FIGS. 4 and 5), then transmits the decided information to the received frame process unit  1602 - j  as destination information DI-j. The detail operation of the header process unit  1700  is described later.  
         [0119]    The received frame process unit  1602 , when receiving destination information DI-j, writes the information decided by the header process unit  1700  in the internal header part  1840  of the frame. In other words, the received frame process unit  1602  writes the VLAN ID of the destination information DI-j in the VLAN ID  1844  of the internal header part  1840 , the output line number is written in the output line number  1842 , the destination site information is written in the destination site information  1843 , and the destination site information bit is written in the destination site information bit  1845  respectively. Then, the received frame process unit  1602  transmits the frame to the frame switch  1603 . The received frame process unit  1602 , when receiving a plurality of pieces of destination information DI-j addressed to one frame, copies the frame and transmits a copy of the frame to the frame switch  1603 . At this time, at least one of the VLAN-ID  1844 , the output line number  1842 , and the destination site information  1843  must be different from the original one set in the internal header part  1840 .  
         [0120]    The frame switch  1603  then transmits the frame to the transmit frame process unit  1604 - j  corresponding to the output line number  1842 . The transmit frame process unit  1604 - j  deletes the internal header part  1840  from and adds the preamble  411 , the SFD  412 , and the VLAN Tag  416  to the frame, thereby the frame format is updated as shown in FIG. 6. In other words, the process unit  1604 - j  writes the value of the VLAN ID  1844  in the VLAN ID  504  of the VLAN Tag  416 , the LSP selection information of the destination site information  1843  in the LSP selection information  505  of the UP  502 , the output line selection information  1847  of the destination site information  1843  in the output line selection information  506  of the UP  502 , and the destination site information bit  1845  in the destination site information bit  507  respectively to change the frame format. The frame is then transmitted to the MC.  
         [0121]    Next, the operation by the header process unit  1700  will be described with reference to FIG. 17.  
         [0122]    The header process unit  1700 , when receiving frame header information FH-j from the received frame process unit  1602 - j , stores the frame header information FH with the frame header information storage. The frame header information FH is obtained by multiplexing a plurality of pieces of information FH-j through a multiplexer  1740 .  
         [0123]    A table access means  1721  of the VLAN ID decision unit  1720  reads an entry  1501 - i  corresponding to the input line number stored in the memory  1760  from the table  1500  (FIG. 4) to decide the VLAN ID information, then transmits the decision result VI to both of the results output unit  1750  and the table access means  1713 .  
         [0124]    The destination information decision unit  1710  refer to the table  1000  (FIG. 5) to decide both the output line number and the destination site information (LSP selection information and output line selection information) corresponding to the DMAC  414  and transmits the destination result (information DI) to the results output unit  1750 .  
         [0125]    More concretely, the table access means  1711  of the destination information decision unit  1710 , when the frame header information FH is stored in the frame header information storage  1760 , reads the entries  1010 - i  one by one from the table  1000  and transmits the read entries  1010 - i  to the comparator  1712 . The comparator  1712  compares the information written in the frame with that set in each entry  1010 - i  so that the DMAC  414  stored in the frame header information storage  1760  is compared with the MAC address  1002 - i  set in each entry  1010 - i  and transmits the result to the table access means  1711 . This comparison is repeated until it is completed for all the entries  1010 - i  in the table  1000 . Each time a “matching” entry is detected in the comparison, the “matching” denoting information is transmitted to the destination information decision circuit  1714  together with the line number  1001 - i  and the destination site information  1003 - i  set in the entry  1010 - i . On the other hand, the table access means  1713  reads the bit map  1310 - i  stored in the table  1300  (FIG. 13) corresponding to the VLAN ID information VI decided by the VLAN ID decision unit  1720  and used for the flooding operation, then transmits the result to the destination information decision circuit  1714 .  
         [0126]    Receiving each “matching” denoting information from the table access means  1711 , the destination information decision circuit  1714  transmits the destination information DI to the results output unit  1750 . In this information DI, the line number  1001 - i , the destination site information  1003 - i , and the destination site information bit “1” are set. When receiving no “matching” information, the destination information decision circuit  1714  transmits the destination information DI to the results output unit  1750 . The information DI includes an output line number obtained by encoding the bit map  1310 - i  used for flooding operation, which is received from the table access means  1713 , the destination site information “00”, and destination site information bit “0”. At this time, the destination information decision circuit  1714  does not transmit the destination information DI with respect to the bit corresponding to the input line number  1814  stored in the frame header information storage  1760 . When the bit map is described so as to transmit the frame to a plurality of output lines  1605 - j , the destination information decision circuit  1714  transmits a plurality of pieces of the destination information DI to the results output unit  1750 .  
         [0127]    Each time receiving destination information DI, the results output unit  1750  transmits the values of the destination information DI and the VLAN ID as the destination information VI DI-j to the received frame process unit  1602 - j  corresponding to the input line number  1841  stored in the frame header information storage  1760 . And, because the value of the VLAN ID information VI is decided by an input line number, the same value is always set in the plurality of pieces of the destination information DI-j.  
         [0128]    While a description has been made so far for a case in which the ME2 recognizes the enterprise B and writes this information in the VLAN ID  504 , the terminal T2 and the CE2 may also write the information of the enterprise B in the VLAN ID  504  to transmit frames. In this connection, the frame format in the ME2 becomes as shown in FIG. 19. At this time, the VLAN ID decision unit  1720  does not decide the VLAN ID information VI and the table access means  1713  reads the bit map  1310 - i  corresponding to the VLAN ID  504  stored in the frame header information storage  1760  and transmits the result to the destination information decision circuit  1714 . The transmit frame process unit  1604 - j  does not overwrite the information of the VLAN ID  1844  on the VLAN ID  504 .  
         [0129]    [ME2 Learning Operation] 
         [0130]    Next, a description will be made for a case in which the ME2 receives frames formatted as shown in FIG. 6 from the MC and performs the learning operation. In this connection, an internal header part  1840  is added to the format of the frames received by the ME2, thereby the frame format comes to differ from that (shown in FIG. 6) of the frames in the ME2. And, both preamble  411  and SFD  412  are deleted from the header part  510  of the frame to form a new header part  1910  (as shown in FIG. 19).  
         [0131]    At first, the operation by the header process unit  1700  will be described. The header process unit  1700 , when receiving frame header information FH-j consisting of an internal header part  1840  and a header part  1910  from the received frame process unit  1602 - j , stores the frame header information FH obtained by multiplexing a plurality of pieces of information FH-j through the multiplexer  1740  with the frame header information storage  1760 .  
         [0132]    The destination information decision unit  1710  refers to the table  1000  (FIG. 5) to check the presence of an entry  1010 - i  corresponding to the SMAC  413  written in the frame. When it is not found, the destination information decision unit  1710  learns the input line number  1841 , the LSP selection information  505  set in the UP  502 , and the output line selection information  506  corresponding to the SMAC  413 .  
         [0133]    More concretely, the table access means  1711  reads the entries  1010 - i  one by one from the table  1000  and transmits the read entries  1010 - i  to the comparator  1712 . The comparator  1712  compares the SMAC  413  stored in the frame header information storage  1760  of the frame with the MAC address  1002 - i  set in each entry  1010 - i  and transmits the result to the table access means  1711 . The table access means  1711  and the comparator  1712  repeat the above operation until the comparison is completed for all the entries  1010 - i  in the table  1000 .  
         [0134]    When a “matching” entry  1010 - i  is detected, the table access means  1711  decides that both line number and destination site information corresponding to the SMAC  413  are already stored in the table  1000 , thereby terminating the learning operation. If no “matching” entry  1010 - i  is detected, the table access means  1711  registers an entry  1010 - i  in the table  1000 . The new entry  1010 - i  includes the line number  1001 - i  as the input line number  1841  stored in the frame header information storage  1760  of the frame, the MAC address  1002 - i  as the SMAC  413  stored in the frame header information storage  1760  of the frame, the destination site information  1013 - i  of the LSP selection information  1003 - i  as the LSP selection information  505  set in the UP  502 , and the output line selection information  1023 - i  of the destination site information  1003 - i  as the output line selection information  506  set in the UP  502  respectively.  
         [0135]    [Node: PE] 
         [0136]    Next, a description will be made for the operation by the PE1/PE3 employed for the network of the present invention with reference to FIGS. 1, 15,  21 , and  20 . FIG. 20 shows a block diagram of a major portion of the PE1/PE3. FIG. 21 shows a block diagram of a header process unit  2300  (Both PE1 and PE3 are the same in configuration).  
         [0137]    In the embodiment to be described below, it is premised that transfer and flooding operations by the PE1 and PE3 for frames from the LAN-B1 terminal T2 to the LAN-B3 terminal T7 and learning operations by the PE3 and PE1 for frames from the terminal T7 to the terminal T2.  
         [0138]    As shown in FIG. 20, the PE1 is configured by a received frame process unit  2002 - k  provided to cope with a plurality of input lines  2001 - k  (k=1 to L) to which frames are inputted, a transmit frame process unit  2004 - k  provided to cope with a plurality of output lines  2005 - k  from which frames are output, a header process unit  2300  for processing the header part of each inputted frame, and a frame switch  2003  for switching frames among output lines. The header process unit  2300  analyzes the header of each frame to decide the output line number and the LSP. The frame switch  2003  switches frames among output lines according to the output line number decided by the header process unit  1700 .  
         [0139]    [Transfer and Flooding Operations by PE1] 
         [0140]    Next, a description will be made for the transfer operation by the PE1 in response to a frame received from the ME3. The format of the frames in the PE1 (shown in FIG. 22) differs from that of the frames received (shown in FIG. 6). An internal header part  2140  is added to the frame format in this case and the preamble  411  and the SFD  412  are deleted from the header part  510  of the frame format in FIG. 6 to form the new header part  2110 . This internal header part  2140  consists of fields of input line number  2141 , output line number  2142 , tunnel label information  2143 , VC label information  2144 , and 3-bit VC EXP information  2145 . This VC EXP information  2145  consists of fields of output line selection information  2147 , VC EXP information bit  2146  for setting valid/invalid of the output line selection information  2147 , and a field  2148  that is not used.  
         [0141]    The received frame process unit  2002 - k , when receiving a frame through an input line  2001 - k , deletes the preamble  411  and the SFD  412  from and adds an internal header part  2140  to the frame, then writes the identifier of the input line  2001 - k  to which the frame is inputted in the input line number field  2141  of the frame. The received frame process unit  2002 - k  then stores the frame once therein and transmits the frame header information FH- k  consisting of the internal header part  2140  and the header part  2110  to the header process unit  2300 . In the frame header information FH-k, the values set in the output line number  2142 , the tunnel label information  2143 , the VC label information  2144 , and the VC EXP information  2145  are all meaningless.  
         [0142]    The header process unit  2300  decides such target information as an output line number, a tunnel label information, a VC label information, and the VC EXP information according to the VLAN ID  504  of the UP  502  set in the frame header information FH- k  by referring to the table  1200  or  2400  (FIGS. 8 and 12), then transmits the decided information to the received frame process unit  2002 - k  as the destination information DI-k. The operation of this header process unit  2300  will be described later more in detail.  
         [0143]    Receiving the destination information DI-k, the received frame process unit  2002 - k  writes the information decided by the header process unit  2300  in the internal header part  2140  of the frame. In other words, the received frame process unit  2002 - k  writes the output line number of the destination information DI-k in the output line number field  2142 , the tunnel label information in the tunnel label information field  2143 , the VC label information in the VC label information field  2144 , and the VC EXP information in the VC EXP information field  2145  located respectively in the internal header part  2140 . The received frame process unit  2002 - k  then transmits the frame to the frame switch  2003 .  
         [0144]    The frame switch  2003  transmits the frame to the transmit frame process unit  2004 - k  corresponding to the output line number  2142 . The transmit frame process unit  2004 - k  deletes the internal header part  2140  from the frame and adds a capsule header part  740  thereto to format the frame as shown in FIG. 9. Concretely, the transmit frame process unit  2004 - k  writes the value of the tunnel label information  2143  in the tunnel label field  801  of the tunnel shim header  446 , the value of the VC label information  2144  in the VC label field  901  of the VC shim header  447  and the value of the VC EXP information  2145  in the VC EXP field  902  respectively to change the frame format. After this, the transmit frame process unit  2004 - k  transmits the frame to the next node.  
         [0145]    Next, the operation by the header process unit  2300  will be described with reference to FIG. 21.  
         [0146]    The header process unit  2300 , when receiving frame header information FH-k from the received frame process unit  2002 - k , stores the frame header information FH obtained by multiplexing a plurality of pieces of information FH-k through the multiplexer  2340  with the frame header information storage  2360 .  
         [0147]    When the ME2 completes the learning and the UP  502  has a meaningful value (“1” is set in the destination site information bit of the UP  502 ), the destination information decision unit  2310  refers to the table  1200  (FIG. 8) and transmits the output line number, the tunnel label information, the VC label information, and the VC EXP information obtained from the table in correspondence with both VLAN ID  504  and UP  502  to the destination information decision circuit  2314 . On the other hand, when the ME2 does not complete the learning yet and the UP  502  has a meaningless value (“0” is set in the destination site information bit of the UP  502 ), the destination information decision unit  2310  transmits a set of one or more output line numbers corresponding to the VLAN ID  504 , the tunnel label information, the VC label information, and the VC EXP information to the destination information decision circuit More concretely, the table access means  2311  of the destination information decision unit  2310 , when the frame header information FH is stored in the frame header information storage  2360 , reads entries  1210 - i  one by one from the table  1200  and transmits the read entries to the comparator  2312 . The comparator  2312 , when “1” is set in the destination site information bit, compares the information written in the frame with that set in each entry  1210 - i  so that the VLAN ID  501  stored in the frame header information storage  2360  of the frame is compared with the VLAN ID  1201 - i  set in each entry  1210 - i  and the LSP selection information written in the frame is compared with the LSP selection information  1202 - i  set in each entry  1210 - i . On the other hand, when “0” is set in the destination site information bit, the comparator  2312  masks the LSP selection information  1202 - i  (regardless of whether or not “matching” is detected with respect to the LSP selection information) to make the comparison, that is, compares the VLAN ID  501  stored in the frame header information storage  2360  of the frame with the VLAN ID  1201 - i  set in each entry  1210 - i  and transmits the result to the table access means  2311 . The above comparison is repeated until it is completed for all the entries  1210 - i  in the table  1200 .  
         [0148]    And, each time a “matching” entry is detected in the comparison, the comparator  2311  transmits the “matching” denoting information to the destination information decision circuit  2314  together with the line number  1204 - i , the tunnel label  1205 - i , and the VC label  1206 - i  set in the “matching” entry  1210 - i . When “1” is set in the destination site information bit, the comparator  2311  sets the 3-bit VC EXP information to the lower one bit of the output line selection information  506  of the UP  502  and sets “1” in the upper second bit in the frame. The “1” denotes that the VC EXP information is valid. When “0” is set in the destination site information bit, the comparator  2312  sets “0” (denoting that the VC EXP information is invalid) in the upper second bit and transmits the result to the destination information decision circuit  2314 . When “1” is set in the destination site information bit  507 , the comparator  2312  decides that “matching” is detected only in the entry  1210 - i  to be transmitted to the VC LSP-B2 and the T-LSP2 in the line connected to the PC2. When “0” is set in the destination site information bit  507 , the comparator  2312  decides that “matching” is also detected in the entry  1210 - i  to be transmitted to the VC LSP-B1 and the T-LSP1 in the line to the PC1.  
         [0149]    Each time receiving “matching” denoting information from the table access means  2311 , the destination information decision circuit  2314  transmits the line number  1201 - i , the tunnel label  1205 - i , the VC label  1206 - i , and the VC EXP information to the object as the destination information DI.  
         [0150]    The results output unit  2350  transmits one or more pieces of the destination information DI to the received frame process unit  2002 - k  corresponding to the input line number  2141  stored in the frame header information storage  2360  as the destination information DI-k.  
         [0151]    [Notifying Operation by PE3] 
         [0152]    Next, the notifying operation by the PE3 will be described.  
         [0153]    The configuration of the PE3 is the same as that of the PE1 (FIG. 20). The PE3, when receiving a frame addressed to the LAN-B1 terminal T2 from the LAN-B3 terminal T7 through the MAN-3, not only transfers the frame just like the PE1 described above, but also decides the output line selection information used for transmitting the frame addressed to the terminal T7 and writes the result in the frame to notify the ME2 of the output line selection information.  
         [0154]    Consequently, the header process unit  2300  decides the output line selection information used for selecting a line to the MAN-3 and adds the output line selection information to the information DI-k in transfer operation by the PE1, then transmits the frame to the received frame process unit  2002 - k . More concretely, each time the PE3 decides a “matching” entry  1210 - i   1  in the above transfer operation, the table access means  2311  reads the entry  1210 - i   2  paired with the entry  1210 - i   1  and decides that the VC label  1206 - i   2  set in the entry  1210 - i   2  is the target VC label 1 and the line number  1204 - i   2  set in the entry  1210 - i   2  is the target output line number 1, then notifies the comparator  2317  of the decision results.  
         [0155]    To read such a pair of entries, for example, the table access means  2311  is just required to assume the addresses of the entries  1210 - i   1  and  1210 - i   2  as consecutive integers (2n and 2n+1) and read the entry  1210 -( i+ 1) from the address 2n+1 when it is decided that the address 2n matches with that of the entry  1210 - i  and read the entry  1210 -( i −1) from the address 2n when it is decided that the address 2n+1 matches with that of the entry  1210 - i . In addition, the table access means  2316  reads the entries  2410 - i  one by one from the table  2400  and transmits the read entries  2410 - i  to the comparator  2317 .  
         [0156]    The comparator  2317  compares the information written in the frame with that set in each entry  1210 - i  so that the input line number  2141  stored in the frame header information storage  2360  of the frame is compared with the input number  2401 - i  set in each entry  2410 - i , the VC label 1 written in the frame is compared with the VC label  2403 - i  set in each entry  2410 - i , and the output line number 1 written in the frame is compared with the output line number  2404 - i  set in each entry  2410 - i . The comparator  2317  then transmits the results to the table access means  2316 . The table access means  2316  and the comparator  2317  repeat the above operation until the comparison is completed for all the entries  2410 - i  in the table.  
         [0157]    The table access means  2316  transmits the output line selection information  2406 - i  set in the VC EXP  2403 - i  field of the “matching” entry  2410 - i  to the results output unit  2350  as the output line selection information LSNI. The results output unit  2350  transmits the above information to the received frame process unit  2002 - k  as a portion of the destination information DI-k.  
         [0158]    The received frame process unit  2002 - k  writes this output line selection information in the output line selection information field  506  of the UP  502  in the frame and transfers the frame to the frame switch  1603 .  
         [0159]    [Transfer and Flooding Operations by PE3] 
         [0160]    Next, how the PE3 transfers each frame received from the PC3 will be described.  
         [0161]    In this case, the frame format in the PE1 differs from that of received frames shown in FIG. 9. An internal header part  2140  is added to each received frame and both preamble  411  and SFD  412  are deleted from the capsule header part  740  to form a new header  2240  as shown in FIG. 23.  
         [0162]    Receiving a frame through an input line  2001 - k , the received frame process unit  2002 - k  adds the internal header part  2140  to the frame and deletes the preamble  411  and the SFD  412  from the header part  2210  of the frame, then writes the identifier of the input line  2001 - k  to which the frame is inputted in the input line number field  2141  of the frame to change the frame format as shown in FIG. 23. The received frame process unit  2002 - k  also stores the frame once therein, then transmits the frame header information FH-k consisting of the internal header part  2140 , the capsule header part  2240 , and header part  2210  to the header process unit  2300 . The header process unit  2300  decides the target output line number according to the frame header information FH-k and transmits the result to the received frame process unit  2002 - k  as the destination information DI-k. The operation by this frame header process unit  2300  will be described later more in detail.  
         [0163]    After this, the received frame process unit  2002 - k  writes the output line number set in the destination information DI-k in the output line number field  2142  of the internal header part  2140  and transmits the frame to the frame switch  2003 . The frame switch  2003  then transmits the frame to the transmit frame process unit  2004 - k  corresponding to the output line number  2142 . The transmit frame process unit  2004 - k  deletes the internal header part  2140  and the capsule header part  2240  from the frame and adds the preamble  411  and the SFD  412  to the frame to change the frame format as shown in FIG. 6, then transmits the frame to the next node.  
         [0164]    Next, the operation by the header process unit  2300  will be described with reference to FIG. 21.  
         [0165]    The header process unit  2300 , receiving a plurality of pieces of frame header information FH-k from the received frame process unit  2002 - k , stores the frame header information FH obtained by multiplexing a plurality of pieces of information FH-k through the multiplexer  2340  with the frame header information storage  2360 .  
         [0166]    The destination information decision unit  2310  refers to the table  2400  (FIG. 12) to decide the target output line number. More concretely, the table access means  2316  reads the entries  2410 - i  one by one from the table  2400  and transmits the read entries  2410 - i  to the comparator  2317 . The comparator  2317  then compares the information written in the frame with that set in each entry  2410 - i  so that, when “1” is set in the VC EXP information bit  906  located in the VC EXP  902 , the input line number  2141  stored in the frame header information storage  2360  of the frame is compared with the input line number  2401 - i  set in each entry  2410 - i , the VC label  901  stored in the frame header information storage  2360  of the frame is compared with the VC label  2402 - i  set in each entry  2410 - i , and the output line selection information  905  of the VC EXP  902  stored in the frame header information storage  2360  of the frame is compared with the output line selection information  2406 - i  of the VC EXP  2403 - i  set in each entry  2410 - i.    
         [0167]    On the other hand, when “0” is set in the VC EXP information bit  906 , the comparator  2317  masks the output line selection information (regardless of whether or not the output line selection information matches with the target) to make the comparison. In other words, the comparator  2317  makes comparisons as described above so that the input line number  2141  stored in the frame header information storage  2360  of the frame is compared with the input line number  2401 - i  set in each entry  2410 - i  and the VC label  901  stored in the frame header information storage  2360  of the frame is compared with the VC label  2402 - i  set in each entry  2410 - i.    
         [0168]    The comparator  2317  transmits the results to the table access means  2316 . The table access means  2316  and the comparator  2317  repeat the above operation until the comparison is completed for all the entries  2410 - i  in the table  2400 .  
         [0169]    Each time “matching” is detected in the above comparison with respect to an entry  2410 - i , the comparator  2316  transmits the “matching” denoting information to the destination information decision circuit  2314  together with the output line number  2404 - i  set in the “matching” entry  2410 - i . When the ME2 completes the learning and the VC EXP  902  has a meaningful value (that is, “1” is set in the VC EXP information bit  906 ), the PE3 decides “matching” only in the entry  2410 - i  to be transmitted to the MAN-3. When the ME2 does not complete the learning and the VC EXP  902  has a meaningless value (that is, “0” is set in the VC EXP information bit  906 ), the ME2 also decides “matching” in the entry  1210 - i  to be transmitted to the MAN-4.  
         [0170]    The destination information decision circuit  2314  transmits one or more line numbers  2404 - i  received from the table access means  2316  to the results output unit  2350  as the destination information DI. The results output unit  2350 , each time receiving the destination information DI, transfers the information to the received frame process unit  2002 - k  corresponding to the input line number  2141  stored in the frame header information storage  2360  as the destination information DI-k.  
         [0171]    [Notifying Operation by PE1] 
         [0172]    Next, the notifying operation of the PE1 will be described.  
         [0173]    The PE1, when receiving a frame addressed to the terminal T2 from the terminal T7, not only transfers the frame just like the PE3 described above, but also decides the LSP selection information used for transmitting the above frame addressed to the terminal T7 and writes the result in the frame to notify the ME2 of the LSP selection information.  
         [0174]    Consequently, the header process unit  2300  decides the LSP selection information and transmits the information to the received frame process unit  2002 - k  as a portion of the destination information DI-k. More concretely, the table access means  2316  reads the entries  2410 - i  one by one from the table  2400  (FIG. 12) and transmits the read entries  2410 - i  to the comparator  2317 . The comparator  2317  then compares the information written in the frame with that set in each entry  2410 - i  so that the input line number  2141  set in the frame header information storage  2360  of the frame is compared with the input line number  2401 - i  set in each entry  2410 - i  and the VC label  901  set in the frame header information storage  2360  of the frame is compared with the VC label  2402 - i  set in each entry  2410 - i . After this, the comparator  2312  transmits the results to the table access means  2316 . The table access means  2316  and the comparator  2317  repeat the above operation until the comparison is completed for all the entries  2410 - i  in the table  2400 .  
         [0175]    The table access means  2316  transmits the LSP selection information  2405 - i  obtained from the “matching” entry  1410 - i  to the results output unit  2350  as the LSP selection information LSPSI. At this time, the VC EXP  2403 - i  is masked, so that “matching” comes to be detected in a plurality of entries  2410 - i  in which the values of the VC EXP2 differs from each other. However, because the value of the LSP selection information  2405 - i  in all those entries  2410 - i  are the same, the value in any of those entries  2410 - i  may be transmitted to the results output unit  2350 .  
         [0176]    The results output unit  2350  then transmits the LSP selection information LSPSI to the received frame process unit  2002 - k  as a portion of the destination information DI-k. When it is required to transmit a plurality of pieces of destination information DI-k, each including a unique output line number, the same value is set in all those pieces of the LSP selection information.  
         [0177]    The received frame process unit  2002 - k  writes the LSP selection information set in the destination information DI-k in the LSP selection information  505  of every frame to be transmitted to the frame switch  1603 , then transfers the frames to the ME2.