Topology Changes and RSTP

Recall that when an 802.1D switch detects a port state change (either up or down), it signals the Root Bridge by sending topology change notification (TCN) BPDUs. The Root Bridge, in turn, must signal the topology change by sending out a TCN message that is relayed to all switches in the STP domain.

RSTP detects a topology change only when a nonedge port transitions to the Forwarding state. This might seem odd because a link failure is not used as a trigger. RSTP uses all of its rapid convergence mechanisms to prevent bridging loops from forming. Therefore, topology changes are detected only so that bridging tables can be updated and corrected as hosts appear first on a failed port and then on a different functioning port.

When a topology change is detected, a switch must propagate news of the change to other switches in the network so that they can correct their bridging tables, too. This process is similar to the convergence and synchronization mechanism; topology change (TC) messages propagate through the network in an ever-expanding wave.

BPDUs, with their TC bit set, are sent out all of the nonedge designated ports. This is done until the TC While timer expires, after two intervals of the Hello time. This notifies neighboring switches of the new link and the topology change. In addition, all MAC addresses associated with the nonedge designated ports are flushed from the content-addressable memory (CAM) table. This forces the addresses to be relearned after the change, in case hosts now appear on a different link.

All neighboring switches that receive the TC messages also must flush the MAC addresses learned on all ports except the one that received the TC message. Those switches then must send TC messages out their nonedge designated ports, and so on.

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