Spanning tree with rapid forwarding database updates

• US 6,330,229 B1
• Filed: 11/09/1998
• Issued: 12/11/2001
• Est. Priority Date: 11/09/1998
• Status: Expired due to Term

• Current Assignee: Hewlett Packard Enterprise Development LP (Hewlett-Packard Enterprise Company)
• Original Assignee: 3Com Corporation (HP Inc.)
• Inventors: Jain, Vipin K., Seaman, Michael John
• Primary Examiner(s): Olms, Douglas
• Assistant Examiner(s): VANDERPUYE, KENNETH N
• Application Number:
  US09/188,852
• Time in Patent Office: 1,128 Days
• Field of Search: 370/401, 370/408, 370/256, 370/255, 370/428, 709/239, 709/242
• US Class Current: 370/256
• CPC Class Codes: H04L 12/4625 Single bridge functionality, e.g. connection of two networks over a single bridge
• CPC Class Codes: H04L 69/40 for recovering from a failure of a protocol instance or entity, e.g. service redundancy protocols, protocol state redundancy or protocol service redirection management of faults, events, alarms or notifications in data switching networks H04L41/06

Abstract

Management of forwarding databases in the case of link failures on bridges according to the all improved spanning tree, limits the propagation of notifications of topology change to only those parts of the network which are affected by the link failures, and trigger partial flushing as opposed to complete forwarding database flushing of learned MAC addresses to relearn the sets of addresses associated with ports affected by the change in topology. When a bridge moves its root port to a new port, the bridge can move the addresses learned through the original root port to the new root port without any relearning. The sets of addresses associated with the designated ports on upstream bridges from the old root port, are subject to flushing. Also when the bridge attaches to the new branch, it triggers a message to the root instructing all bridges in the new path to the root to flush addresses learned on their root ports.

Claims

1. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm, to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports, the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of destination addresses of stations learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles; and

logic to update the forwarding database in response to changes in the roles, the logic including for a port changing from the alternate port role to the root port role updating the forwarding database by transferring the set of addresses associated with the port in the root port role before the change to the port having the root port role after the change.

2. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm, to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports, the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of destination addresses of stations learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles; and

logic to update the forwarding database in response to changes in the roles;

wherein the resources to forward frames incoming on a particular port, includes logic coupled to the forwarding database to block packets having destination addresses in the set of addresses associated with the particular port.

3. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm, to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports, the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of destination addresses of stations learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles;

logic to update the forwarding database in response to changes in the roles; and

logic responsive to notification through a particular port in a designated port role that a port on a different network device accessible through the particular port has changed a port in the alternate port role to the root port role, to update the set of addresses associated with the port in the root port role in the network device to remove at least the addresses of stations accessible through the different network device.

4. The network device of claim 3, further including logic to forward the notification through the port in the root port role in the network device.

5. The network device of claim 3, wherein the topology management resources include resources to indicate whether the topology management resources act as the root of the active topology, and if the topology management resources do not act as the root, then to forward the notification through the port in the root port role in the network device.

6. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm, to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports, the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of destination addresses of stations learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles; and

logic to update the forwarding database in response to changes in the roles;

wherein the topology management resources include resources to determine that a port on a different network device accessible through a particular port in the designated port role has become disconnected from the particular port, and if the port on a different network device has become disconnected, then to update the set of addresses associated with the particular port by removing at least the set of addresses accessible through the different network device from the set of addresses associated with the particular port, and to forward a notification through the port in the root port role in the network device.

7. The network device of claim 6, wherein the resources to determine that a port on a different network device accessible through a particular port in the designated port role has become disconnected from the particular port comprise logic to receive a message addressed to the topology management resources carrying a notification of the disconnection.

8. The network device of claim 6, wherein the resources to determine that a port on a different network device accessible through a particular port in the designated port role has become disconnected from the particular port comprise logic to poll network devices accessible through ports in the designated port role to detect the disconnection.

9. The network device of claim 7, including logic responsive to notification through a particular port in a designated port role that a port on a different network device has changed to a root port role for the different network device to determine a second network device having a port in a designated port role which had been coupled to a port in the root port role on the different network device before the change, and to notify the second network device that the different network device had been disconnected.

10. The network device of claim 9, wherein the resources to notify the second protocol entity that the different network device had been disconnected, comprise logic to determine an address of the second network device from the different network device, and to send a message to the second network device.

11. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm compliant with IEEE Standard 802.1D, to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports, the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of addresses of stations learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles;

logic to compute states for ports in the plurality of ports in response to the parameters, including placing the port providing the root port role in a forwarding state, placing the one or more ports providing the designated port roles in a forwarding state, and placing the one or more ports providing the alternate port roles in a blocking state;

logic to manage transition of the states of the ports in the plurality of ports from a blocking state to a forwarding state in response to a change in the active topology, including for a port changing from the alternate port role to the root port role updating the forwarding database by transferring the set of addresses associated with the port in the root port role before the change to the port having the root port role after the change.

12. The network device of claim 11, wherein the resources to forward frames incoming on a particular port, includes logic coupled to the forwarding database to block packets having destination addresses in the set of addresses associated with the particular port.

13. The network device of claim 11, wherein logic to issue notification to other network devices in the network of changes in the roles, includes logic to notify devices about the change through the port in the root port role after the change.

14. The network device of claim 11, including logic responsive to notification through a particular port in a designated port role that a port on a different network device accessible through the particular port has changed a port in the alternate port role to the root port role, to update the set of addresses associated with the port in the root port role in the network device to remove at least the addresses of stations accessible through the different network device.

15. The network device of claim 14, further including logic to forward the notification through the port in the root port role in the network device.

16. The network device of claim 14, wherein the topology management resources include resources to indicate whether the topology management resources act as the root of the active topology, and if the topology management resources do not act as the root, then to forward the notification through the port in the root port role in the network device.

17. The network device of claim 11, wherein the topology management resources include resources to determine that a port on a different network device accessible through a particular port in the designated port role has become disconnected from the particular port, and if the port on a different network device has become disconnected, then to update the set of addresses associated with the particular port by removing at least the set of addresses of stations accessible through the different network device from the set of addresses associated with the particular port, and to forward a notification through the port in the root port role in the network device.

18. The network device of claim 17, wherein the resources to determine that a port on a different network device accessible through a particular port in the designated port role has become disconnected from the particular port comprise logic to receive a message addressed to the topology management resources carrying a notification of the disconnection.

19. The network device of claim 17, wherein the resources to determine that a port on a different network device accessible through a particular port in the designated port role has become disconnected from the particular port comprise logic to poll network devices accessible through ports in the designated port role to detect the disconnection.

20. The network device of claim 18, including logic responsive to notification through a particular port in a designated port role that a port on a different network device has changed to a root port role for the different network device to determine a second network device having a port in a designated port role which had been coupled to a port in the root port role on the different network device before the change, and to notify the second network device that the different network device had been disconnected.

21. The network device of claim 20, wherein the resources to notify the second protocol entity that the different network device had been disconnected, comprise logic to determine an address of the second network device from the different network device, and to send a message to the second network device.

22. The network device of claim 11, including logic coupled with the forwarding database to learn addresses of end stations accessible through particular ports, and to add learned addresses to the respective set of addresses.

23. The network device of claim 22, including logic to remove a given address from a set of addresses associated with a particular port in the forwarding database, if a packet carrying the given address as a source address is not received on the particular port within an aging interval.

24. The network device of claim 11, wherein the network includes stations which are descendants accessible through ports in the designated port role, and stations which are not descendants accessible through the port in the root port role, and the forwarding database includes a set of addresses associated with the port in the root port role including addresses of stations which are not descendants, and respective sets of addresses associated with ports in the designated port role including addresses of descendants which are accessible through the respective ports.

25. The network device of claim 11, wherein the forwarding database includes entries including a MAC address and port identifier for the MAC address, and the logic to update the forwarding database includes logic to change the port identifier of entries in the forwarding database.

26. For a network comprising a plurality of local area network LAN segments interconnected by network devices according to an active topology established by a spanning tree protocol which provides at network devices a protocol entity managing a plurality for ports according to a blocking state, a listening state, a learning state and a forwarding state, and including a forwarding database storing for the ports in the plurality of ports, respective sets of addresses of stations learned from frames incoming on respective ports, an improvement comprising:

storing parameters for specifying the active topology, the parameters including information for an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for paths between the root and respective LAN segments coupled to the one or more ports, and identification of one or more ports in the plurality of ports for alternate port roles; and

for a particular port changing from the alternate port role to the root port role, updating the forwarding database by transferring the set of addresses associated with the port in the root port role before the change to the port having the root port role after the change.

27. The improvement of claim 26, including for the particular port changing from the alternate port role to the root port role, causing transition from the blocking state to the forwarding state without requiring transition through the listening and learning states.

28. The improvement of claim 26, including signaling other network devices in the spanning tree protocol about the transition.

29. The improvement of claim 26, wherein the active topology includes a root device, and including sending a signal to other network devices accessed through the port in the root port role on said network device before the change up to the root device, to flush the set of addresses associated with respective ports in the designated port roles of addresses descendant from said network device.

30. The improvement of claim 26, wherein the active topology includes a root device, and including sending a signal to other network devices accessed through the port in the root port role on said network device after the change up to the root device, to flush the set of addresses associated with respective ports in the root port roles of addresses descendant from said network device.

31. The improvement of claim 26, wherein said network device comprises a protocol entity compliant with IEEE Standard 802.1D, and the active topology includes a root device, and including:

sending a signal to other network devices accessed through the port in the root port role on said network device before the change up to the root device, to flush the set of addresses associated with respective ports in the designated port roles of addresses descendant from said network device;

sending a signal to other network devices accessed through the port in the root port role on said network device after the change up to the root device, to flush the set of addresses associated with respective ports in the root port roles of addresses descendant from said network device.

32. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm, to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports, the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of station addresses learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles;

logic to notify other network devices in the network which have forwarding databases needing to be updated in response to changes in the roles; and

logic to update the forwarding database includes for a port changing from the alternate port role to the root port role updating the forwarding database by transferring the set of addresses associated with the port in the root port role before the change to the port having the root port role after the change.

33. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm, to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports, the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of station addresses learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles; and

logic to notify other network devices in the network which have forwarding databases needing to be updated in response to changes in the roles;

wherein the resources to forward frames incoming on a particular port, includes logic coupled to the forwarding database to block packets having destination addresses in the set of addresses associated with the particular port.

34. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm, to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports, the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of station addresses learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles;

logic to notify other network devices in the network which have forwarding databases needing to be updated in response to changes in the roles; and

logic responsive to notification through a particular port in a designated port role that a port on a different network device accessible through the particular port has changed a port in the alternate port role to the root port role, to update the set of addresses associated with the port in the root port role in the network device to remove at least the addresses of stations accessible through the different network device.

35. The network device of claim 34, further including logic to forward the notification through the port in the root port role in the network device.

36. The network device of claim 34, wherein the topology management resources include resources to indicate whether the topology management resources act as the root of the active topology, and if the topology management resources do not act as the root, then to forward the notification through the port in the root port role in the network device.

37. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports, the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of station addresses learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles; and

logic to notify other network devices in the network which have forwarding databases needing to be updated in response to changes in the roles, the logic including resources to indicate active status of the port in the root port role, such that an other network device on the first branch receives notification of the change by loss of indication of active status; and

wherein the topology management resources include resources to determine that a port on a different network device accessible through a particular port in the designated port role has become disconnected from the particular port, and if the port on a different network device has become disconnected, then to update the set of addresses associated with the particular port by removing at least the set of addresses accessible through the different network device from the set of addresses associated with the particular port, and to forward a notification through the port in the root port role in the network device up the first branch of the tree.

38. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm, to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of station addresses learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles; and

logic to notify other network devices in the network which have forwarding databases needing to be updated in response to changes in the roles, the logic including resources to indicate active status of the port in the root port role, such that an other network device on the first branch receives notification of the change by loss of indication of active status; and

wherein the resources to determine that a port on a different network device accessible through a particular port in the designated port role has become disconnected from the particular port comprise logic to receive a message addressed to the topology management resources carrying a notification of the disconnection.

40. The network device of claim 38, including logic responsive to notification through a particular port in a designated port role that a port on a different network device has changed to a root port role for the different network device to determine a second network device having a port in a designated port role which had been coupled to a port in the root port role on the different network device before the change, and to notify the second network device that the different network device had been disconnected.

41. The network device of claim 40, wherein the resources to notify the second network device that the different network device had been disconnected, comprise logic to determine an address of the second network device from the different network device, and to send a message to the second network device.

39. For a network comprising a plurality of local area network LAN segments, a network device comprising:

a plurality of ports coupled to LAN segments in the network;

topology management resources which manage the plurality of ports according to a spanning tree algorithm, to set an active topology for the plurality of ports; and

resources, responsive to the active topology, which forward frames incoming through ports in the plurality of ports and which block forwarding of frames incoming through other ports in the plurality of ports, the resources including a forwarding database storing for the ports in the plurality of ports, respective sets of station addresses learned from frames incoming on respective ports, and wherein the topology management resources comprise

logic to compute parameters for managing the active topology, the parameters including an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a preferred path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for preferred paths between the root and respective LAN segments coupled to the one or more ports in the plurality of LAN segments, and identification of one or more ports in the plurality of ports for alternate port roles; and

logic to notify other network devices in the network which have forwarding databases needing to be updated in response to changes in the roles, the logic including resources to indicate active status of the port in the root port role, such that an other network device on the first branch receives notification of the change by loss of indication of active status; and

wherein the resources to determine that a port on a different network device accessible through a particular port in the designated port role has become disconnected from the particular port comprise logic to poll network devices accessible through ports in the designated port role to detect the disconnection.

42. For a network comprising a plurality of local area network LAN segments interconnected by network devices according to an active topology established by a spanning tree protocol which provides at network devices a protocol entity managing a plurality for ports according to a blocking state, a listening state, a learning state and a forwarding state, and including a forwarding database storing for the ports in the plurality of ports, respective sets of addresses of stations learned from frames incoming on respective ports, an improvement comprising:

detecting a change in the active topology at a particular network device in the network, and causing a set of other network devices in the network to update their respective forwarding databases in response to the change, and in response to their respective positions in the active topology;

storing parameters for specifying the active topology, the parameters including information for an identification of a root of the network, identification of a port in the plurality of ports for a root port role to be used for a path to the root, identification of one or more ports in the plurality of ports for designated port roles to be used for paths between the root and respective LAN segments coupled to the one or more ports, and identification of one or more ports in the plurality of ports for alternate port roles; and

for a particular port changing from the alternate port role to the root port role, updating the forwarding database by transferring the set of addresses associated with the port in the root port role before the change to the port having the root port role after the change.

43. For a network comprising a plurality of local area network LAN segments interconnected by network devices according to an active topology established by a spanning tree protocol which provides at network devices a protocol entity managing a plurality for ports according to a blocking state, a listening state, a learning state and a forwarding state, and including a forwarding database storing for the ports in the plurality of ports, respective sets of addresses of stations learned from frames incoming on respective ports, an improvement comprising:

detecting a change in the active topology at a particular network device in the network, and causing a set of other network devices in the network to update their respective forwarding databases in response to the change, and in response to their respective positions in the active topology;

wherein the active topology includes a root device, and the step of causing a set of other network devices to update their respective forwarding databases includes sending a signal to other network devices accessed through the port in the root port role on said network device before the change up to the root device, to flush the set of addresses associated with respective ports in the designated port roles of addresses descendant from said network device.

44. For a network comprising a plurality of local area network LAN segments interconnected by network devices according to an active topology established by a spanning tree protocol which provides at network devices a protocol entity managing a plurality for ports according to a blocking state, a listening state, a learning state and a forwarding state, and including a forwarding database storing for the ports in the plurality of ports, respective sets of addresses of stations learned from frames incoming on respective ports, an improvement comprising:

detecting a change in the active topology at a particular network device in the network, and causing a set of other network devices in the network to update their respective forwarding databases in response to the change, and in response to their respective positions in the active topology;

wherein the active topology includes a root device, and the step of causing a set of other network devices to update their respective forwarding databases includes sending a signal to other network devices accessed through the port in the root port role on said network device after the change up to the root device, to flush the set of addresses associated with respective ports in the root port roles of addresses descendant from said network device.

45. For a network comprising a plurality of local area network LAN segments interconnected by network devices according to an active topology established by a spanning tree protocol which provides at network devices a protocol entity managing a plurality for ports according to a blocking state, a listening state, a learning state and a forwarding state, and including a forwarding database storing for the ports in the plurality of ports, respective sets of addresses of stations learned from frames incoming on respective ports, an improvement comprising:

detecting a change in the active topology at a particular network device in the network, and causing a set of other network devices in the network to update their respective forwarding databases in response to the change, and in response to their respective positions in the active topology;

wherein said network device comprises a protocol entity compliant with IEEE Standard 802.1D, and the active topology includes a root device, and the step of causing a set of other network devices to update their respective forwarding databases includes;

sending a signal to other network devices accessed through the port in the root port role on said network device before the change up to the root device, to flush the set of addresses associated with respective ports in the designated port roles of addresses descendant from said network device;

sending a signal to other network devices accessed through the port in the root port role on said network device after the change up to the root device, to flush the set of addresses associated with respective ports in the root port roles of addresses descendant from said network device.