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. Once the Root Bridge is aware of a change in the topology of the network, it sets the Topology Change (TC) flag on the sent BPDUs. The BPDUs are then relayed to all 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 stale information.

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

• RSTP bridge starts the TC while timer with a value equal to twice the Hello Time for all its non-edge designated ports and its root port, if necessary.
• It flushes the MAC addresses associated with all non-edge designated ports.
• As long as 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.