When a Rapid Spanning Tree Protocol (RSTP) switch detects a topology change, it starts the TC While timer with a value equal to twice the hello timer for all its non-edge designated ports and its root port. Additionally, the switch flushes the Media Access Control (MAC) addresses associated with its non-edge designated ports and its root port from the Content Addressable Memory (CAM) table. A switch detects a topology change by receiving a bridge protocol data unit (BPDU) with the topology change (TC) bit set? these BPDUs are called topology change notification (TCN) messages.

With RSTP, topology changes are detected for the sole purpose of updating RSTP switching tables. RSTP does not consider loss of connectivity a topology change? consequently, only non-edge ports that transition into the forwarding state are considered topology changes, which results in TCN messages being disseminated throughout the network. When a switch detects a topology change, it starts the TC While timer and generates TCN messages for all its non-edge designated ports and its root port. Additionally, all MAC addresses linked to the non-edge designated ports and the root port are flushed from the CAM table.

Flushing the CAM table requires that the MAC addresses be relearned after the topology change in the event that a host now appears on a different link. It is important to note that a flood of TCN messages could cause repeated flushing of the CAM table and a spike in CPU utilization, which could cause performance problems on the switch.

RSTP is used to significantly increase convergence speed after a topology change. RSTP is based on the 802.1w standard developed by the Institute of Electrical and Electronics Engineers (IEEE) to address the slow transition of a Spanning Tree Protocol (STP) port to the forwarding state. Unlike STP, which has five port states, RSTP has only three: discarding, learning, and forwarding. The disabled, blocking, and listening states of STP are combined into the discarding state in RSTP. RSTP uses the STP root port and designated port roles but splits up the STP blocking port role into the alternate port and backup port roles. An alternate port is a blocked port that receives more useful BPDUs from a port on another device, and a backup port is a blocked port that receives more useful BPDUs from a port on the same device. RSTP is backward compatible with switches that can only use STP, but the convergence benefits provided by RSTP are lost when RSTP interacts with STP devices.

RSTP does not flush the MAC addresses associated with its edge ports. Because the edge port is connected to a single host, the port cannot form a loop and is immediately placed into the forwarding state. If an edge port ever receives a BPDU, the port will lose its edge port designation. When a switch detects a topology change, it does not start the TC While timer with a value equal to the hello timer, nor does it start the TC While timer with a value equal to three times the hello timer. The switch will start the TC While timer with a value equal to twice the hello timer and generate TCN messages for its non-edge designated ports and its root port.