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#### Topology Change and Topology Change Detection

When an 802.1D bridge detects a topology change, the bridge first notifies the root bridge using a reliable mechanism. Once the root bridge is aware of a change in the topology of the network, the root bridge sets the topology change flag on the BPDUs sent out, which are then relayed to all the bridges in the network. When a bridge receives a BPDU with the TC flag bit set, the bridge reduces its bridging-table aging time to forward delay seconds, which ensures a relatively quick flushing of stale information.

In RSTP, only non-edge ports moving to the forwarding state cause a topology change. Consequently, a loss of connectivity is no longer considered a topology change, which is contrary to 802.1D. That is, a port moving to a blocking state no longer generates a TC. When an RSTP bridge detects a topology change, the following happens:

1. RSTP bridge starts the TC while the timer with a value equal to twice the hello time for all the non-edge designated ports and its root port, if necessary.

2. RSTP bridge flushes the MAC addresses associated with all these non-edge designated ports.

3. When the TC while timer is running on a port, the BPDUs sent out of that port have the TC bit set. The BPDUs are also sent on the root port while the timer is active.

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