Abstract:
Disclosed is a method, apparatus and computer-readable medium for selecting a root bridge when a spanning tree is configured in an Ethernet network including a plurality of legacy switches and a plurality of synchronous switches. The method comprises the steps of providing in advance identification information in a configuration BPDU message of the synchronous switch, the identification information representing that a corresponding switch is an AV switch, broadcasting configuration BPDU messages by the legacy switches and the synchronous switches; and receiving configuration BPDU messages by the synchronous switch from other switches, confirming bridge priorities according to the synchronous switches through the received configuration BPDU message and the identification information to select a synchronous root bridge, receiving configuration BPDU messages by the legacy switch from other switches, and confirming bridge priorities through the received configuration BPDU messages to select a legacy root bridge.

Description:
CLAIM OF PRIORITY 
       [0001]    This application claims the benefit of the earlier filing date, pursuant to 35 USC 119, to that patent application entitled “Method For Selecting Root Bridge In Configuration Of Spanning Tree,” filed in the Korean Intellectual Property Office on Mar. 3, 2006 and assigned Serial No. 2006-20665, the contents of which are hereby incorporated by reference. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present invention relates to an Ethernet network, and more particularly to a method for selecting a root bridge in an Ethernet network when a spanning tree is configured. 
         [0004]    2. Description of the Related Art 
         [0005]    Ethernet is technology which is the most universally and familiarly accessible when data is exchanged among different terminals. However, Ethernet has been known as a technology not suitable for multimedia data transmission because of multimedia data sensitive to transmission time delay. Recently, technology has been actively discussed, which can transmit multimedia data, including images and voice, while being compatible with existing Ethernet. Such technology is being currently standardized in an Institute of Electrical and Electronics Engineers (IEEE) 802.1, which is referred to herein as Audio Video (AV) bridging or residential bridging. 
         [0006]      FIG. 1  is an exemplary view illustrating the general construction of an Ethernet network including a plurality of bridges. As illustrated in  FIG. 1 , an Ethernet network may include a plurality of Local Area Networks (LANs) referred to as LAN A to LAN E, to which a plurality of terminating devices (not shown) are respectively connected, and a plurality of bridges  101  to  105  providing connections among the illustrated LANs. For example, a message transferred may be bridged from one LAN to any of the other LANs according to an IEEE 802.1D standard stipulating Media Access Control (MAC) bridging. 
         [0007]    In the network connected by the bridges as described above, if a multipath exists among certain bridges, a loop is formed and thus a message sent from one bridge may be repeatedly circulated along the loop. In order to prevent this problem, only one path must principally exist between certain two bridges (in  FIG. 1 , a third bridge  103  and a fourth bridge  104 , etc.). Accordingly, each bridge supports a Spanning Tree Algorithm (STA) for executing a Spanning Tree Protocol (STP) based on an IEEE 802.1D standard, thereby preventing a loop from being generated. 
         [0008]    An STP currently standardized in an IEEE 802.1D protocol is a protocol for an existing bridge, i.e. a legacy bridge, supporting only best effort traffic, and blocks one of two bridges to prevent a loop from being generated. To this end, bridges negotiate with one another to allow only one of two bridges to be activated. Further, an STA prevents a loop from being generated among bridges in a network connected by the bridges, selects the most economic path, and forwards a message through the path. 
         [0009]      FIG. 2  is a block diagram illustrating the Ethernet network of  FIG. 1  using a spanning tree structure. In order to configure the spanning tree as illustrated in  FIG. 2 , information is exchanged through a spanning tree configuration message broadcasted by a bridge, i.e. a configuration Bridge Protocol Data Unit (BPDU) message. 
         [0010]    A spanning tree based on a root bridge (a first bridge  101  in  FIG. 2 ) is configured by using such a configuration BPDU message. As illustrated in  FIG. 2 , the root bridge is a bridge located at the top of the spanning tree. It is most probable that communication among bridges will be performed via the root bridge. Each bridge determines a port to be disabled through a BPDU message exchanged therebetween. In an IEEE 802.1D protocol, a disabled port is defined as a blocking state and an enabled port is defined as a forwarding state. The configuration BPDU message includes a root Identification (ID), a root path cost, a transmission bridge ID, etc. 
         [0011]    The root ID is an identifier of a bridge considered as a root bridge located in the center of a logical tree bridge topology configured in order to compute a spanning tree path. In the course of configuring a spanning tree, the root ID is gradually converged to one value. The path cost has a priority value recommended according to traffic transmission speed as illustrated in table 1. The transmission bridge ID is an identifier of a bridge transmitting the configuration BPDU message, and a unique ID such as a MAC address is allocated to each bridge. 
         [0000]    
       
         
               
             
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Root Path Cost Parameter 
               
             
          
           
               
                   
                 Connection 
                 Recommended 
                 Recommended 
                 Allocable 
               
               
                 Parameter 
                 speed 
                 value 
                 range 
                 range 
               
               
                   
               
             
          
           
               
                 Path cost 
                 4 
                 Mb/s 
                 250 
                 100 to 1000 
                 1 to 65535 
               
               
                 Path cost 
                 10 
                 Mb/s 
                 100 
                 50 to 600 
                 1 to 65535 
               
               
                 Path cost 
                 16 
                 Mb/s 
                 62 
                 40 to 400 
                 1 to 65535 
               
               
                 Path cost 
                 100 
                 Mb/s 
                 19 
                 10 to 60  
                 1 to 65535 
               
               
                 Path cost 
                 1 
                 Gb/s 
                 4 
                 3 to 10 
                 1 to 65535 
               
               
                 Path cost 
                 10 
                 Gb/s 
                 2 
                 1 to 5  
                 1 to 65535 
               
               
                   
               
             
          
         
       
     
         [0012]    As illustrated in table 1, the path cost includes values recommended according to bandwidths and allocable ranges. In the recommended value, 250 is set for a link speed of 4 Mbps, 100 is set for a link speed of 10 Mbps, 62 is set for a link speed of 16 Mbps, 19 is set for a link speed of 100 Mbps, 4 is set for a link speed of 1000 Mbps, and 2 is set for a link speed of 10000 Mbps. The allocable range is set from 1 to 65535. 
         [0013]    A selection process of a root bridge will now be described in more detail. Each bridge initially considers itself a root bridge, employs a root ID as its own bridge ID, i.e. a transmission bridge ID, and broadcasts a configuration BPDU message to all ports. Then, each bridge receives such a configuration BPDU message from each port, and compares the root ID value of the received configuration BPDU message with its own root ID value. When the root ID value of the received configuration BPDU message is greater than the root ID value. i.e. when the root ID value has a higher priority, each bridge drops the received configuration BPDU message, and continues to transmit its own configuration BPDU message. However, when the root ID value of the received configuration BPDU message is smaller than the root ID value, each bridge stops transmitting its own configuration BPDU message. After a known time passes using such a process, only one bridge having the smallest root ID value transmits a configuration BPDU message within the network. Herein, a corresponding bridge becomes a root bridge in a corresponding network. 
         [0014]    In the meantime, when the priorities of root IDs are equal to one another, the configuration BPDU messages are compared in a sequence of the lowest root path cost, the lowest bridge ID and the lowest port ID so as to determine their priorities. 
         [0015]    As described above, each bridge receives a configuration BPDU message from each port, selects a root bridge, selects an optimal path to the selected root bridge, selects a Root Port (RP) having the lowest path cost up to the root bridge, and selects a Designated Port (DP) taking charge of message (frame) transfer to a corresponding segment. Remaining ports except for the RP and DP are determined as blocking ports. As illustrated in  FIGS. 1 and 2 , one RP and a plurality of DPs may exist in one bridge. 
         [0016]    The STP technology as described above has been developed for application of a bridge supporting best effort traffic, but it is not suitable for AV bridging due to the following problems. This will be described in more detail with reference to  FIGS. 3 to 7 . 
         [0017]      FIG. 3  is one exemplary view illustrating the general construction of an Ethernet network including a plurality of legacy Ethernet switches and synchronous Ethernet switches. As illustrated in  FIG. 3 , an AV bridging topology is classified as an AV cloud ensuring QoS and a legacy area not ensuring QoS. An AV cloud is an area including only switches supporting AV bridging, and an AV switch can transmit/receive synchronous packets usable for multimedia data broadcasting.  FIG. 3  illustrates a state in which devices (AV devs) capable of both performing an existing Ethernet communication function and reproducing multimedia data are connected to AV switches, etc. A term “switch” may be generally considered as a device developed from a bridge, but the switch illustrated in  FIG. 3 , etc., is used as a term including a bridge and a switch belonging to the AV bridging topology. 
         [0018]      FIG. 4  is one exemplary view illustrating a case in which a synchronous Ethernet switch is selected as a root bridge when a spanning tree is configured in an Ethernet network including a plurality of legacy Ethernet switches and synchronous Ethernet switches.  FIG. 5  is a block diagram illustrating the Ethernet network of  FIG. 4  using a spanning tree structure, and additionally shows a communication state among legacy Ethernet termination devices for descriptions about problems of  FIG. 4 . 
         [0019]    The Ethernet network illustrated in  FIGS. 4 and 5  may include a first and a second legacy switch  401  and  402 , a first to a fourth AV switch  411  to  414 , a first and a second legacy device  501  and  502  connected to the first and the second legacy switches  401  and  402 , respectively, and a first to a third AV device  511  to  513  connected to the second to the fourth AV switches  412  to  414 , respectively. In  FIGS. 4 and 5 , the first legacy switch  401  or the second legacy switch  402  is connected to the first AV switch  411 , and only a connection for transmission of asynchronous packets is generally implemented between the first legacy switch  401  or the second legacy switch  402  and the first AV switch  411 . 
         [0020]    In  FIGS. 4 and 5 , the first AV switch  411  is selected as a root bridge. When the root bridge exists in the AV cloud as described above, it is inevitably necessary to pass through the first AV switch  411  in order to communicate with the first legacy device  501  or the second legacy device  502 . In such a case, since the AV switch accommodates both time sensitive traffic (AV traffic) and best effort traffic as defined in AV browsing (synchronous Ethernet), the AV switch can partially pass through best effort traffic generated in the first legacy device  501  or the second legacy device  502 . Therefore, the first AV switch  411  causes a bottleneck and thus may deteriorate the performance of an entire network. 
         [0021]      FIG. 6  is one exemplary view illustrating a case where a legacy Ethernet switch is selected as a root bridge when a spanning tree is configured in an Ethernet network including a plurality of legacy Ethernet switches and synchronous Ethernet switches.  FIG. 7  is a block diagram illustrating the Ethernet network of  FIG. 6  by using a spanning tree structure, and additionally shows a communication state among AV devices for descriptions about problems of  FIG. 6 . 
         [0022]    The Ethernet network illustrated in  FIGS. 6 and 7  may include a first and a second legacy switch  601  and  602 , a first to a fourth AV switch  611  to  614 , a first and a second legacy device  701  and  702  connected to the first and the second legacy switches  601  and  602 , respectively, and a first to a third AV device  711  to  713  connected to the second to the fourth AV switches  612  to  614 , respectively. 
         [0023]    In  FIGS. 6 and 7 , the first legacy switch  601  is selected as a root bridge. When the switch is selected as the root bridge in the legacy area as described above, it is inevitably necessary to pass through the legacy switch in order to communicate with the first AV device  711  or the second AV device  712 . In such a case, communication is impossible because the legacy switch does not support AV traffic (time sensitive traffic). 
       SUMMARY OF THE INVENTION 
       [0024]    Accordingly, the present invention has been made to solve the above-mentioned problems occurring in the prior art an provides additional advantages, by providing a method for selecting a root bridge when a spanning tree is configured that can allow communication of each traffic to be efficiently performed in AV browsing (synchronous Ethernet). 
         [0025]    According to an embodiment of the present, there is provided a method for selecting a root bridge when a spanning tree is configured in an Ethernet network including a plurality of legacy switches and a plurality of synchronous switches, the method including the steps of providing identification information in a configuration Bridge Protocol Data Unit (BPDU) message of the synchronous switch, the identification information representing that a corresponding switch is an AV switch, broadcasting configuration BPDU messages by the legacy switches and the synchronous switches and receiving configuration BPDU messages by the synchronous switch from other switches, confirming bridge priorities according to the synchronous switches through the received configuration BPDU message and the identification information to select a synchronous root bridge, receiving configuration BPDU messages by the legacy switch from other switches, and confirming bridge priorities through the received configuration BPDU messages to select a legacy root bridge. 
         [0026]    According to another embodiment of the present, there is provided a method for selecting a root bridge in a synchronous switch when a spanning tree is configured in an Ethernet network including a plurality of legacy switches and a plurality of synchronous switches, the method including the steps of Broadcasting a configuration Bridge Protocol Data Unit (BPDU) frame including identification information at a preset hello time, the identification information representing that a corresponding switch is a synchronous switch, when configuration BPDU frames are received from other switches, determining if the identification information is confirmed from the received configuration BPDU frames, comparing a priority of the received configuration BPDU information with a priority of BPDU information of the synchronous switch, when the received configuration BPDU information has a higher priority, stopping broadcasting the configuration BPDU frame, and updating information of a network according to the received configuration BPDU information; and when the received configuration BPDU information has a lower priority, dropping corresponding received configuration BPDU frames. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0027]    The above features and advantages of the present invention will be more apparent from the following detailed description taken in conjunction with the accompanying drawings, in which: 
           [0028]      FIG. 1  is one exemplary view illustrating the general construction of an Ethernet network including a plurality of bridges; 
           [0029]      FIG. 2  is a block diagram illustrating the Ethernet network of  FIG. 1  by using a spanning tree structure; 
           [0030]      FIG. 3  is one exemplary view illustrating the general construction of an Ethernet network including a plurality of legacy Ethernet switches and synchronous Ethernet switches; 
           [0031]      FIG. 4  is another exemplary view illustrating a case where a synchronous Ethernet switch is selected as a root bridge when a spanning tree is configured in an Ethernet network including a plurality of legacy Ethernet switches and synchronous Ethernet switches; 
           [0032]      FIG. 5  is a block diagram illustrating the Ethernet network of  FIG. 4  by using a spanning tree structure; 
           [0033]      FIG. 6  is further another exemplary view illustrating a case where a legacy Ethernet switch is selected as a root bridge when a spanning tree is configured in an Ethernet network including a plurality of legacy Ethernet switches and synchronous Ethernet switches; 
           [0034]      FIG. 7  is a block diagram illustrating the Ethernet network of  FIG. 6  by using a spanning tree structure; 
           [0035]      FIG. 8  is one exemplary view illustrating the general construction of an Ethernet network including a plurality of legacy Ethernet switches and synchronous Ethernet switches according to one embodiment of the present invention; 
           [0036]      FIG. 9  is a block diagram illustrating the Ethernet network of  FIG. 8  by using a spanning tree structure; 
           [0037]      FIG. 10  is block diagram illustrating the internal construction format of a configuration BPDU frame used in the present invention; 
           [0038]      FIG. 11  is a block diagram illustrating the internal construction of the synchronous Ethernet switch of  FIGS. 8 and 9  to which the present invention is applied; and 
           [0039]      FIG. 12  is a flow diagram illustrating an operation for root bridge selection in spanning tree configuration in a synchronous Ethernet switch according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0040]    Hereinafter, an exemplary embodiment according to the present invention will be described with reference to the accompanying drawings. In the following description, many particular items, such as detailed elements, are shown, but these are provided for helping the general understanding of the present invention, and it will be understood by those skilled in the art that these particular items can be modified without departing from the spirit and scope of the present invention. 
         [0041]      FIG. 8  is one exemplary view illustrating the general construction of an Ethernet network including a plurality of legacy Ethernet switches and synchronous Ethernet switches according to one embodiment of the present invention.  FIG. 9  is a block diagram illustrating the Ethernet network of  FIG. 8  by using a spanning tree structure. The Ethernet network of  FIGS. 8 and 9  according to one embodiment of the present invention may include a first and a second legacy switch  801  and  802 , a first to a fourth AV switch  811  to  814 , a first and a second legacy device  901  and  902  connected to the first and the second legacy switches  801  and  802 , respectively, and a first to a third AV device  911  to  913  connected to the second to the fourth AV switches  812  to  814 , respectively. In  FIGS. 8 and 9 , the first legacy switch  801  or the second legacy switch  802  is connected to the first AV switch  811 , and only a connection for transmission of asynchronous packets is generally implemented between the first legacy switch  801  or the second legacy switch  802  and the first AV switch  811 . 
         [0042]    As illustrated in  FIGS. 8 and 9 , in the Ethernet network according to the present invention, two root bridges (the first AV switch  811  and the first legacy switch  801  in  FIGS. 8 and 9 ) are selected in an AV cloud and a legacy area, respectively. In such a case, two spanning trees for time sensitive traffic and best effort traffic are configured as illustrated in  FIG. 9 . In this way, if two spanning trees are configured, i.e. if both a spanning tree for time sensitive traffic and a spanning tree for best effort traffic are configured, the first legacy switch  801  becomes a root bridge in communication between the legacy devices  901  and  902 . In such a configuration, performance deterioration of a network due to a bottleneck does not occur because an AV switch is not included within a corresponding spanning tree. Further, in communication among the AV devices  911  to  913 , for example, since a spanning tree in which the first AV switch  811  becomes a root bridge is implemented, communication error does not occur. In this way, the scheme based on the present invention is applied, so that it is possible to construct a network capable of optimizing and processing both time sensitive traffic and best effort traffic. 
         [0043]    As described above, in order to select respective root bridges in the AV cloud and the legacy area, the first and the second legacy switches  801  and  802  and the first to the fourth AV switches  811  to  814  of the present invention perform the following procedure. First, specific information for representing that a corresponding switch is an AV switch is provided in a BPDU message. In one aspect of the invention, the specific information representing the corresponding switch as an AV switch is provided in advance. Such specific information includes a root ID with the first priority for comparison, which is set as a predetermined specific value in selection of a root bridge, thereby allowing a corresponding switch to be identified as an AV switch. 
         [0044]    In such a case, using the fact that a root ID having a lower value than others is selected as a root bridge, when a switch exists in an AV cloud, a root ID value is set as a value higher than 0X8000 (a minimum basic recommended value), such as 0XFFFF so as to use it as a value for identifying an AV switch. In this case, one of the legacy switches is primarily selected as a root bridge according to the conventional root bridge selection procedure without applying separate modification to the conventional root bridge selection procedure. That is, when a legacy root bridge is selected, since the specific root ID determined for an AV bridge has a high value (e.g. 0XFFFF), the legacy bridges drop this value when received through a BPDU message. As a result, a bridge having a low root ID value is selected as a root bridge from the legacy bridges. 
         [0045]      FIG. 10  is block diagram illustrating the internal construction format of a configuration BPDU frame used in the present invention. Referring to  FIG. 10 , a configuration BPDU frame proposed in an IEEE 802.1D protocol includes a “protocol ID” area  1105 , a “protocol version” area  1106 , a “BPDU type” area  1107 , a “flags” area  1108 , a “root ID” area  1109 , a “root path cost” area  1110 , a “bridge ID” area  1111 , a “port ID” area  1112 , a “message age” area  1113 , a “MAX age” area  1114 , a “hello time” area  1115 , and a “forward delay” area  1116 . The “protocol ID” area  1105  stores information for identifying protocols, the “protocol version” area  1106  stores information about the version of a protocol, the “BPDU type” area  1107  stores information about a BPDU type, and the “flags” area  1108  stores information about a flag. The “root ID” area  1109  stores information about a root identifier, the “root path cost” area  1110  stores information about root path cost, the “bridge ID” area  1111  stores information about a bridge identifier, and the “port ID” area  1112  stores information about a port identifier. The “message age” area  1113  stores information about a message age, the “MAX age” area  1114  stores information about a maximum age, the “hello time” area  1115  stores information about a hello time, and the “forward delay” area  1116  stores information about a forward delay. 
         [0046]    The “root ID” area  1109  includes a “bridge priority” area  1109 - 1  of two bytes for storing information about a bridge priority and a “bridge MAC address” area  1109 - 2  of eight bytes for storing information about a bridge MAC address. The bridge priority is a value adjustable by a manager, and the bridge MAC address uses one of the port MAC addresses of a bridge. The bridge priority has a basic value of 0X8000, and the number 1 port MAC address is recommended as the basic value of the bridge MAC address. 
         [0047]    According to the embodiment of the present invention, in the BPDU frame having the construction as described above, a value higher than 0X8000 (a minimum basic recommended value) such as 0XFFFF is set in the bridge priority” area  1109 - 1  of two bytes for storing information about a bridge priority, and this can be used as a value for identifying an AV switch. 
         [0048]    As described above, the specific information is provided in advance in the BPDU message in order to select respective root bridges in the AV cloud and the legacy area, and then each of the AV switches confirms AV switch identification information within corresponding BPDU messages, confirms bridge priorities according to AV switches, and performs a root bridge selection operation. Herein, in a case where each of the AV switches has set a root ID to have a value of a specific value such as 0XFFFF as AV switch identification information, the AV switches have the same root ID value. Accordingly, each of the AV switches compare the BPDU messages in a sequence of the lowest root path cost, the lowest bridge ID and the lowest port ID, which is the subsequent step of bridge priority determination, thereby selecting an AV root bridge. 
         [0049]      FIG. 11  is a block diagram illustrating the internal construction of a synchronous Ethernet switch (AV switch) to which the present invention is applied, and representatively discloses the construction of the first AV switch  811 . For convenience of description,  FIG. 11  discloses function units related to the processing of a configuration BPDU message. Referring to  FIG. 11 , the first AV switch  811  to which the present invention is applied includes a frame forwarding unit  8113 , a sentence structure analyzer  8112 , a memory unit  8114 , and an STP controller  8111 . The frame forwarding unit  8113  includes a plurality of ports, a frame storage queue, etc., to transmit/receive Ethernet frames to/from other switches connected through a LAN. The sentence structure analyzer  8112  analyzes the sentence structure of frames received in the frame forwarding unit  8113 , and transfers a configuration BPDU frame to the STP controller  8111 . The memory unit  8114  stores a configuration parameter related to a network, and stores operation programs of the STP controller  8111 , etc. The STP controller  8111  generally controls these function units to perform an operation for configuring a spanning tree. 
         [0050]    The memory unit  8114  may be an aggregation of core system memories or memory elements, which may store a spanning tree configuration program  8114 - 1  and spanning tree configuration information  8114 - 2  for the operation of the STP controller  8111 . In addition, the memory unit  8114  includes a forwarding table (not shown), which is referred to by the frame forwarding unit  8113 , etc. The spanning tree configuration information  8114 - 2  is information related to spanning tree configuration, and the memory unit  8114  updates and stores configuration BPDU information of both a legacy switch having the highest priority and an AV switch having the highest priority when a root bridge is selected according to characteristics of the present invention, as well as its own priority information stored in advance for root bridge selection. 
         [0051]    The STP controller  8111  confirms the configuration BPDU message, which is received in the frame forwarding unit  8113 , through the sentence structure analyzer  8112  for root bridge selection, and continues to update the configuration BPDU information of the legacy switch having the highest priority. Further, the STP controller  8111  confirms AV switch identification information within the configuration BPDU message through the sentence structure analyzer  8112 , continues to update the configuration BPDU information of the AV switch having the highest priority, and selects respective root bridges from legacy switches and AV switches. 
         [0052]      FIG. 12  is a flow diagram illustrating an operation for root bridge selection in spanning tree configuration in a synchronous Ethernet switch (AV switch) according to one embodiment of the present invention. Hereinafter, the root bridge selection operation in the AV switch according to one embodiment of the present invention will be described with reference to  FIG. 12 . For root bridge selection, in step  1201 , the AV switch transmits (broadcasts) its own configuration BPDU message through all ports at a preset hello time (basic value is two seconds). When a configuration BPDU message is received from a different switch, the AV switch confirms the received configuration BPDU frame in step  1202 . 
         [0053]    In step  1203 , the AV switch determines if identification information used for identifying an AV switch is confirmed in a corresponding received configuration BPDU frame. For example, this is a step for determining if a root ID value is 0XFFFF, and it may be determined that the corresponding configuration BPDU frame relates to an AV switch. In step  1203 , if the identification information used for identifying an AV switch is confirmed in the corresponding received configuration BPDU frame, step  1210  is performed. Otherwise, step  1220  is performed. 
         [0054]    In step  1220 , the AV switch compares the priority of the received configuration BPDU information with the priority of its own BPDU information in order to select a legacy root bridge. In step  1220 , the AV switch determines a priority by comparing two types of BPDU information in a sequence of a root ID, root path cost, a bridge ID, and a port ID. In step  1221 , if the received configuration BPDU information has a higher priority as a result of the comparison in step  1220 , step  1222  is performed. Otherwise, step  1224  is performed. 
         [0055]    In step  1222 , the AV switch stops transmitting its own configuration BPDU information. In step  1223 , the AV switch updates information about a legacy network (i.e. updates the value of a BPDU received up to now and having the highest priority). In step  1224 , the AV switch drops corresponding received information because the received BPDU information has a lower priority, and repeats the above-described steps after returning to step  1201 . 
         [0056]    In step  1210 , the AV switch compares the priority of the received configuration BPDU information with the priority of its own BPDU information in order to select an AV root bridge. Similarly to step  1220 , in step  1210 , the AV switch determines a priority by comparing two types of BPDU information in a sequence of a root ID, root path cost, a bridge ID, and a port ID. However, in the embodiment of the present invention, when a root ID has been set to have a specific value such as 0XFFFF as AV switch identification information, AV switches have the same root ID value. Accordingly, the AV switch compares the priorities by comparing corresponding BPDU information in a sequence of the lowest root path cost, the lowest bridge ID and the lowest port ID, which is the subsequent step of bridge priority determination. In step  1211 , if the received BPDU information has a higher priority as a result of the comparison in step  1210 , step  1212  is performed. Otherwise, step  1224  is performed. 
         [0057]    In step  1212 , the AV switch stops transmitting its own configuration BPDU information. In step  1223 , the AV switch updates information about an AV network (i.e. updates the value of a BPDU received up to now and having the highest priority). 
         [0058]    As illustrated in  FIG. 12 , the operation for root bridge selection in the spanning tree configuration in the AV switch can be performed. Herein, each AV switch continues to receive a configuration BPDU message from a different switch even after stopping the transmission of its own configuration BPDU message, compares the received configuration BPDU message with the stored configuration BPDU information with the highest priority, and continues to perform an operation for updating the information about the legacy network or AV network. Once a certain time passes through such a process, only two switches, i.e. a legacy switch with the highest priority and an AV switch with the highest priority, transmit configuration BPDU information, so that root bridges are selected. 
         [0059]    According to the present invention as described above, when implementing AV bridging being currently conducted in an IEEE 802.1, a scheme for root bridge selection in spanning tree configuration is compatible with an existing STP, and solves both a bottleneck, which may be caused by processing of best effort traffic when an AV switch is selected as a root bridge, and the inability to process AV traffic when a legacy switch is selected as a root bridge, so that it is possible to prevent the conventional unreasonable topology and improve the efficiency of a synchronous Ethernet network. 
         [0060]    The above-described methods according to the present invention can be realized in hardware or as software or computer code that can be stored in a recording medium such as a CD ROM, a RAM, a ROM, a floppy disk, a hard disk, or a magneto-optical disk or downloaded over a network, so that the methods described herein can be executed by such software or computer code using a general purpose computer, or a special processor or in programmable hardware, such as an ASIC or FPGA, or dedicated hardware. As would be understood in the art, the computer, the processor or the programmable hardware include memory components, e.g., RAM, ROM, Flash, etc. that may store or receive software or computer code which when accessed and executed by the computer, processor or hardware implement the processing methods described herein. 
         [0061]    Although a preferred embodiment of the present invention have been described for illustrative purposes, those skilled in the art will appreciate that various modifications, additions and substitutions are possible, without departing from the scope and spirit of the invention as disclosed in the accompanying claims, including the full scope of equivalents thereof. For example, in the above description, a root ID may have a value of 0XFFFF as identification information for identifying an AV switch. However, other root ID values may be set as corresponding identification information.