Spanning Tree Protocol: RSTP Topology Change Events

802.1D defines four distinct events as topology change events.

802.1W simplifies topology change events by limiting it to Non-Edge ports that transition from a non-Forwarding state to a Forwarding state.

The newly opened port can provide a better path to certain MAC addresses than what was previously available and the CAM table required updating.

Loss of a forwarding port is no longer a cause for a topology change since the MACs learned from that port are no longer available unless some other port in the topology becomes forwarding and possibly provides a better path to those MACs.

RSTP does not use TCN BPDUs to notify of the event, instead RSTP immediately floods BPDUs with the TC flag set when a switch detects a topology change on a port or learns about a topology change on a port from a BPDU with the TC flag bit set.

RSTP performs the following when a topology change is detected:

  • Set the tcWhile timer to the value of the Hello time plus one second on all remaining non-edge designated ports and Root port except the port on which the TC was detected or learned.
  • Immediately clear the MACs from the CAM table learned from these ports
  • Send BPDUs with the TC flag set on these ports every Hello seconds until the tcWhile timer expires

This way the topology change notification is propagated throughout the network rapidly in the form of BPDUs with the TC flag bit set.

Edge ports do not cause topology change events and MACs learned on them are not flushed during the topology change event handling.




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