Topology Change and Topology Change Detection

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

In RSTP, only non edge ports moving to the forwarding state cause a topology change. Thus, a loss of connectivity is not considered as a topology change any more, contrary to 802.1D (that is, a port moving to the blocking state no longer generates a TC). When an RSTP bridge detects a topology change, the following happens:

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

• It flushes the MAC addresses associated with all of the non-edge designated ports.

• As long as the TC-while timer is running on a port, the BPDUs sent out have the TC bit set. The BPDUs are also sent on the root port while the timer is active.