Abstract:
Disclosed is a communication control method for recovering communication failures caused by a packet loop that occurs as a result of L2-level misconnection at relay switches in a communication system where data transmission/reception is carried out between a plurality of end hosts connected to the relay switches within a subnet. The method includes the steps of consecutively sending packets each having, as its originator address, a MAC address other than an originator MAC address of an end host desired to be communicated making up the subnet; thereby stopping the packet loop; and then sending packet data using, as a destination MAC address, a MAC address of a destination end host.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2005-172678, filed on Jun. 13, 2005, the entire contents of which are incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a communication control method and communication apparatus employing the method. More particularly, the invention relates to a communication control method for securing communications when an L2 loop occurs, and a communication apparatus employing the method. 
     2. Description of the Related Art 
     In networks is known an L2 loop (broadcast storm) which is one of extremely frequent faults which may occur as a result of mal-connections of LAN cables (see, e.g., Japanese Patent Application Laid-open Publication No. 2002-252625). When this L2 loop occurs, the transmission paths are not only subjected to high loads all over the subnets, but also there appears phenomenon called black hole phenomenon by which packets are forwarded to the point where the loop is present. 
     Following is a description of the mechanism by which such an L2 loop occurs.  FIGS. 1A and 1B  are diagrams describing the L2 loop occurring mechanism. 
     As shown in  FIG. 1A , terminal  1  broadcasts packets with the originator address of “A”, which is the MAC address of the terminal  1 , and then, the packets are forwarded by relay switches SW 1 , SW 2  and SW 3  such that the packets reach all the terminals  2  and  3  within a subnet. 
     At that time, the relay switches SW 1 , SW 2  and SW 3  learn “A” which is a packet issuing MAC address at a port receiving the packet. In other words, the switches learn that a terminal having the MAC address “A” is present ahead of this port. This enables each of the relay switches to determine a port to which the packet is forwarded based on the result of this learning when a packet having a destination MAC address “A” is next sent from another terminal. 
     However, as shown in  FIG. 1B , when the cables become looped as a result of mal-connection of the LAN cables at the relay switch SW 3  for example, a broadcast packet sent from the terminal  1  loops. Thus, each time the packet sent from the terminal  1  loops at the relay switch SW 3 , the packet from the terminal  1  is sent from the looping point such that the relay switches SW 1 , SW 2  and SW 3  within the subnet erroneously learn the MAC addresses as the terminal  1  of the MAC address “A” being present at the looping point, based on the same principle as that used in the previous description. 
     As a result, even though another terminal intends to send a packet destined for the terminal  1 , the packet disappears into the looping point formed at the relay switch SW 3 , and therefore a packet cannot normally arrive at the terminal  1 , disabling the communication to the terminal  1 . 
     Since apparatuses, personal computers and routers using Windows (OS provided by Microsoft Corp.) often send a broadcast packet, there occurs phenomena that the terminal which has once sent the broadcast packet becomes prevented from performing communications within the subnet. 
     Although Japanese Patent Application Laid-open Publication No. 2002-252625 discloses a technique for detecting the point where such a loop occurs, there has hitherto been no technique for correcting the addresses which have erroneously been learned after the occurrence of the loop. It has hitherto been difficult to communicate with a terminal lying within the loop subnet unless the loop is fundamentally ceased by disconnecting the loop occurring source through physical works such as removing the cables. 
     Such a disconnection work has led disadvantageously to high time and human costs due to the need to identify the looping point. 
     SUMMARY OF THE INVENTION 
     It is therefore an object of the present invention to provide a communication control method enabling communications within a subnet by securing communication paths between hosts within a loop subnet or between a host within the loop subnet and a host outside the loop subnet even though the L2 loop broadcast storm occurs. 
     In order to achieve the above object, according to an aspect of the present invention there is provided a communication control method for recovering communication failures caused by a packet loop that occurs as a result of L2-level misconnection at relay switches in a communication system where data transmission/reception is carried out between a plurality of end hosts connected to the relay switches within a subnet, the method comprising the steps of consecutively sending packets each having, as its originator address, a MAC address other than an originator MAC address of an end host desired to be communicated making up the subnet; thereby stopping the packet loop; and then sending packet data using, as a destination MAC address, a MAC address of a destination end host. The destination of the packets to be consecutively sent may be a broadcast address. The packets to be consecutively sent may be directed to a unicast address using as a destination an address not existing in the subnet. The packets to be consecutively sent may be multicast using as an originator MAC address an address not existing in the subnet. The MAC address other than an originator MAC address of an end host desired to be communicated may be a MAC address other than an existent MAC address acquired by monitoring communication and an existent MAC address identified in advance. The communication control method may further comprise the step of setting a MAC address not existing within the subnet as the originator address to consecutively send the packets to be broadcast. The communication control method may further comprise the steps of setting an MAC address not existing within the subnet as the originator address; sending packets whose destination is a broadcast address, to which the end host desired to be communicated respond in unicast; and thereby allowing the relay switch within the subnet to correctly learn the MAC address of the end host. The communication control method may further comprise the steps of setting an MAC address not existing within the subnet as the originator address; sending unicast packets whose destination is an address not existing within the subnet; and thereby allowing the relay switch within the subnet to correctly learn the MAC address of the end host. The communication control method may further comprise the steps of setting an MAC address not existing within the subnet as the originator address; sending packets whose destination is a multicast address, to which the end host desired to be communicated respond in unicast; and thereby allowing the relay switch within the subnet to correctly learn the MAC address of the end host. 
     In order to attain the above object, according to another aspect of the present invention there is provided a communication control method for recovering communication failures caused by a packet loop that occurs as a result of L2-level misconnection at relay switches in a communication system where data transmission/reception is carried out between a plurality of end hosts connected to the relay switches within a subnet, the method comprising the steps of consecutively sending packets each having, as its originator address, a MAC address of an end host desired to be communicated; thereby causing to delete a MAC learning table of the relay switch in the event that broadcast packets have the same originator address; and sending packet data using, as a destination MAC address, a MAC address of a destination end host. 
     In the above aspect, the communication control method may further comprise the steps of sending ARP (Address Resolution Protocol) broadcast packets; and thereafter sending packets intended to correct mis-learning to acquire a MAC address from an IP address of the end host. The consecutive sending of the packets may be executed by an apparatus disposed at a remote site. 
     Thus, even though the L2 loop broadcast storm occurs, the communication paths are secured between the hosts within a loop subnet or between a host within the loop subnet and a host outside the loop subnet, enabling the communications within a subnet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above and other objects, aspects, features and advantages of the present invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings, in which: 
         FIGS. 1A and 1B  are explanatory Views of the mechanism with which an L2 loop (broadcast storm) occurs; 
         FIG. 2  shows an exemplary configuration of a subnet for describing an embodiment of the present invention implementing the principle of the invention; 
         FIG. 3  shows an exemplary functional configuration of an invention apparatus  100  according to the present invention; 
         FIG. 4  shows an operation flow of the invention apparatus  100  corresponding to the embodiment of  FIG. 2 ; 
         FIG. 5  shows signal sequences in the subnet; 
         FIG. 6  shows an exemplary apparatus configuration of the invention apparatus  100  of a second embodiment; 
         FIG. 7  shows an operation flow of the invention apparatus  100  of the second embodiment; 
         FIG. 8  shows a signal sequence flow of a third embodiment; 
         FIGS. 9A and 9B  show packet formats; 
         FIG. 10  shows signal sequences corresponding to a fourth embodiment; 
         FIG. 11  shows an exemplary functional configuration of the invention apparatus that is applied to a fifth embodiment; 
         FIG. 12  is an explanatory view describing the concept of mis-learning in a sixth embodiment; and 
         FIG. 13  shows signal sequences corresponding to the sixth embodiment. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Embodiments of the present invention will now be described with reference to the accompanying drawings. It is to be appreciated that the embodiments are given for the purpose of understanding the present invention and that the technical scope of the present invention is not intended to be limited thereto. 
     The fundamental principle of the present invention lies in correcting the erroneous address learning upon the loop occurrence by consecutively sending packets from remote. This enables communications with terminals within a subnet without disconnecting the loop occurring source and enables remarkable reduction of the time and human costs for recovering the communications, which have hitherto been needed. 
     It is thus sufficient that there exist no looped packets having MAC addresses of both communication sender and destination terminals as source addresses, and hence the principle of the present invention lies in that packets meeting these conditions are consecutively broadcast for replacement with intentionally looped packets. 
     According to the present invention, at relay switch ports making up a loop, packets are consecutively input and while simultaneously existing loop packets are also input, whereupon two (2) inputs are present on a single output port. Since this consecutively occurs, input or output queues overflow, as a result of which looped packets are discarded by the queue overflow at the relay switch, enabling the state where only packets sent according to the present invention are becoming looped to be created. 
     The above principle prevents packets causing mis-learning of the MAC addresses from being sent from the loop, enabling the communication with the terminals (end hosts) within a subnet. 
       FIG. 2  shows an example of the subnet configuration for describing the present invention implementing the above principle. A relay switch SW 1  connects to end hosts  200 A and  200 B and to a communication apparatus  100  (hereinafter referred to as the invention apparatus  100 ) to which the method of the present invention is applied as a concept of one host, for recovering the communications at the L2 looping. The state is shown where a cable-connected relay switch SW 2  is ahead of the relay switch SW 1  with L2 loop occurring as a result of cable mal-connection at the relay switch SW 2 . 
     First Embodiment 
       FIG. 3  is a diagram showing an example of the functional configuration of the invention apparatus  100  according to the present invention.  FIG. 4  is an operation flow of the invention apparatus  100  corresponding to the embodiment of  FIG. 3 , and  FIG. 5  is a diagram showing sequences of signals in the subnet of  FIG. 2 . 
     Reference will be made to these figures to describe a first embodiment of the present invention. 
     The invention apparatus  100  has a function of sending specified packets (Ether header destination address, originator address, protocol, payload corresponding to the protocol, etc.) In a specified number of times and sends consecutively into the subnet packets that have been broadcast with the MAC addresses not existing in the subnet as the originator addresses. This presents an effect of stopping the end host MAC address to be communicated being erroneously learned by the relay switches SW 1  and SW 2 . 
     In the functional block of the invention apparatus  100  shown in  FIG. 3 , an input unit  101  acquires an MAC address (=MC) of the invention apparatus  100  (step S 1  of  FIG. 4 ) and further acquires control parameters such as the number of packets sent, the size, header and payload information thereof (step S 2 ). Then, a packet transmission unit  102  creates packets having the MAC address (=MC) as the originator address in accordance with the input control parameters and sends the packets through a network interface (step S 3 ). 
     Delivered to the packet transmission unit  102  as the control parameters of the invention apparatus  100  are protocol, destination MAC address, originator MAC address, packet length and parameters proper to the upper protocol of the MAC packet. These parameters of the packets to be sent may all be altered, may be default values or may be unalterable at fixed values. The packet transmission unit  102  instructed to send packets creates the packets based on the control parameters and send them via the interface  103  to a network  110 . If an instruction to stop is given, the packet transmission unit  102  stops the transmission of the packets. 
     In this case, each packet is configured such that the destination address and originator address are written to its header portion. For example, using the default parameters from the input unit  101 , the packet transmission unit  102  sends 10,000 packets for example with ARP (Address Resolution Protocol) request frame for Ethernet longest packet of 1,514 bytes, with the destination MAC address in the form of FF-FF-FF-FF-FF-FF MAC broadcast, the originator address being the MAC address of the invention apparatus  100 , the address required by the ARP being IP address(=IPC) of the invention apparatus  100 . This enables the looped packets to be replaced with packets sent by the invention apparatus  100 . 
     For example, a case is contemplated in  FIG. 2  where a broadcast (BC) packet PKT 1  sent from the end host  200 A loops at the relay switch SW 2 , with the result that the MAC address (=MA) of the end host  200 A is continuously erroneously learned at the relay switches SW 1  and SW 2  (see P 1  of  FIG. 5 , where MA is the looping direction). 
     In this state, the invention apparatus  100  issues a large number of broadcast packets PKT 2  (see P 2  of  FIG. 5 ). This results in the looping direction represented by the MAC address (=MC) of the invention apparatus  100  to clear away the looped packets PKT 1  sent from the end host  200 A. Thus, there can be stopped the state where the packets PKT 1  become looped, which may cause the mis-learning. 
     It is to be noted that this state merely stops the looping of the packets from the end host  200 A causing the mis-learning and that the mis-learning remains unsolved. 
     Afterwards, for example, the end host  200 A that is a communication originator sends a unicast packet PKT 3  to the end host  200 B not performing any voluntary mis-learning. This allows the address of the end host  200 A to arrive at the end host  200 B while being correctly learned on the relay switch SW 1  (see P 4  of  FIG. 5 ). 
     As a result, communication from the end host  200 B to the end host  200 A also arrives at the end host  200 A through a path where a response packet PKT 4  is correctly learned (see P 5  of  FIG. 5 ). 
     The above mechanism ensures the correction of mis-learning such that communication between the end hosts  200 A and  200 B is enabled even in the state where the L2 loop occurs. 
     In this case, available as the destination MAC address of the MAC packet PKT 2  to be sent is a multicast address of the MAC packet or a unicast address of the MAC packet not used in the subnet, in addition to the broadcast (=BC) address of the MAC packet. 
     The originator address of the MAC packet to be sent need not be the MAC address (=MC) of the invention apparatus  100  but can be any address other than the MAC address possessed by the end host to be communicated. The originator address may be a unicast address of the MAC packet not used in the subnet, a unicast address of the MAC packet being used in the subnet, a MAC broadcast address, or a MAC multicast address. 
     If, for example, ARP is sent with the originator address in the form of any MAC address not existing in the subnet, then in case the packets sent from the invention apparatus  100  become looped, the sent packets will be continued to be resent from the looping point to the overall areas of the subnet. At that time, the MAC address learned at each of the relay switches is an earlier created MAC address of a really non-existent end host, and hence no influence can be imparted to any really existent hosts inclusive of the invention apparatus  100  even though the MAC address is erroneously learned at each relay switch as the MAC address being present at the looping point. 
     The sent protocol is not intended to be limited to the ARP, and any upper protocols are also available such as IP (Internet Protocol), ICMP (Internet Control Message Protocol) and IPX (Internet Packet Exchange). Alternatively, there may also be used presently unused protocol that is not interpretable by the end host or normally uninterpretable packet. The sent packet length and the number of packets to be sent can be any values as long as packets capable of being actually sent over the network are used. 
     The invention apparatus  100  need not necessarily be disposed within the looping subnet. For example, in the case where it is disposed within a network different from the looping subnet, the same applies thereto by consecutively sending the ICMP Echo Request having as its destination the IP broadcast address destined for the looping subnet, or by consecutively sending packets to which routers such as RIP within the looping subnet such as RIP respond with broadcast packets or by consecutively sending packets to which the end host within the NetBIOS looping subnet respond with the broadcast packets. The invention apparatus  100  may send a MAC packet with a VLAN (Virtual Local Area Network) tag attached thereto such that the packets can be sent to any appropriate subnet by specifying VLANtag used in the looping subnet. 
     Second Embodiment 
     A second embodiment will hereinafter be described where the looped reception packets are sent while being monitored. 
     The configuration of the subnet in the second embodiment is as shown in  FIG. 2 .  FIG. 6  shows an example of the apparatus configuration of the invention apparatus  100  according to the second embodiment.  FIG. 7  is a flowchart of the operation thereof. 
     In the exemplary configuration of the invention apparatus  100  show in  FIG. 6 , the input unit  101  acquires check parameters and the packet transmission unit  102  sets the number of packets to be sent, the size and destination addresses thereof (step S 11 ). 
     An address set H of a host to be communicated is acquired or set (step S 12 ). A MAC address list is then acquired including the MAC address of the invention apparatus  100  and MAC addresses on-subnet(step S 13 ). 
     An address X is then created that does not appear in the thus acquired MAC address list (step S 14 ). 5,000 ARP packets are issued for an example, with the originator address in the form of the thus created address X (step S 15 ). 
     On the contrary, a monitor unit  106  records therein all of the originator addresses of the Ethernet packets received during a certain period of time, e.g., for one (1) second. After the reception of one (1) second, a set S of the received originator MAC addresses are recorded therein (step S 16 ). 
     Comparison is then made between the set S of the originator MAC addresses of the packets received during the period and the address set H of the host to be communicated (step S 17 ). If the address set H is not included in the set S in this comparison (N of step S 17 , then the really existent end hosts  200 A and  200 B are not subjected to any influence of the mis-learning such that communication therewith becomes enabled, ending the procedure. If included (Y of step S 17 ), then the packet sending with X as the originator address (S 15 ) is continued until the set H becomes not included in the set S. 
     Although in this embodiment the sending operation is repeated infinitely as long as even only one MAC address of the really existent end host is included in the originator MAC addresses of the received packets (the set H is included in the set S: Y of step S 17 ), the sending operation may be stopped when reaching the limited number of times. 
     Alternatively, the receiving operation need not necessarily be continued during a certain period of time. That is, each time the packet is received, comparison may be made at step S 17  of whether the originator address matches the MAC address of the end host really existing within the subnet. If affirmative, the packet with any non-existent MAC address as the originator address may consecutively be issued in the same manner as the first embodiment. 
     Alternatively, the received originator MAC address of the broadcast packet may first be acquired and recorded received by the monitor unit  106  during a certain period of time. Then, in case the MAC address of the end host to be communicated matches the received originator MAC address recorded by the monitor unit  106 , the above sending operation may be executed such that the sending operation may not be executed unless the MAC address of the end host to be communicated is erroneously learned. This prevents the network from being subjected to any unnecessary load. 
     In case the broadcast packet whose originator address comes from the end host to be communicated is not currently looped but was looped in the past, the MAC address may possibly be erroneously learned. For this reason, the above sending operation (step S 15 ) may be executed in case the end host cannot be communicated when it is verified that no loop occurs for the certain period of time. 
     Thus, with the above state, by sending and receiving the packets intended for communication between the end hosts, the communication can advantageously be recovered even though any mis-learning takes place. 
     Third Embodiment 
     Description will be made of a third embodiment where the host causes responding broadcast packets to loop. 
     The third embodiment is characterized by stopping all of the states where the packets causing mis-learning are looped and correcting the mis-learning to ensure normal learning. The network configuration is as shown in  FIG. 2 , and  FIG. 8  is a signal sequence flow in this embodiment. 
       FIGS. 9A and 9B  show formats of the MAC packet, and more specifically the MAC header has three fields consisting of destination MAC, originator MAC and type. In the diagrams showing the associated MAC packet formats besides  FIGS. 9A and 9B , the type of the MAC header is not shown for the purpose of simplification. 
     First, in conformity with the first and second embodiments, the invention apparatus  100  consecutively issues PAC packets PKT 2  of the format shown in  FIG. 9A  to obtain the state where there disappears a loop LP 1  caused by the MAC packet PKT 1  originating from the end host  200 A to be communicated (P 1 ). At this time, a loop LP 2  caused by the MAC packet PKT 2  issued from the invention apparatus  100  remains not disappeared for a while. 
     While an ARP request packet is then issued in this embodiment, the destination IP address requested is set to an IP address (=IPA) of the end host  200 A within the ARP request packet of the format structure shown in  FIG. 9B , with requester IP and MAC addresses within the ARP request packet being set to an IP address (=IPX) and a MAC address (=MX) that are not used in the subnet in the same manner as the second embodiment. A single or a plurality of packets are then sent with the ARP requesting originator MAC address within the MAC header portion being set to MX and with the destination MAC address set to a broadcast MAC address (=BC). This enables the sent ARP request packet to be broadcast while looping (LP 3 ). 
     When receiving a single ARP request packet, the end host  200 A sends a single ARP response packet to the destination MAC address (=MX) (P 2 ). 
     This MAC address (=MX) is erroneously learned by all of the relay switches SW 1  and SW 2  as long as it exists at the looping point, as a result of which the ARP response is relayed toward the loop so that the MAC address (=MA) of the end host  200 A is normally learned at each relay switch along the relay path (P 3 ). 
     In the above state, by sending and receiving the packets PKT 3  and PKT 4  intended for communication between the end hosts  200 A and  200 B, the communication can be recovered even when the mis-learning occurs (P 4 ). Since the end host  200 A continues to respond to the ARP resulting in a high load, the loop packet may be replaced with one not subjecting the host to such a high load after the normal learning so that the response is ceased. This approach is applicable not only to the ARP but also to various protocols such as IP multicast and IP broadcast protocols as long as the end host responds to the broadcast packet. 
     Fourth Embodiment 
     The feature of a fourth embodiment lies in that mis-learning is corrected without stopping the packets causing mis-learning from looping. 
       FIG. 10  shows sequences of signals corresponding to such a fourth embodiment. In certain type of Ethernet relay switches, when packets having the same originator address (in this case, MAC address (=MA) of the end host  200 A) often arrive at different ports (P 1 -P 2 ), the ports learning the address MA are frequently changed and the normal learning is regarded as being difficult so that the corresponding MAC addresses are temporarily deleted from the learning table, with the result that the address MA becomes unlearned (P 3 ). 
     In that event, when a packet directed to the end host  200 A intended for communication arrives at a relay switch, the packet is regarded as destination unknown due to its unlearned MA, and is broadcast to all the ports in spite of the unicast packet (P 4 ). This enables the packet to be forwarded to correct ports as well although normal learning is not achieved, resulting in the same effect as obtained when the mis-learning has been corrected. 
     In order to present such a state, consecutive sending has only to be made of MAC packets having the address MA as the originator address. By way of example, in the third embodiment, the same effect can be obtained as the case where the mis-learning has been corrected at the relay switch (see P 3  of  FIG. 8 ) by allowing the end host to respond to the packets consecutively broadcast by the loop, instead of completely stopping the packets causing the mis-learning from looping in advance. 
     As alternative, the invention apparatus  100  may be disposed in a subnet different from the looping subnet such that unicast packets are consecutively sent to a host within the looping subnet by way of a router that is a gateway to the looping subnet. Even though a broadcast packet sent from the router loops, this embodiment temporarily allows the packet directed to the router to be normally forwarded from the host, enabling communication with the invention apparatus  100  via the router. 
     With the above state, by sending and receiving the packets intended for communication between the end hosts, such an effect can be obtained that the communication is recovered even when the mis-learning takes place. In case of the above example, the destination address need not necessarily be the unicast address but instead can be any addresses such as the MAC broadcast address and MAC multicast address. 
     Fifth Embodiment 
     Description will be made of a fifth embodiment where the address mis-learning is stopped and corrected remotely by allowing an end host belonging to a remote subnet different from a looping subnet to remotely control the invention apparatus within or outside the looping subnet. 
       FIG. 11  shows an example of the functional configuration of the invention apparatus, which is applied to such a fifth embodiment. To and from the invention apparatus  100  previously disposed within a looping subnet as an end host within the subnet, a remotely existing end host sends a control message through communication means such as telnet or SNMP (Simple Network Management Protocol) and receives the control message through the packet reception unit  105  thereof. The invention apparatus  100  interprets the received control message as input of the control parameter through a remote control unit  104  thereof to effect the same operations as those in any of the first to fourth embodiments. No specific limitations are imposed on the control message as long as any existing communication means are used. For example, in case of using TCP/IP, the socket communication inclusive of telnet or SNMP may carry the control message. 
     In the event of the invention apparatus  100  acting as a host connecting to a looping subnet, as shown in  FIG. 11 , the interface  103  sending and receiving a control message is identical to the interface sending a broadcast packet intended for stopping and correcting the mis-learning. Alternatively, a plurality of physically different interfaces may be employed as long as they operate as a logically single end host. 
     The interface may be implemented as part of the logical function of a router (gateway) at the boundary via which the looping subnet connects to an external network. In this case, the network interface  103  of  FIG. 11  is divided into two portions, i.e., the subnet side and the external network side although as the logical function the router-directed packets are once received by the packet reception unit  105 , effecting substantially the same subsequent operations. 
     The invention apparatus  100  need not necessarily be disposed within a looping subnet. The same effect can be obtained by, as in the first embodiment, by consecutively sending ICMP Echo Request packets whose destination is IP broadcast address from the outside of the subnet to the interior of the looping subnet. 
     Sixth Embodiment 
     The feature of a sixth embodiment lies in that the MAC address can be acquired from the IP address upon the occurrence of loop. 
       FIG. 12  is an explanatory view of the concept of mis-learning in the sixth embodiment.  FIG. 13  shows signal sequences corresponding to the sixth embodiment. 
     As is apparent from  FIG. 12 , when the invention apparatus  100  intends to communicate with another end host  200  within the subnet upon the occurrence of loop, the invention apparatus  100  needs to send an ARP request to acquire a MAC address (=MA) of the end host  200  even if it knows the IP address (=IPA) of the end host  200 . 
     However, when issuing a broadcast packet such as the ARP request upon the occurrence of loop (P 1 ), the packet becomes looped leading to mis-learning of the MAC address (MC) of the invention apparatus  200  (P 2 ). This prevents a response to the ARP request to normally return to the invention apparatus  100 , with the result that the MAC address (MA) of the end host  200  cannot be acquired resulting in the communication disabled. 
     The state at that time will hereinafter be described in detail. As a result of looping of an ARP request (I) from the invention apparatus  100 , the end host  200  often receives the ARP request originating from the invention apparatus and, for each reception, issues an ARP response (II). However, the MAC address (=MC) of the invention apparatus  100  is erroneously learned as being present at the looping point by the switches SW 1  and SW 2 . 
     For this reason, all the ARP responses (II) sent from the end host  200  are consequently forwarded to the looping point so that mis-learning at the looping point leads to ARP response packets becoming looped at the looping point (P 3 ). 
     Ordinarily, as shown in  FIG. 12 , unicast packets are looped in the opposite direction (broken line of  FIG. 12 ) to the direction (solid line of  FIG. 12 ) where the broadcast packet turns. Thus, as shown in  FIG. 13 , the ARP request packets are caused to disappear from the loop by sending a large number of broadcast packets (III) having a non-existent MAC address (=MX) as the originator MAC address, as in the first or third embodiment. 
     When the invention apparatus  100  then issues any MAC packet (IV) with MC of the invention apparatus as the originator MAC address and with MX as the destination MAC address as in the third embodiment for example, mis-learning at the switches SW 1  and SW 2  is corrected based on the principle of the third embodiment. As a result, the looped ARP response packet (II) sent from the end host  200  is forwarded toward the invention apparatus  100  which finally receives the ARP response to acquire the MAC address (=MA) of the end host  200  (P 5 ). 
     Thus, even in the event of looping, the MAC address of the end host  200  can be acquired using the ARP protocol. 
     As set forth hereinabove based on the embodiments in conjunction with the accompanying drawings, even when the L2 loop broadcast storm takes place, the present invention secures the communication path between the hosts within a looping subnet or between a host within the looping subnet and a host outside the looping subnet, enabling the communication within the subnet. 
     This ensures reliability of the communication qualities, contributing to industry to a great extent. 
     While the illustrative and presently preferred embodiments of the present invention have been described in detail herein, it is to be understood that the inventive concepts may be otherwise variously embodied and employed and that the appended claims are intended to be construed to include such variations except insofar as limited by the prior art.