Topology Change Because of a Link Failure

By default, bridges keep MAC addresses in the bridge table for 5 minutes. When a topology change occurs, the bridge temporarily lowers this timer to the same as the forward delay timer (default: 15 seconds). This allows the STP network to react to changes in topology by having the bridges quickly relearn the MAC address changes that occur when links change state. Without this, network devices could be unreachable for up to 5 minutes while the bridge ages the MAC address out. This is typically called a black hole because data is forwarded toward a bridge that no longer can reach the network device. Topology change BPDUs are a mechanism to overcome this. A common misconception is that topology change BPDUs cause STP to recalculate. The purpose of topology change BPDUs is to avoid black holes and allow the bridges to have up-to-date bridge tables. STP recalculations only occur only when the bridge sees BPDUs with better paths through the bridged network or when the bridge no longer receives configuration BPDUs from the root bridge.