Identifying the Rstp Tcn Process

This topic describes the process that RSTP uses to notify all bridges in the network of a TC.

RSTP Topology Change Mechanism

In 802.1D, any port state change generates a TCN. When an 802.1D bridge detects TC, it sends TCNs toward the root bridge. The root bridge sets the TC flag on the outbound BPDUs that are relayed to switches down from the root.

When a bridge receives a BPDU with the TC flag bit set, the bridge reduces its bridge-table aging time to forward delay seconds. This ensures a relatively quick flushing of the MAC address table.

In RSTP, only nonedge ports moving to the forwarding state cause a TC. Loss of connectivity is not considered to be a TC, and, under these conditions, a port moving to the blocking state does not generate a TC BDPU.

© 2006 Cisco Systems, Inc.

Implementing Spanning Tree


RSTP Actions

When an RSTP bridge detects a TC, it performs these actions.





The RSTP bridge starts the TC-While timer.

RTSP sets the TC-While timer with a value equal to twice the hellotime for all its nonedge designated ports and the root port, if necessary.


The RSTP bridge flushes the MAC addresses associated with all these ports.


The TC flag bit is set on all outbound BPDUs.

BPDUs are sent on the root port as long as the TC-While timer is active.


The bridge receives a BPDU with the TC bit set from a neighbor and clears the MAC addresses on all ports.

The port that received the TC BPDU retains learned MAC addresses.


The bridge starts the TC-While timer and sends BPDUs, with a TC bit set, out of all its designated ports and root port.

RSTP does not use the specific TCN BPDU, unless a legacy bridge needs to be notified.

The TCN is flooded across the entire network, one switch at a time, from the switch that is the source of the change rather than from the root bridge. The TC propagation is now a one-step process. There is no need for each switch port to wait for the root bridge to be notified and then maintain the TC state for the value of the max age plus forward delay seconds.

If the port consistently keeps receiving BPDUs that do not correspond to the current operating mode for two periods of hellotime, the port switches to the mode that is indicated by the BPDUs.

3-46 Building Cisco Multilayer Switched Networks (BCMSN) v3.0 © 2006 Cisco Systems, Inc.

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