Apparatus and method for rapid detection of unidirectional breaks in a network ring

• US 20050201409A1
• Filed: 05/10/2005
• Published: 09/15/2005
• Est. Priority Date: 05/06/2003
• Status: Active Grant

• Current Assignee: Adtran Networks SE (ADTRAN Holdings, Inc.)
• Original Assignee: Overture Networks, Inc. (ADTRAN Holdings, Inc.)
• Inventors: Lynch, Robert Leroy, Griswold, David Stephen, Swope, Glenn Russell, Pate, Prayson Will
• Granted Patent: US 7,355,965 B2
• Time in Patent Office: Days
• US Class Current: 370/445
• CPC Class Codes: H04L 12/42 Loop networks
• CPC Class Codes: H04L 12/437 Ring fault isolation or reconfiguration for SDH/SONET ring networks H04J3/085

Abstract

Normal 802.3 Ethernet requires a tree topology. If a ring or a loop exists, then packets will be forwarded around the ring indefinitely. If the ring is broken, then there is no possibility of packets being propagated forever. This invention shows how to quickly impose a virtual break in the ring such that all nodes can communicate with each other, and how to remove the virtual break when a real failure occurs. This is accomplished by placing intelligent nodes on the ring that work together to virtually break and restore the ring. An embodiment is disclosed that handles a unidirectional break in a communication link where the unidirectional break is not sensed as an OPER DOWN state. This abstract is provided as an aid to those performing prior art searches and not a limitation on the scope of the claims.

Claims

1. A ring relay for use in a bidirectional network ring for a ring that is manipulated to provide a virtual break in order to operate within a network requiring a tree topology, the network ring carrying data packets;

the ring relay comprising the following attributes;

having a first ring port in data communication with a first incoming link and a first outgoing link and a second ring port in data communication with a second incoming link and a second outgoing link connected to the network ring;

operating to pass HELLO messages received on the first incoming link to the second outgoing link and to pass HELLO messages received on the second incoming link to the first outgoing link; and

after detecting an OPER DOWN failure on the second incoming link, reacting to the detection of the network failure on the second incoming link by;

imposing a break on the second incoming link so that data packets are blocked from passing into the ring relay but the imposed break does not block any incoming HELLO messages;

imposing a break on the second outgoing link so that data packets are blocked from leaving the ring relay on that link;

sending a type of HELLO message and a control signal out the second outgoing link such that an adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed;

detecting the cessation of the OPER DOWN failure on the second incoming link then resuming operating to pass HELLO messages received on the first incoming link to the second outgoing link; and

delaying the removal of the imposed break for user data on the second outgoing link and on the second incoming link for a time period sufficient to allow the bidirectional network ring to impose a virtual break at a device other than the ring relay.

2. The ring relay of claim 1 wherein the action of sending a type of HELLO message and a control signal out the second outgoing link such that an adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed is accompanied by sending a type of HELLO message and a control signal out the first outgoing link such that an adjacent node receives a HELLO message to provide assurance that the first outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed.

3. The ring relay of claim 1 wherein the act of sending a type of HELLO message and a control signal out the second outgoing link such that the adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed is achieved by sending a type of HELLO message that includes an indication that the ring relay is imposing a bidirectional break.

4. The ring relay of claim 1 wherein the act of sending a type of HELLO message and a control signal out the second outgoing link such that the adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed is achieved by sending a series of control messages not part of a HELLO message wherein the control message conveys the control signal to the node other than the ring relay to remove the virtual break and keep the virtual break removed as long as the series of control messages are received by the node other then the ring relay.

5. The ring relay of claim 1 wherein the act of sending a type of HELLO message and a control signal out the second outgoing link such that the adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed is achieved by sending a control message not part of a HELLO message wherein the control message conveys the control signal to the node other than the ring relay to remove the virtual break until another control message is sent conveying the need to re-impose the virtual break.

6. A ring relay for use in a bidirectional network ring for a ring that is manipulated to provide a virtual break in order to operate within a network requiring a tree topology, the network ring carrying data packets;

the ring relay comprising the following attributes;

having a first ring port in data communication with a first incoming link and a first outgoing link and a second ring port in data communication with a second incoming link and a second outgoing link connected to the network ring;

operating to pass HELLO messages received on the first incoming link to the second outgoing link and to pass HELLO messages received on the second incoming link to the first outgoing link; and

after detecting a gap in the receipt of HELLO messages on the second incoming link in excess of a set threshold, reacting to the detection of the network failure on the second incoming link by;

imposing a break on the second incoming link so that data packets are blocked from passing into the ring relay but the imposed break does not block any incoming HELLO messages;

imposing a break on the second outgoing link so that data packets are blocked from leaving the ring relay on that link;

sending a type of HELLO message and a control signal out the second outgoing link such that an adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed;

after receiving a HELLO message on the second incoming link, resuming operating to pass HELLO messages received on the first incoming link to the second outgoing link; and

delaying the removal of the imposed break for user data on the second outgoing link and on the second incoming link for a time period sufficient to allow the bidirectional network ring to impose a virtual break at a device other than the ring relay.

7. The ring relay of claim 6 wherein the act of sending a type of HELLO message and a control signal out the second outgoing link such that an adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed is accompanied by sending a type of HELLO message and a control signal out the first outgoing link such that an adjacent node receives a HELLO message to provide assurance that the first outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed.

8. The ring relay of claim 6 wherein the act of sending a type of HELLO message and a control signal out the second outgoing link such that the adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed is achieved by sending a type of HELLO message that includes an indication that the ring relay is imposing a bidirectional break.

9. The ring relay of claim 6 wherein the act of sending a type of HELLO message and a control signal out the second outgoing link such that the adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed is achieved by sending a series of control messages not part of a HELLO message wherein the control message conveys the control signal to the node other than the ring relay to remove the virtual break and keep the virtual break removed as long as the series of control messages are received by the node other then the ring relay.

10. The ring relay of claim 6 wherein the act of sending a type of HELLO message and a control signal out the second outgoing link such that the adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring relay is operating and to convey to a node other than the ring relay that the ring relay is imposing a bidirectional break so that the virtual break should be removed is achieved by sending a control message not part of a HELLO message wherein the control message conveys the control signal to the node other than the ring relay to remove the virtual break until another control message is sent conveying the need to re-impose the virtual break.

11. The ring relay of claim 6 wherein the delaying of the removal of the imposed break for user data on the second outgoing link and on the second incoming link for a time period sufficient to allow the bidirectional network ring to impose a virtual break at the device other than the ring relay is accomplished by delaying the removal of the imposed break for user data until a control signal is received from the device other than the ring relay that the virtual break has been imposed.

12. A ring device for use in a bidirectional network ring for a network ring that is manipulated to provide a virtual break in order to operate within a network requiring a tree topology. the network ring carrying data packets;

the ring device comprising the following attributes;

having a first ring port in data communication with a first incoming link and a first outgoing link and a second ring port in data communication with a second incoming link and a second outgoing link connected to the network ring; and

operating to detect an absence of HELLO protocol packets on the second incoming link in excess of a specified time duration then;

blocking the second outgoing link for user data such that a virtual break is imposed on the second outgoing link;

generating and sending a series of modified HELLO protocol packets on the second outgoing link the sending of modified HELLO protocol packets onto the first and second outgoing link used to trigger immediate corrective action by the bidirectional network ring to remove an existing virtual break from a device other than this ring device; and

detecting a HELLO protocol packet on the second incoming link then;

ceasing the generation and sending of the series of modified HELLO protocol packets on the second outgoing link; and

delaying the removal of the blocking of user data on the second outgoing link for a time period sufficient to allow the bidirectional network ring to impose a virtual break at a device other than this ring device.

13. The ring device of claim 12 wherein the act of generating and sending a series of modified HELLO protocol packets on the second outgoing link is accompanied by generating and sending a series of modified HELLO protocol packets on the first outgoing link and the act of ceasing the generation and sending of the series of modified HELLO protocol packets on the second outgoing link is accompanied by ceasing the generation and sending of the series of modified HELLO protocol packets on the first outgoing link.

14. A slave arbiter for use in a bidirectional network ring for a ring that is manipulated to provide a virtual break in order to operate within a network requiring a tree topology, the network ring carrying data packets;

the slave arbiter comprising the following attributes;

having a first ring port in data communication with a first incoming link and a first outgoing link and a second ring port in data communication with a second incoming link and a second outgoing link connected to the network ring;

operating to receive HELLO messages on the first incoming link and on the second incoming link and to send HELLO messages on the first outgoing link and the second outgoing link;

maintaining an imposed virtual break on either the first ring port or the second ring port to block the passage of data packets when the network ring is fully functional so as to prevent a data loop; and

reacting to the detection of the link failure on the second incoming link by;

imposing a break on the second incoming link so that data packets are blocked from passing into the slave arbiter but the imposed break does not block any incoming HELLO messages;

imposing a break on the second outgoing link so that data packets are blocked from leaving the slave arbiter on that link;

removing the imposed virtual break from either the first ring port or the second ring port; and

detecting the cessation of the link failure on the second incoming link;

then imposing a virtual break on either the first ring port or the second ring port before removing the imposed break on the second incoming link and the second outgoing link.

15. The slave arbiter of claim 14 wherein the link failure detected on the second incoming link was an OPER DOWN.

16. The slave arbiter of claim 14 wherein the link failure detected on the second incoming link was a gap in the receipt of HELLO messages beyond a set threshold.

17. A slave arbiter for use in a bidirectional network ring for a ring that is manipulated to provide a virtual break in order to operate within a network requiring a tree topology, the network ring carrying data packets;

the slave arbiter comprising the following attributes;

having a first ring port in data communication with a first incoming link and a first outgoing link and a second ring port in data communication with a second incoming link and a second outgoing link connected to the network ring;

operating to receive HELLO messages on the first incoming link and on the second incoming link and to send HELLO messages on the first outgoing link and the second outgoing link;

maintaining an imposed virtual break on either the first ring port or the second ring port to block the passage of data packets when the network ring is fully functional so as to prevent a data loop; and

after receiving a control signal on the second incoming link indicating that another node on the network ring is imposing a bidirectional break, reacting to the received control signal by;

removing the imposed virtual break from either the first ring port or the second ring port; and

detecting that the node on the network ring that imposed the bidirectional break will soon remove the bidirectional break and responding by imposing a virtual break on either the first ring port or the second ring port.

18. The slave arbiter of claim 17 wherein:

the slave arbiter receives the control signal on the second incoming link indicating that another node on the network ring is imposing a bidirectional break by discerning between a first type of HELLO message that indicates a lack of network problems over the path traveled by the HELLO signal and a second type of HELLO message that conveys the control signal indicating that a node on the network ring is imposing a bidirectional break; and

the slave arbiter detects that the node on the network ring that imposed the bidirectional break will soon remove the bidirectional break by detecting the resumption of receipt of the first type of HELLO message on the second incoming link.

19. The slave arbiter of claim 17 wherein:

the slave arbiter receives the control signal on the second incoming link indicating that another node on the network ring is imposing a bidirectional break in a separate first type of control message not part of a HELLO message; and

the slave arbiter detects that the node on the network ring that imposed the bidirectional break will soon remove the bidirectional break by detecting the cessation of receipt of the first type of control message on the second incoming link.

20. The slave arbiter of claim 17 wherein:

the slave arbiter receives the control signal on the second incoming link indicating that another node on the network ring is imposing a bidirectional break in a separate first type of control message not part of a HELLO message; and

the slave arbiter detects that the node on the network ring that imposed the bidirectional break will soon remove the bidirectional break by detecting the receipt of a second type of control message on the second incoming link.

21. A ring device for use in a bidirectional network ring for a ring that is manipulated to provide a virtual break in order to operate within a network requiring a tree topology, the network ring carrying data packets;

the ring device comprising the following attributes;

having a first ring port in data communication with a first incoming link and a first outgoing link and a second ring port in data communication with a second incoming link and a second outgoing link connected to the network ring;

operating to send HELLO messages onto the first outgoing link and the second outgoing link, the receipt of the HELLO messages on a remote device on the network ring indicating the functioning of the network between the ring device and the remote device; and

reacting to the discernment of the link failure on the second incoming link by;

imposing a break on the second incoming link so that data packets are blocked from passing into the ring device but the imposed break does not block any incoming HELLO messages;

imposing a break on the second outgoing link so that data packets are blocked from leaving the ring device on that link;

sending a type of HELLO message and a control signal out the second outgoing link such that the adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring device is operating and to convey to the remote device (even if the remote device is not the adjacent node) the control signal that the ring device is imposing a bidirectional break so that the virtual break should be removed;

detecting the cessation of the link failure on the second incoming link then resuming operating to send HELLO messages onto the second outgoing link; and

delaying the removal of the imposed break for user data on the second outgoing link and on the second incoming link for a time period sufficient to allow the bidirectional network ring to impose a virtual break at the remote device.

22. The ring device of claim 21 wherein the link failure is the detection of a problem with the communication signal.

23. The ring device of claim 22 wherein the link failure is discerned by detecting a gap in the receipt of a HELLO message on the second incoming link for a period in excess of a specified value.

24. The ring device of claim 22 wherein the operation of sending the type of HELLO message and the control signal out the second outgoing link such that the adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring device is operating and to convey to the remote device (even if the remote device is not the adjacent node) the control signal that the ring device is imposing a bidirectional break so that the virtual break should be removed is achieved by sending a second type of HELLO message that differs from a first type of HELLO message in that the second type of HELLO message indicates that the ring device is imposing a bidirectional break.

25. The ring device of claim 24 wherein the ring device is adapted to respond to the receipt of a second type of HELLO message on the first incoming link by sending a second type of HELLO message on the first outgoing link.

26. The ring device of claim 22 wherein the operation of sending the type of HELLO message and the control signal out the second outgoing link such that the adjacent node receives a HELLO message to provide assurance that the second outgoing link from that ring device is operating and to convey to the remote device (even if the remote device is not the adjacent node) the control signal that the ring device is imposing a bidirectional break so that the virtual break should be removed is achieved by sending a HELLO message and a separate control message outside of a HELLO message that indicates that the ring device is imposing a bidirectional break.

27. A network ring that is manipulated to provide a virtual break in order to operate within a network requiring a tree topology, the network carrying data packets, the network ring comprising:

A) a Master Arbiter comprising;

a MA first port;

with a first MA incoming link and a first MA outgoing link; and

a MA second port;

with a second MA incoming link and a second MA outgoing link;

B) a Slave Arbiter comprising;

a SA first port normally connected to the MA first port by a full duplex communication connection;

a SA second port normally connected to the MA second port by a full duplex communication connection; and

a SA control system, the SA control system comprising controls over the introduction and removal of a virtual break to prevent the formation of a communication ring for data packets within the network requiring a tree topology through the use of at least one non-forwarding state wherein control traffic is forwarded but data packets are not forwarded;

the SA control system comprising;

at least one rule causing the operation of a SA Port in a slave-forwarding state wherein the port forwards data packets and control traffic received at the port;

at least one rule causing the operation of a SA Port in one of at least one non-forwarding states wherein control traffic is forwarded but data packets are not forwarded;

at least one rule causing the Slave Arbiter to operate when the ring is fully operational such that one SA Port is in a slave-forwarding state and the other SA Port is in a non-forwarding state wherein control traffic is forwarded but data packets are not forwarded;

at least one rule for changing the state of the Slave Arbiter to have both the SA first port and the SA second port acting to forward data packets after receipt of a control signal indicating that a device other than the Slave Arbiter is imposing a bidirectional break in the network ring; and

at least one rule for changing the state of one of the two SA Ports to a non-forwarding state wherein control traffic is forwarded but data packets are not forwarded after discerning that the device other then the Slave Arbiter is going to remove the imposed bidirectional break in the network ring.

28. The network ring of claim 27 wherein the control signal received by the Slave Arbiter indicating that a device other than a Slave Arbiter is imposing a bidirectional break is received in a HELLO message that conveys to the Slave Arbiter that the incoming link to the Slave Arbiter that received the HELLO message is working between the Slave Arbiter and an adjacent network device (which may be the Master Arbiter).

29. The network ring of claim 27 wherein the SA control system further comprises:

at least one rule for detecting a time gap between receipt of HELLO messages in excess of a set value and responding to that detected time gap by imposing a bidirectional break to block data packets but not HELLO messages on both the incoming link with the detected time gap and the outgoing link on that same port then changing the state of the Slave Arbiter to have both the SA first port and the SA second port acting to forward data packets; and

at least one rule for changing the state of one of the two SA Ports to a non-forwarding state wherein control traffic is forwarded but data packets are not forwarded after discerning the receipt of a HELLO message on the incoming link with the detected time gap before removing the imposed bidirectional break.

30. The network ring of claim 27 wherein the SA control system further comprises:

at least one rule for detecting a problem on either the first incoming link or the second incoming link and responding to that detected problem by imposing a bidirectional break to block data packets but not HELLO messages on both the incoming link with the detected problem and the outgoing link on that same port then changing the state of the Slave Arbiter to have both the SA first port and the SA second port acting to forward data packets; and

at least one rule for changing the state of one of the two SA Ports to a non-forwarding state wherein control traffic is forwarded but data packets are not forwarded after discerning the cessation the problem on the incoming link with the detected problem before removing the imposed bidirectional break.

31. The network ring of claim 27 further comprising a ring relay positioned between the MA first port and the SA first port, the ring relay comprised of:

a RR MA-side port with a RR MA incoming link and a RR MA outgoing link; and

a RR SA-side port with a RR SA incoming link and a RR SA outgoing link;

such that the full duplex communication between the SA first port and the MA first port runs through the ring relay and the ring relay operates after detecting a problem with the RR-MA incoming link to send out a control signal on both the RR MA-side port and on the RR SA-side port and acts to impose a bidirectional break on both the RR MA incoming link and the RR MA outgoing link.

32. The network ring of claim 27 further characterized by:

a ring relay positioned between the MA first port and the SA first port, the ring relay comprised of.

A RR MA-side port with a RR MA incoming link and a RR MA outgoing link; and

A RR SA-side port with a RR SA incoming link and a RR SA outgoing link;

such that the full duplex communication between the SA first port and the MA first port runs through the ring relay and the ring relay operates after detecting a problem with the RR-MA incoming link to send out a control signal on the RR MA-outgoing link and imposing a bidirectional break on both the RR MA incoming link and the RR MA outgoing link; and

the Master Arbiter further characterized by reacting to the receipt of the control signal on the first MA incoming link by sending a control signal out on the first MA outgoing link so that the control signal can be passed through the ring relay towards the Slave Arbiter.

33. The network ring of claim 32 wherein control signal indicating that a device other than the Slave Arbiter is imposing a bidirectional break in the network ring is conveyed to the Slave Arbiter in a second type of HELLO message that is different from the first type of HELLO message used when not sending the control signal that a device other than the Slave Arbiter is imposing a bidirectional break in the network ring;

and after a HELLO message of the second type is sent from the ring relay to the Master Arbiter, a HELLO message of the second type is sent from the Master Arbiter to the ring relay, and a HELLO message of the second type is transmitted from the ring relay towards the Slave Arbiter.

34. The network ring of claim 33 wherein:

the ring relay blocks HELLO messages of the first type from passing from the RR SA incoming link to the RR MA outgoing link; and

the Master Arbiter creates a new HELLO message of the second type for transmission on the first MA outgoing link and the ring relay passes the new HELLO message of the second type from the RR MA incoming link to the RR SA outgoing link.

35. A network device comprising:

a first port with a first incoming link and a first outgoing link;

a second port with a second incoming link and a second outgoing link;

at least one user port; and

a control system for the network device;

the control system adapted to allow this network device to provide redundant access for the at least one user port to an existing network through a first access device connected to the first port and a second access device in data communication with the first access device and connected to the second port such that a HELLO message transmitted out the first outgoing link can travel via the first access device then the second access device to the second incoming link and a HELLO message transmitted out the second outgoing link can travel via the second access device then the first access device to the first incoming link;

the control system further characterized by;

at least one rule to impose a virtual break on either the first port or the second port when HELLO messages are being received on both ports, the virtual break blocking data packets but not control messages such as HELLO messages;

at least one rule to detect and react to a HELLO timeout on both ports by removing the virtual break from either the first port or the second port so that the at least one user port can have access to the existing network through the first access device connected to the first port if that is possible or through the second access device connected to the second port if that is possible;

at least one rule to detect and react to a HELLO timeout on one port and an OPER DOWN on the other port ports by removing the virtual break from either the first port or the second port so that the at least one user port can have access to the existing network through the first access device connected to the first port if that is possible or through the second access device connected to the second port if that is possible, this at least one rule not necessarily distinct from the rule to detect and react to a HELLO timeout on both ports;

at least one rule to detect a unidirectional break characterized by an OPER DOWN or a HELLO timeout on one port but not the other port and to react by imposing a bidirectional break on the incoming link and the outgoing link on the port with incoming link experiencing an OPER DOWN or a HELLO timeout before removing the virtual break from the port with the virtual break; and

at least one rule to detect and react to the cessation of the unidirectional break characterized by an OPER DOWN or a HELLO timeout by imposing a virtual break on one port before removing bidirectional break.

36. The network device of claim 35 wherein the control system uses a delay timer to delay the removal of a virtual break a time period sufficient to allow the imposition of the bidirectional break so that a data loop is not created when the virtual break is removed.

37. A method for operating a network ring that is manipulated to provide a virtual break in order to operate within a network requiring a tree topology, the network ring carrying data packets;

the method comprising;

imposing a virtual break at a first network device, the network device comprised of a first port with a first incoming link and a first outgoing link and a second port with a first incoming link and a second outgoing link, the virtual break imposed whenever the first network device is receiving HELLO messages of a first type on both the first incoming link and the second outgoing link;

removing the virtual break at the first network device upon receipt of a control signal from another device on the network indicating that the another device is imposing a bidirectional break; and

discerning that the another network device is going to stop imposing the bidirectional break and imposing a virtual break.

38. The method of claim 37 wherein:

the step of discerning that the another network device is going to stop imposing the bidirectional break is further characterized by discerning that the another network device is going to stop imposing the bidirectional break upon receipt of a control signal from the first network device that the first network device has imposed the virtual break; and

the step of imposing a virtual break is further characterized by sending a control signal to the another network device after the imposition of the virtual break to trigger the removal of the bidirectional break.

39. The method of claim 37 wherein:

the another network device is comprised of a first port with a first incoming link and a first outgoing link and a second port with a second incoming link and a second outgoing link and the another device imposes a bidirectional break and sends the control signal to the first network device when the another device ceases to receive HELLO messages of any type for more than a prescribed period on the first incoming link; and

the another network device responds to the resumption of receipt of HELLO messages on the first incoming link;

by indicating to the first network device that the another network device will stop imposing the bidirectional break so that the first network device can impose a virtual break before the bidirectional break is removed; and

by removing the bidirectional break.

40. The method of claim 37 wherein the control signal is sent to the first network device in a HELLO message of a second type different from the HELLO message of the first type.

41. The method of claim 40 wherein the HELLO message of the second type sent by the another network device are received by a third network device between the first network device and the another network device and the receipt of the HELLO message of the second type on a first incoming link for the third device precludes the third network device from reacting on a lack of HELLO messages of any type on the first incoming link for more than a prescribed period.

42. The method of claim 40 wherein a third network device receives the HELLO message of the second type from the another network device and responds by sending a HELLO message of the second type back towards the another network device which in turn sends the HELLO message of the second type away from the third network device and towards the first network device.

43. The method of claim 37 wherein the control signal is sent to the first network device in a control message that is not a type of HELLO message.

44. The method of claim 37 wherein the first outgoing link of the first network device is connected to the second incoming link on the adjacent network node which is the another network device, and the second outgoing link on the another network device is connected to the first incoming link on the first network device and the response to a bidirectional break between the first port of the first network device and the second port of the another network device is the imposition of bidirectional breaks at both the first port of the first network device and at the second port of the another network device and the removal of the virtual break at the first network device.