Replay apparatus capable of preventing mistaken learning of MAC addresses learning table

0001This application is based upon and claims the benefit of priority from Japanese patent application No. <patcit id="pcit0001" dnum="JP2007142723A"><text>2007-142723, filed on May 30, 2007</text></patcit>.

Background of the Invention:

0002This invention relates to a relay apparatus, a network, and a frame relay method and, in particularly, to technologies for working around a loop on the network and for ensuring redundancy in a case where a plurality of relay apparatuses are connected in a loop fashion to compose the network.

0003In a case where a plurality of relay apparatuses are connected to a network, there are ring redundancy methods as technologies for working around a loop on the network and for ensuring redundancy. One of the ring redundancy methods is disclosed in an informal paper under the title of "<nplcit id="ncit0001" npl-type="s"><text>Extreme Network' Ethernet (registered trademark) Automatic Protection Switching (EAPS) Version 1", RFC (Request For Comments) 3619 (October 2003</text></nplcit>).

0004However, in a case of carrying out a path switching in the ring redundancy method, there is a possibility that reversal of arrival order of a frame occurs and it results in having a large effect on frame transfer of a higher layer. In addition, when the reversal of the arrival order of the frame occurs, an improper learning occurs in a MAC (Media Access Control) address table and there are a possibility that communication cannot be carried out after occurrence of the reversal of the arrival order.

0005In a case where a plurality of relay apparatuses are connected in a ring-shaped fashion by using a ring redundancy method, a control protocol is required to release a loop of a frame relay path. However, a current control protocol may occur reversal of arrival order of a frame in a transient state on a path switching and may have a deleterious effect on frame transfer of an upper layer. In addition, when the reversal of the arrival order occurs, a mistaken learning of the MAC address table occurs. It is therefore a possibility that it is impossible to carry out communication on or after occurrence of the reversal of the arrival order.

0006When a failure occurs in a link of a part in ring structure of the Ethernet (registered trademark), a phenomenon in which arrival order of the frames reverses occurs. By the reversal of the arrival order, it gives rise to problems as follows: <ol id="ol0001" compact="compact"><li>(1) it has adverse effect on the upper layer because the order of data reverses; and</li><li>(2) when a frame arrives through an old route after a frame arrives through a new route and a route learning is carried out, the old route is mistakenly learned in the address learning table and there is a possibility that it compromises communications after this.</li></ol>

0007Various arts related to this invention are already proposed. By way of illustration, Japanese Unexamined Patent Application Publication of Tokkai No. <patcit id="pcit0002" dnum="JP2001127782A"><text>2001-127,782</text></patcit> or <patcit id="pcit0003" dnum="JP2001127782A"><text>JP-A 2001-127782</text></patcit> (which will be also called a first patent document), which corresponds to <patcit id="pcit0004" dnum="US6952396B"><text>U.S. Pat. No. 6,952,396</text></patcit> discloses a control system which enables transport of payload data across a dual counter rotating ring (DCRR) network having two or more nodes. Each node provides access to the DCRR network by a respective local area network (LAN) subtending the node. The control system comprises a topology learning entity operative within each node for monitoring individual, links of the DCRR, discovering the active topology of the DCCR, communicating topology changes to other nodes in the DCCR, and informing frame forwarding and learning processes of the topology to enable failover to redundant resource upon detection of a network component failure. A frame forwarding process selects a shortest path route between a source and a destination node in the DCCR, and forwards frames to the destination node via the selected shortest path route. An address leaning process inspects a source medium access control (MAC) address in each received frame to learn a shortest path route for each MAC address.

0008In addition, Japanese Unexamined Patent Application Publication of Tokkai No. <patcit id="pcit0005" dnum="JP2001308893A"><text>2001-308893</text></patcit> or <patcit id="pcit0006" dnum="JP2001308893A"><text>JP-A 2001-308893</text></patcit> (which will later be called a second patent document) discloses a routing information dynamic reconfiguration system in a loop topology network that automatically performs routing of a substitute path, at on the occurrence of a fault in a communication cable and a node. The loop topology network disclosed in <patcit id="pcit0007" dnum="JP2001308893A"><text>JP-A 2001-308893</text></patcit> comprises first through fourth nodes which are connected in form of a loop. The first and the fourth nodes comprise first and fourth network interface cards (NICs) which detect a fault of a third communication cable between the first and the fourth nodes, and inform first and fourth control software items of the first and the fourth nodes about the result of detection. The first and the fourth control software items inform the second and the third nodes about fault information. The second and the third nodes eliminate all routing information via the third communication cable, transmit an alive notice packet denoting the normality of the own nodes to the first and the fourth nodes. When the first and the fourth nodes are marked as being a communication disabled state in the information of the packet, the first and the fourth nodes add the information of the own nodes to the information of the packet, transmit the resulting information to an opposite node to reconfigure the routing information to a target node.

0009Japanese Unexamined Patent Application Publication of Tokkai No. <patcit id="pcit0008" dnum="JP2004147172A"><text>2004-147172</text></patcit> or <patcit id="pcit0009" dnum="JP2004147172A"><text>JP-A 2004-147172</text></patcit> (which will later be called a third patent document), which corresponds to <patcit id="pcit0010" dnum="US20040081082A"><text>U. S. Patent Application Publication No. 2004/0081082</text></patcit>, discloses a ring switchover method which enables a fast ring switchover in a packet processing at the time of ring configuration. In the third patent document, a system switchover function can be implemented in an Ethernet switch or the like by the ring switchover method in a network constituted of layer 2 switches connected to a ring shape. Each layer 2 switch has a path control function and a failure detection function. The ring switchover method includes the steps of providing in each layer 2 switch and address leaning table in which a Media Access Control (MAC) address and a corresponding port are stored; on detecting of a link failure between mutually neighboring layer 2 switches, transmitting a failure notification frame packet from each neighboring layer 2 switch; and in the layer 2 switch having received the failure notification frame, recording a Media Access Control (MAC) address of said layer 2 switch into the failure notification frame, and transferring the failure notification frame to a neighboring layer 2 switch.

0010Japanese Unexamined Patent Application Publication of Tokkai No. <patcit id="pcit0011" dnum="JP2005027039A"><text>2005-27,039</text></patcit> or <patcit id="pcit0012" dnum="JP2005027039A"><text>JP-A 2005-27039</text></patcit> (which will later be called a fourth patent document) discloses a duplex ring network performing uninterruptible transmission which is equivalent to a line switching technology without affecting the transmission efficiency of a packet. The duplex ring network disclosed in the fourth patent document comprises first through seventh nodes which are connected in a ring-shaped fashion. When the first node transmits a unicast data packet to the second through the seventh nodes, the first node selects a ring with the shortest path. It will be assumed that the first node acts as a transmission source node while the sixth node acts as a destination node. When the failure occurs in the selected ring with the shortest path, the first node (the transmission source node) selects a ring that does not pass through a failure place In this case, the transmission source node stores a unicast data packet for all nodes at a normal time ahead of the failure occurrence and transmits the stored unicast data packet the ring that does not pass through the failure place when the destination node (the sixth node) is not on the ring with the shortest path when the transmission source node selects the ring that does not pass through the failure place. Thus, the unicast data packet for the destination node can be transmitted through the second through the fifth nodes with no short break.

0011Japanese Unexamined Patent Application Publication of Tokkai No. <patcit id="pcit0013" dnum="JP2005252672A"><text>2005-252,672</text></patcit> or <patcit id="pcit0014" dnum="JP2005252672A"><text>JP-A 2005-252672</text></patcit> (which will later be called a fifth patent document) discloses a switching hub, in which relay having no flooding immediately, when switching communication path by a barrier generating is made possible or to which the communication can return immediately. <patcit id="pcit0015" dnum="JP2005252672A"><text>JP-A 2005-252672</text></patcit> discloses a ring-shaped network comprising first through eighth switching hubs which are connected to a transmission path in a ring-shaped fashion. The first switching hub having a function for logically disconnecting a loop by logically blocking a clockwise side port thereof. It will be assumed that a failure occurs in the transmission path between the fourth and the fifth switching hub. In this event, the fourth switching hub re-registers fifth through eighth address learning information pieces into a clockwise side port of the opposite side. The fifth through the eighth address learning information pieces are transmitted from the clockwise side port of the fourth switching hub. When the third switching port receives the fifth through the eighth address learning information pieces, the fifth through the eighth address learning information pieces are re-registered to a clockwise side port of the opposite side.

0012However, any of the first through the fifth patent documents neither discloses nor teaches reversal of arrival order of a frame and mistaken learning of a MAC address learning table.

0013<patcit id="pcit0016" dnum="US2006120279A"><text>US Patent Application US 2006120279</text></patcit> (A1) discloses a network including a plurality of redundant nodes and ordinary nodes. A first redundant node from among the redundant nodes sets a common address and setting information in a message packet and broadcasts the message packet over the network. The common address is commonly used by all the redundant nodes on the network. The setting information is indicative of the fact that the first redundant node is a redundant node. All the nodes have a topology table. The nodes that receive the message packet set in their topology table information indicating of presence of the first redundant node and the common address shared by the redundant nodes. When transmitting a packet to the first redundant node, a node sets the common address as a destination address of the packet.

0014In ITU-T DRAFT STUDY PERIOD 2005-2008, INTERNATIONAL TELECOMMUNICATION UNION, GENEVA ; CH, vol. STUDY GROUP 13, 25 April 2005 (2005-04-25), pages 1-6, XP017406363, Akio Endo from Fujitsu Limited Japan studies the necessity of the four types of Loopback state options for the ETH-LS defined in the draft Recommendation Y.17ethoam and the effective situation for each option from the service status point of view, i.e. in-service or out-of-service.

Summary of the Invention

0015The present invention is defined in the independent claims.

0016It is an exemplary object of this invention to provide a relay apparatus, a network, and a frame relay method which are capable of preventing reversal of arrival order of a frame.

0017It is another exemplary object of this invention to provide a relay apparatus, a network, and a frame relay method which are capable of preventing mistaken leaning of a MAC address learning table.

0018Other exemplary objects of this invention will become clear as the description proceeds.

0019According to a first exemplary aspect of this invention, a relay apparatus includes a MAC (Media Access Control) address learning table for recording transfer destinations for frames. The relay apparatus is connected to other relay apparatuses to compose a network of a ring redundancy method. The relay apparatus includes a port move detecting circuit for detecting a port move which occurs in a case where a frame arrives from other than a path learned in the MAC address learning table on a path changing on ring redundancy.

0020According to a second exemplary aspect of this invention, a network of a ring redundancy method includes a plurality of relay apparatuses which are connected to each other in a ring fashion. Each relay apparatus includes a MAC (Media Access Control) address learning table for recording transfer destinations for frames. Each of the relay apparatuses includes a port move detecting circuit for detecting a port move which occurs in a case where a frame arrives from other than a path learned in the MAC address learning table on a path changing on ring redundancy.

0021According to a third exemplary aspect of this invention, a frame relay method is for use in a relay apparatus including a MAC (Media Access Control) address learning table for recording transfer destinations for frames. The relay apparatus is connected to other relay apparatuses in a ring-shaped connection to compose a network of a ring redundancy method, where a part of a route is systematically interrupted to release a loop state of said network. The frame relay method comprises, in the relay apparatus, detecting a port move which occurs in a case where a frame arrives from other than a path learned in said MAC address learning table on a path changing on the ring redundancy, said path changing being a route changing carried out by opening said interrupted part after occurrence of a failure in the ring-shaped connection; counting a count when the port move is detected; controlling renewal of said MAC address learning table on the basis of the count of said counter; said renewal controlling step renewing said MAC address learning table when the port move occurs once, and said renewal controlling step not renewing said MAC address leaning table and inhibiting transfer of the frame for making said port move occur when the port move occurs twice or more; and measuring a time interval by a timber since a failure occurs in the ring-shaped connection, whereby said timer cancels contents stored in said MAC address learning table when said timer detects a lapse of a predetermined time interval.

Brief Description of the Drawings:

0022<ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Fig. 1</figref> is a block diagram showing a configuration example of a relay apparatus according to an exemplary embodiment of this invention;</li><li><figref idref="f0002">Fig. 2</figref> is a view for use in describing operation of a network according to an exemplary embodiment of this invention;</li><li><figref idref="f0003">Fig. 3</figref> is a view for use in describing operation of a network according to the exemplary embodiment of this invention;</li><li><figref idref="f0004">Fig. 4</figref> is a flow chart for use in describing operation of the relay apparatus according to the exemplary embodiment of this invention;</li><li><figref idref="f0005">Fig. 5</figref> is a flow chart for use in describing operation of the relay apparatus according to the exemplary embodiment of this invention;</li><li><figref idref="f0006">Fig. 6</figref> is a sequence chart for use in describing a first operation example of the network according to the exemplary embodiment of this invention;</li><li><figref idref="f0007">Fig. 7</figref> is a sequence chart for use in describing a second operation example of the network according to the exemplary embodiment of this invention;</li><li><figref idref="f0008">Fig. 8</figref> is a sequence chart for use in describing a third operation example of the network according to the exemplary embodiment of this invention;</li><li><figref idref="f0009">Fig. 9</figref> is a sequence chart for use in describing a fourth operation example of the network according to the exemplary embodiment of this invention; and</li><li><figref idref="f0010">Fig. 10</figref> is a sequence chart for use in describing an operation of an network according to a related art.</li></ul>

Description of the Exemplary Embodiment:

0023<figref idref="f0001">Fig. 1</figref> is a block diagram showing structure of a relay apparatus 1 according to an exemplary embodiment of this invention. In <figref idref="f0001">Fig. 1</figref>, the relay apparatus 1 comprises a MAC (Media Access Control) address learning table 11, a timer 12, a frame switching circuit 13, first through N-th ports 14-1 to 14-N, a port move detecting circuit 15, and a counter 16, where N represents a positive integer which is not less than two.

0024The MAC address learning table 11 is connected to the frame switching circuit 13 and records or stores destination MAC addresses for frames and port names indicative of ports for transferring the frames. When the MAC address learning table 11 is supplied from the timer 12 with a notice of MAC address learning table cancellation, the MAC address learning table 11 cancels all of contents recorded therein.

0025The timer 12 starts timing or clocking by the frame switching circuit 13 at a timing start time instant. When a predetermined time interval elapses from the timing start time instant, the timer 12 notifies the MAC address learning table 11 of the MAC address learning table cancellation.

0026When a frame is transferred to the frame switching circuit 13 through the port move detecting circuit 15, the frame switching circuit 13 refers the MAC address learning table 11 to carry out processing of the frame. When a MAC address learning table cancellation frame is transferred from the port move detecting circuit 15 to the frame switching circuit 13, the frame switching circuit 13 makes the timer 12 start the timing or the clocking thereof.

0027The port move detecting circuit 14 is connected to the first through the N-th ports 14-1 to 14-N. Regardless of whether the frame transferred from the first port 14-1 has the same transmission source MAC address, the port move detection circuit 15 detects a port move transferred from the N-th port 14-N which is another port.

0028When the port move detecting circuit 15 receives frames from the first through the N-th ports 14-1 to 14-N, the port move detecting circuit 15 refers the MAC address learning table 11 to detect the presence or absence in the occurrence of the port move. When the port move detecting circuit 15 detects the port move, the port move detecting circuit 15 notifies the counter 16 that the port move occurs. The port move detecting circuit 15 transfers, to the frame switching circuit 13, the frames transferred from the first through the N-th ports 14-1 to 14-N.

0029Every when the port move detecting circuit 15 notifies the counter 16 that the port move occurs, the counter 16 counts a count notified every transmission source MAC address of the frame as a port move occurrence count of the transmission source MAC address of the frame.

0030The frame switching circuit 13 refers the counter 16 and does not carry out transfer of the frame if the port move occurrence count of the transmission source MAC address of the frame is equal to two or more.

0031The first through the N-th ports 14-1 to 14-N are connected to first through N-th other relay apparatuses #1 to #N and send communication contents to the port move detecting circuit 15 as it is.

0032<figref idref="f0002">Figs. 2</figref> and <figref idref="f0003">3</figref> are views for use in describing operation of a network according to an exemplary embodiment of this invention.

0033Referring now <figref idref="f0002">Fig. 2</figref>, the description will be made as regards operation in a case where any failure does not occurs in a ring connection of the relay apparatuses related to this invention.

0034The illustrated network is a network where first through fourth relay apparatuses 100, 101, 102, and 103 are connected to each other in a ring fashion in a counterclockwise direction. In the example being illustrated, the first through the fourth relay apparatus 100 to 103 are referred to as a relay apparatus A, a relay apparatus B, a relay apparatus C, and a relay apparatus D, respectively. In <figref idref="f0002">Fig. 2</figref>, the first through the fourth relay apparatuses 100 to 103 are depicted at A, B, C, and D, respectively.

0035Inasmuch as the first through the fourth apparatuses 100 to 103 are connected to each other in the ring fashion as it is, the frame is transmitted in a loop fashion when the frame is transmitted. As a result, a phenomenon in which a communication path or route is saturated with unnecessary frames occurs.

0036Therefore, a part of the communication path or route, that is a secondary port 104 of the second relay apparatus 101, is systematically broken or interrupted to release a loop state of the network. In the example being illustrated in <figref idref="f0002">Fig. 2</figref>, a breaking or an interrupting is made between the first relay apparatus 100 and the second relay apparatus 101. Under the circumstances, it will be assumed that the first relay apparatus 100 transfers, to the second relay apparatus 101, a frame (communication 105 between the first relay apparatus 100 and the second relay apparatus 101) in a clockwise direction. In this event, the frame advances the path or the route of the first relay apparatus 100, the fourth relay apparatus 103, the third relay apparatus 102, and the first relay apparatus 101 in the clockwise direction to arrive at a destination MAC address.

0037Referring to <figref idref="f0003">Fig. 3</figref>, the description will be made as regards operation in a case where a failure 107 occurs between the first relay apparatus 100 and the fourth relay apparatus 103 in the network which is described in conjunction with <figref idref="f0002">Fig. 2</figref>.

0038It will be presumed that communication between first relay apparatus 100 and the second relay apparatus 101 occurs. Under the circumstances, when the failure 107 occurs in the communication path or route in <figref idref="f0002">Fig. 2</figref>, it is impossible to carry out the communication between the first relay apparatus 100 and the second relay apparatus 101. Therefore, by opening the secondary port 104 of the second relay apparatus 101 where the communication is broken or interrupted, it is possible to carry out communication 106 between the first relay apparatus 100 and the second relay apparatus 101 in a counterclockwise direction.

0039Referring to <figref idref="f0010">Fig. 10</figref>, the description will proceed to an example where a mistaken learning occurs in a case where the failure 107 occurs. <figref idref="f0010">Fig. 10</figref> is a sequence chart for use in describing an operation of a network of a related art.

0040A zeroth frame #0 is transferred from the first relay apparatus 100 toward the second relay apparatus 101 in the clockwise direction (at a first event e1 in <figref idref="f0010">Fig. 10</figref>). Therefore, the MAC address learning table 11 in each relay apparatus is renewed and a path or a route passing through the first relay apparatus 100, the fourth relay apparatus 103, the third relay apparatus 102, and the second relay apparatus 101 in the clockwise direction is recoded in the MAC address learning table 11. In addition, a path or a route between the first relay apparatus 100 and the second relay apparatus 101 is broken or interrupted by the secondary port 104 of the second relay apparatus 101 in order to release the loop state of the network (at a second event e2 in <figref idref="f0010">Fig. 10</figref>).

0041A first frame # 1 is transferred from the first relay apparatus 100 toward the second relay apparatus 101 in the clockwise direction (at a third event e3 in <figref idref="f0010">Fig. 10</figref>).

0042At a time instant when the first frame #1 is transferred to the fourth relay apparatus 103, the failure 107 occurs between the first relay apparatus 100 and the fourth relay apparatus 103 (at a fourth event e4 in <figref idref="f0010">Fig. 10</figref>).

0043Ports bordering a zone where the above-mentioned failure 107 (at the fourth event e4 in <figref idref="f0010">Fig. 10</figref>) occurs are broken or interrupted.

0044Communication between the first relay apparatus 100 and the second relay apparatus 101 that is broken or interrupted (at the second event e2 in <figref idref="f0010">Fig. 10</figref>) by the secondary port 104 is opened (at a seventh event e7 in <figref idref="f0010">Fig. 10</figref>).

0045Inasmuch as the failure 107 occurs (at the fourth event e4 in <figref idref="f0010">Fig. 10</figref>), a MAC address learning table cancellation frame is transmitted from the first relay apparatus 100 in the counterclockwise direction (at a fifth event e5 in <figref idref="f0010">Fig. 10</figref>).

0046Each of the first through the fourth relay apparatuses 100 to 103 cancels contents stored in the MAC address learning table 11 (at sixth, eighth, tenth, and eleventh events e6, e8, e10, and e11 in <figref idref="f0010">Fig. 10</figref>) upon receipt of the MAC address learning table cancellation frame (at the fifth event e5 in <figref idref="f0010">Fig. 10</figref>).

0047The first relay apparatus 100 transmits a second frame #2 toward the second relay apparatus 101 in the counterclockwise direction (at a ninth event e9 in <figref idref="f0010">Fig. 10</figref>). The second frame #2 is directly transmitted from the first relay apparatus 100 to the second relay apparatus 101. This is because the contents stored in the MAC address learning table 11 are cancelled and a flooding occurs caused by absent of the path or the route.

0048When the second frame #2 arrives in the second relay apparatus 101 (at the ninth event e9 in <figref idref="f0010">Fig. 10</figref>), the MAC address learning table 11 in the second relay apparatus 101 is renewed by using information in the second frame #2 by the frame switching circuit 13 (at the ninth event e9 in <figref idref="f0010">Fig. 10</figref>).

0049Thereafter, the first frame # 1 arrives in the second relay apparatus 101 from the fourth relay apparatus 103 in the clockwise direction and the MAC address learning table 11 in the second relay apparatus 10 is renewed by using information in the first frame # 1 (at the third event e3 in <figref idref="f0010">Fig. 10</figref>) which is old mistaken information.

0050Inasmuch as the MAC address learning table 11 in the second relay apparatus 101 make a mistake, it is impossible to carry out communication in a case where the communication is carried out through the second relay apparatus 101.

0051In a case where the first through the fourth relay apparatuses 100 to 103 are connected in the ring-shaped fashion by using the ring redundancy method, a control protocol is required to release the loop of a frame relay path or route. However, a current or a conventional control protocol may occur reversal of arrival order of a frame in a transient state on a path or route switching and may have a deleterious effect on frame transfer of an upper layer. In addition, when the reversal of the arrival order occurs, a mistaken learning of the MAC address learning table 11 occurs. It is therefore a possibility that it is impossible to carry out communication on or after occurrence of the reversal of the arrival order, as mentioned in the preamble of the instant specification.

0052When the failure 107 occurs in a link of a part in ring structure of the Ethernet (registered trademark), a phenomenon in which arrival order of the frames reverses occurs. By the reversal of the arrival order, it gives rise to problems as follows: <ol id="ol0002" compact="compact"><li>(1) it has adverse effect on the upper layer because the order of data reverses; and</li><li>(2) when a frame arrives through an old path or route after a frame arrives through a new path or route and a path or route learning is carried out, the old path or route is mistakenly learned in the MAC address learning table 11 and there is a possibility that it compromises communications after this.</li></ol>

0053<figref idref="f0004">Figs. 4</figref> and <figref idref="f0005">5</figref> show flowcharts for use in describing operation of the relay apparatus 1 according to the exemplary embodiment of this invention.

0054Referring to <figref idref="f0001 f0002 f0003 f0004 f0005">Figs. 1 to 5</figref>, the description will be made as regards operation in a case where the failure 107 occurs between the first relay apparatus 100 and the fourth relay apparatus 103 in the network.

0055It will be assumed that the failure 107 occurs in the network (at a step S 1 in <figref idref="f0004">Fig. 4</figref>). In this event, a master switch, which detects that the failure 107 occurs, transmits a MAC address learning table cancellation frame (at a step S 1 in <figref idref="f0004">Fig. 4</figref>). When each relay apparatus receives the MAC address learning table cancellation frame, each relay apparatus determines operation in accordance with whether or not the relay apparatus is adjacent to a failure zone (at steps S3 and S4 in <figref idref="f0004">Fig. 4</figref>).

0056It will be assumed that the relay apparatus 1 is adjacent to the failure zone at the step S4 in <figref idref="f0004">Fig. 4</figref>. In this event, the step S4 is followed by a step S5 at which the relay apparatus 1 immediately cancels contents stored in the MAC address learning table 11.

0057It will be presumed that the relay apparatus 1 is not adj acent to the failure zone at the step S4 in <figref idref="f0004">Fig. 4</figref>. Under the circumstances, the step S4 is succeeded by a step S6 at which the relay apparatus 1 starts the timer 12 and initializes the counter 16. Thereafter, the relay apparatus 1 transfers the MAC address learning table cancellation frame to other relay apparatuses (at step S7 in <figref idref="f0004">Fig. 4</figref>). Activated by the MAC address learning table cancellation frame, after a lapse of the predetermined time interval in the timer 12 (at step S8 in <figref idref="f0004">Fig. 4</figref>), the timer 12 of the relay apparatus 1 cancels the contents stored in the MAC address learning table 11 (at a step S9 in <figref idref="f0004">Fig. 4</figref>).

0058Turning to <figref idref="f0005">Fig. 5</figref>, it will be assumed that the timer 12 does not start (in NO at a step S 11 in <figref idref="f0005">Fig. 5</figref>). In this event, the relay apparatus 1 transfers, to other relay apparatuses, frames where each relay apparatus should transfer to the other relay apparatuses (at a step S 18 in <figref idref="f0005">Fig. 5</figref>).

0059It will be presumed that the timer 12 starts (in YES at the step S 11 in <figref idref="f0005">Fig. 5</figref>). Under the circumstances, the relay apparatus 1 carries out the following operation.

0060It will be assumed that the relay apparatus 1 receives the frame (at a step S12 in <figref idref="f0005">Fig. 5</figref>). In this event, the port move detecting circuit 15 determines whether or not a port move occurs (at a step S 13 in <figref idref="f0005">Fig. 5</figref>). When the port move does not occur (NO at the step S 13), the step S 13 proceeds to the step S 18 at which the relay apparatus 1 transfers the frame to the other relay apparatuses in the usual way.

0061It will be presumed that the port move occurs (YES at the step S 13 in <figref idref="f0005">Fig. 5</figref>). Under the circumstances, the relay apparatus 1 refers the occurrence count of the port move recorded in the counter 16. It will be assumed that the occurrence count of the port move is zero (YES at a step S 14 in <figref idref="f0005">Fig. 5</figref>). In this event, the port move detecting circuit 15 of the relay apparatus 1 renews the value or the count (the occurrence count) in the counter 16 so as to increment by one (at a step S15 in <figref idref="f0005">Fig. 5</figref>), the frame switching circuit 13 of the relay apparatus 1 renews the MAC address learning table 11 (at a step S16 in <figref idref="f0005">Fig. 5</figref>) and transfers the frame to the other relay apparatuses through the port move detecting circuit 15 (at the step S 18 in <figref idref="f0005">Fig. 5</figref>).

0062It will be presumed that the occurrence count of the port move is not zero (NO at the step S 14 in <figref idref="f0005">Fig. 5</figref>). Under the circumstances, the frame switching circuit 13 of the relay apparatus 1 deletes the frame (at a step S 19 in <figref idref="f0005">Fig. 5</figref>). Therefore, the frame switching circuit 13 of the relay apparatus 1 does not transfer the frame to the other relay apparatuses.

0063<figref idref="f0006 f0007 f0008 f0009">Figs. 6 through 9</figref> are sequence charts for use in describing first through fourth operation examples of the network according to the exemplary embodiment of this invention.

0064Referring to <figref idref="f0006">Figs. 6</figref> thorough 9, the description will be made as regards operations and advantages of the network according to the exemplary embodiment of this invention.

0065Referring now <figref idref="f0006">Fig. 6</figref>, the description will proceeds to the first operation example in a case where the failure 107 occurs and an arrival order of a frame reverses when a transfer path or route is recorded or stored in the MAC address learning table 11. Herein, each of the first through the fourth relay apparatuses 100, 101, 102, and 103 is similar in structure to the relay apparatus illustrated in <figref idref="f0001">Fig. 1</figref>.

0066A zeroth frame #0 is transferred from the first relay apparatus 100 toward the second relay apparatus 101 in a clockwise direction (at a first event a1 in <figref idref="f0006">Fig. 6</figref>). Therefore, the MAC address learning table 11 in each relay apparatus is renewed and a path or route passing through the first relay apparatus 100, the fourth relay apparatus 103, the third relay apparatus 102, and the second relay apparatus 101 in the clockwise direction is recoded in the MAC address learning table 11. In addition, a path or route between the first relay apparatus 100 and the second relay apparatus 101 is broken or interrupted by the secondary port 104 of the second relay apparatus 101 in order to release the loop state of the network (at a second event a2 in <figref idref="f0006">Fig. 6</figref>).

0067A first frame # 1 is transferred from the first relay apparatus 100 toward the second relay apparatus 101 in the clockwise direction (at a third event a3 in <figref idref="f0006">Fig. 6</figref>).

0068At a time instant when the first frame #1 is transferred to the fourth relay apparatus 103, the failure 107 occurs between the first relay apparatus 100 and the fourth relay apparatus as shown in <figref idref="f0003">Fig. 3</figref> (at a fourth event a4 in <figref idref="f0006">Fig. 6</figref>).

0069Communication between the first relay apparatus 100 and the second relay apparatus 101 in a counterclockwise direction that is broken or interrupted (at the second event a2 in <figref idref="f0006">Fig. 6</figref>) is opened by the secondary port 104 of the second relay apparatus 101 (at a seventh event a7 in <figref idref="f0006">Fig. 6</figref>).

0070Inasmuch as the failure 107 occurs, a MAC address learning table cancellation frame is transmitted from the first relay apparatus 100 in the counterclockwise direction (at a fifth event a5 in <figref idref="f0006">Fig. 6</figref>) and the first and the fourth relay apparatuses 100 and 103, which are adjacent a failure zone, cancel contents stored in the MAC address learning table 11 upon receipt of the MAC address learning table cancellation frame (at sixth and thirteenth events a6 and a13 in <figref idref="f0006">Fig. 6</figref>).

0071A second frame #2 is transmitted from the first relay apparatus 100 toward the second relay apparatus 101 in the counterclockwise direction (at an eighth event a8 in <figref idref="f0006">Fig. 6</figref>). The second frame #2 is directly transmitted from the first relay apparatus 100 to the second relay apparatus 101. This is because the contents stored in the MAC address learning table 11 is cancelled and a flooding occurs caused by absent of the path or route.

0072When the second frame #2 arrives in the second relay apparatus 101, the port move occurs because the second frame #2 arrives through a path or route which is different from that through which the zeroth frame #0 arrives. Under the circumstances, the MAC address learning table 11 in the second relay apparatus 101 is renewed by the frame switching circuit 13 using information in the second frame #2 and the counter 16 in the second relay apparatus 101 is counted up by the port move detecting circuit 15.

0073Thereafter, the first frame # 1 arrives in the second relay apparatus 10 1 in the clockwise direction. However, inasmuch as the port move for the second time occurs, the MAC address learning table 11 in the second relay apparatus 10 is not renewed and the first frame #1 is not transferred.

0074When the MAC address learning table cancellation frame arrives at the second relay apparatus 101 and the third relay apparatus 102 in the counterclockwise direction, contents stored in the MAC address learning table 11 in the second and the third relay apparatuses 101 and 102 are cancelled (at tenth and twelfth events a10 and a12 in <figref idref="f0006">Fig. 6</figref>) after a lapse of the predetermined time interval (at ninth and eleventh events a9 and a11 in <figref idref="f0006">Fig. 6</figref>).

0075Referring now <figref idref="f0007">Fig. 7</figref>, the description will proceeds to the second operation example in a case where the failure 107 occurs and a frame arrives in a transmission order when a transfer path or route is recorded or stored in the MAC address learning table 11.

0076A zeroth frame #0 is transferred from the first relay apparatus 100 toward the second relay apparatus 101 in a clockwise direction (at a first event b 1 in <figref idref="f0007">Fig. 7</figref>). Therefore, the MAC address learning table 11 in each relay apparatus is renewed by the frame switching circuit 13 and a path or route passing through the first relay apparatus 100, the fourth relay apparatus 103, the third relay apparatus 102, and the second relay apparatus 101 in the clockwise direction is recoded in the MAC address learning table 11. In addition, a path or route between the first relay apparatus 100 and the second relay apparatus 101 is broken or interrupted by the secondary port 104 of the second relay apparatus 101 in order to release the loop state of the network (at a second event b2 in <figref idref="f0007">Fig. 7</figref>).

0077A first frame # 1 is transferred from the first relay apparatus 100 toward the second relay apparatus 101 in the clockwise direction (at a third event b3 in <figref idref="f0007">Fig. 7</figref>).

0078At a time instant when the first frame # 1 is transferred to the fourth relay apparatus 103, the failure 107 occurs between the first relay apparatus 100 and the fourth relay apparatus 103 as shown in <figref idref="f0003">Fig. 3</figref> (at a fourth event b4 in <figref idref="f0007">Fig. 7</figref>).

0079Communication between the first relay apparatus 100 and the second relay apparatus 101 that is broken or interrupted in the counterclockwise direction (at the second event b2 in <figref idref="f0007">Fig. 7</figref>) is opened by the secondary port of the second relay apparatus 101 (at a seventh event b7 in <figref idref="f0007">Fig. 7</figref>) because the failure 107 occurs.

0080Inasmuch as the failure 107 occurs, a MAC address learning table cancellation frame is transmitted from the first relay apparatus 100 in the counterclockwise direction (at a fifth event b5 in <figref idref="f0007">Fig. 7</figref>) and the first and the fourth relay apparatuses 100 and 103, which are adjacent a failure zone, cancel contents stored in the MAC address learning table 11 upon receipt of the MAC address learning table cancellation frame (at sixth and thirteenth events b6 and b13 in <figref idref="f0007">Fig. 7</figref>).

0081A second frame #2 is transmitted from the first relay apparatus 100 toward the second relay apparatus 101 in the counterclockwise direction (at an eighth event b8 in <figref idref="f0007">Fig. 7</figref>). The second frame #2 is directly transmitted from the first relay apparatus 100 to the second relay apparatus 101. This is because the contents stored in the MAC address learning table 11 is cancelled and a flooding occurs caused by absent of the path or route.

0082The first frame #1 arrives in the second relay apparatus 101. In this event, a port move does not occur because the first frame #1 arrives in the second relay apparatus 101 through a path or route in the clockwise direction that is similar to that through the zeroth frame #0 arrives.

0083Thereafter, the second frame #2 arrives in the second relay apparatus 101. In this event, the frame switching circuit 13 of the second relay apparatus 101 carries out renewal of the MAC address learning table 11 in the second relay apparatus 101.

0084When the MAC address learning table cancellation frame arrives at the second relay apparatus 101 and the third relay apparatus 102, contents stored in the MAC address learning table 11 in the second and the third relay apparatuses 101 and 102 are cancelled (at tenth and twelfth events b10 and b12 in <figref idref="f0007">Fig. 7</figref>) after a lapse of the predetermined time interval (at ninth and eleventh events b9 and b11 in <figref idref="f0007">Fig. 7</figref>).

0085Subsequently referring <figref idref="f0008">Fig. 8</figref>, the description will proceeds to the third operation example in a case where the failure 107 occurs and an arrival order of a frame reverses when a transfer path or route is not recorded or not stored in the MAC address learning table 11.

0086A first frame # 1 is transferred from the first relay apparatus 100 toward the second relay apparatus 101 in a clockwise direction (at a first event c1 in <figref idref="f0008">Fig. 8</figref>). In addition, a path or route between the first relay apparatus 100 and the second relay apparatus 101 is broken or interrupted by the secondary port 104 of the second relay apparatus 101 in order to release the loop state of the network (at a second event c2 in <figref idref="f0008">Fig. 8</figref>).

0087At a time instant when the first frame # 1 is transferred to the fourth relay apparatus 103, the failure 107 occurs between the first relay apparatus 100 and the fourth relay apparatus as shown in <figref idref="f0003">Fig. 3</figref> (at a third event c3 in <figref idref="f0008">Fig. 8</figref>).

0088Communication between the first relay apparatus 100 and the second relay apparatus 101 in the counterclockwise direction that is broken or interrupted (at the second event c2 in <figref idref="f0008">Fig. 8</figref>) is opened by the secondary port 104 of the second relay apparatus 101 (at a sixth event c6 in <figref idref="f0008">Fig. 8</figref>).

0089Inasmuch as the failure 107 occurs, a MAC address learning table cancellation frame is transmitted from the first relay apparatus 100 in the counterclockwise direction (at a fourth event c4 in <figref idref="f0008">Fig. 8</figref>) and the first and the fourth relay apparatuses 100 and 103, which are adjacent a failure zone, cancel contents stored in the MAC address learning table 11 upon receipt of the MAC address learning table cancellation frame (at fifth and twelfth events c5 and c12 in <figref idref="f0008">Fig. 8</figref>).

0090A second frame #2 is transmitted from the first relay apparatus 100 toward the second relay apparatus 101 in the counterclockwise direction (at a seventh event c9 in <figref idref="f0008">Fig. 8</figref>). The second frame #2 is directly transmitted from the first relay apparatus 100 to the second relay apparatus 101. This is because the contents stored in the MAC address learning table 11 is cancelled and a flooding occurs caused by absent of the path or route.

0091When the second frame #2 arrives in the second relay apparatus 101, the MAC address learning table 11 in the second relay apparatus 101 is renewed by the frame switching circuit 13 using information in the second frame #2 because path or route information is not recorded or not stored in the MAC address learning table 11.

0092Thereafter, the first frame #1 arrives in the second relay apparatus 101 in the clockwise direction. However, inasmuch as the port move for the second time occurs, the MAC address learning table 11 in the second relay apparatus 10 is not renewed and the first frame #1 is not transferred.

0093When the MAC address learning table cancellation frame arrives at the second relay apparatus 101 and the third relay apparatus 102, contents stored in the MAC address learning table 11 in the second and the third relay apparatuses 101 and 102 are cancelled by the timer 12 (at ninth and eleventh events c9 and c11 in <figref idref="f0008">Fig. 8</figref>) after a lapse of the predetermined time interval (at eighth and tenth events c8 and c10 in <figref idref="f0008">Fig. 8</figref>).

0094Furthermore referring now <figref idref="f0009">Fig. 9</figref>, the description will proceeds to the fourth operation example in a case where the failure 107 occurs and a frame arrives in a transmission order when a transfer or route path is not recorded or not stored in the MAC address learning table 11.

0095A first frame # 1 is transferred from the first relay apparatus 100 toward the second relay apparatus 101 in a clockwise direction (at a first event d1 in <figref idref="f0009">Fig. 9</figref>). In addition, a path or route between the first relay apparatus 100 and the second relay apparatus 101 is broken or interrupted by the secondary port 104 of the second relay apparatus 10 1 in order to release the loop state of the network (at a second event d2 in <figref idref="f0009">Fig. 9</figref>).

0096At a time instant when the first frame # 1 is transferred to the fourth relay apparatus 103, the failure 107 occurs between the first relay apparatus 100 and the fourth relay apparatus 103 as shown in <figref idref="f0003">Fig. 3</figref> (at a third event d3 in <figref idref="f0009">Fig. 9</figref>).

0097Communication between the first relay apparatus 100 and the second relay apparatus 101 in a counterclockwise direction that is broken or interrupted (at the second event d2 in <figref idref="f0009">Fig. 9</figref>) is opened by the secondary port 104 of the second relay apparatus 101 (at a sixth event d6 in <figref idref="f0009">Fig. 9</figref>) because the failure 107 occurs.

0098Inasmuch as the failure 107 occurs, a MAC address learning table cancellation frame is transmitted from the first relay apparatus 100 in the counterclockwise direction (at a fourth event d4 in <figref idref="f0009">Fig. 9</figref>) and the first and the fourth relay apparatuses 100 and 103, which are adjacent a failure zone, cancel contents stored in the MAC address learning table 11 upon receipt of the MAC address learning table cancellation frame (at fifth and twelfth events d5 and d 12 in <figref idref="f0009">Fig. 9</figref>).

0099A second frame #2 is transmitted from the first relay apparatus 100 toward the second relay apparatus 10 1 in the counterclockwise direction (at a seventh event c7 in <figref idref="f0009">Fig. 9</figref>). The second frame #2 is directly transmitted from the first relay apparatus 100 to the second relay apparatus 101. This is because the contents stored in the MAC address learning table 11 is cancelled and a flooding occurs caused by absent of the path or route.

0100When the first frame #1 arrives in the second relay apparatus 101, the MAC address learning table 11 in the second relay apparatus 101 is renewed by the frame switching circuit 13 using information in the first frame # 1 because the path or route information is not recorded or not stored in the MAC address learning table 11. In addition, the counter 16 is counted up by the port move detecting circuit 15.

0101Thereafter, the second frame #2 arrives in the second relay apparatus 101. However, inasmuch as occurrence of the port move is twice, the MAC address learning table 11 is not renewed and the second frame #2 is not transferred.

0102When the MAC address learning table cancellation frame arrives at the second relay apparatus 101 and the third relay apparatus 102, contents stored in the MAC address learning table 11 in the second and the third relay apparatuses 101 and 102 are cancelled by the timer 12 (at ninth and eleventh events d9 and d11 in <figref idref="f0009">Fig. 9</figref>) after a lapse of the predetermined time interval (at eighth and tenth events d8 and d10 in <figref idref="f0009">Fig. 9</figref>). Therefore, in the MAC address learning table 11, mistaken path or route information, which is recorded by using the first frame #1 arrived later, is deleted.

0103In the manner which is described above, according to the exemplary embodiment of this invention, in the network using the ring redundancy method which is technique for ensuring redundancy with a loop worked around in a case the first through the fourth relay apparatuses 100 to 103 are connected, when a frame arrives through a path or route rather than a learned path or route before and after a path or route switching, the exemplary embodiment comprises detecting it and carrying out a suitable processing. It is therefore possible to prevent reversal of arrival order of frames in a transient state during the path or route switching and communication failure which may occur caused by the reversal of arrival order.

0104The relay apparatus 1 according to the exemplary embodiment includes the port move detecting circuit 15 for detecting the port move occurring in the case where the frame arrives through a path or route rather than the learned path or route stored in the MAC address learning table 11, the counter 16 for counting a count where the port move is detected by the port move detecting circuit 15, the frame switching circuit 13 for determining processing when the frame is transferred to the relay apparatus 1, the MAC address learning table 11 for recording transfer destinations for the frames, and the timer 12 for measuring a time interval elapsed since the frame switching circuit 13 detects that failure occurs in the ring connection.

0105The relay apparatus 1 keeps watch on the port move by the port move detecting circuit 15 when the failure 107 is detected. When the port move occurs one, the MAC address learning table 11 is renewed by the frame switching circuit 13. When the port move occurs twice or more, the MAC address learning table 11 is not renewed and the frame for making the port move occur is not transferred. In the manner which is described above, according to the exemplary embodiment, the reversal of arrival order is prevented by doing not transfer the frame.

0106In addition, the timer 12 of the relay apparatus 1 cancels contents stored in the MAC address learning table 11 when the timer 12 detects that the predetermined time interval elapses since the failure 107 is detected. In the exemplary embodiment, mistaken path or route information is not continuously stored in the MAC address learning table 11 by cancelling the contents stored in the MAC address learning table 11.

0107Furthermore, in the relay apparatus 1, the timer 12 and the counter 16 do not operate until the failure 107 occurs again after the contents stored in the MAC address learning table 11 is cancelled.

0108In the relay apparatus of the first exemplary aspect of this invention, the above-mentioned relay apparatus may include a counter for counting a count where the port move is detected by the port move detecting circuit on the path switching on the ring redundancy, and a frame switching circuit for controlling renewal of the MAC address learning table on the basis of the count of the counter. The frame switching circuit may renew the MAC address learning table when the port move occurs once. The frame switching circuit may do not renew the MAC address learning table and may inhibit transfer of the frame for making the port move occur when the port move occurs twice or more. The relay apparatus further may include a timer for measuring a time interval since a failure occurs in a ring connection on the path switching on the ring redundancy. In this event, the timer cancels contents stored in the MAC address learning table when the timer detects a lapse of a predetermined time interval.

0109In the network of the second exemplary aspect of this invention, each of the relay apparatuses may include a counter for counting a count where the port move is detected by the port move detecting circuit on the path switching on the ring redundancy, and a frame switching circuit for controlling renewal of the MAC address learning table on the basis of the count of the counter. The frame switching circuit may renew the MAC address learning table when the port move occurs once. The frame switching circuit may do not renew the MAC address learning table and may inhibit transfer of the frame for making the port move occur when the port move occurs twice or more. Each of the relay apparatuses further may include a timer for measuring a time interval since a failure occurs in a ring connection on the path switching on the ring redundancy. In this event, the timer cancels contents stored in the MAC address learning table when the timer detects a lapse of a predetermined time interval.

0110In the frame relay method of the third exemplary aspect of this invention, the above-mentioned frame relay method may further include, in said relay apparatus, counting a count where the port move is detected on the path switching on the ring redundancy, and controlling renewal of the MAC address learning table on the basis of the count of the counter. The renewal controlling step may renew the MAC address learning table when the port move occurs once. The renewal controlling step may do not renew the MAC address learning table and may inhibit transfer of the frame for making the port move occur when the port move occurs twice or more. The frame relay method further may include, in the relay apparatus, measuring a time interval by a timer since a failure occurs in a ring connection on the path switching on the ring redundancy. In this event, the timer cancels contents stored in the MAC address learning table when the timer detects a lapse of a predetermined time interval.

0111While this invention has been particularly shown and described with reference to exemplary embodiments thereof, the invention is not limited to these embodiments. It will be understood by those of ordinary skill in the art that various changes in form and details may be therein without departing from the scope of the present invention as defined by the claims.