STP Topology Changes

If an upstream device is not immediately notified of changes in the network topology, such as a link disconnection or a port Down event, the upstream device may send packets to an unreachable link when the topology changes. This is because the aging time of MAC address entries is 5 minutes by default. In Figure 1, the link between S1 and S2 is blocked. Traffic from A to B flows through S1, S3, S4, and S2. If the link between S3 and S4 fails, traffic is directly forwarded from S1 to S2. However, a MAC address entry pointing to S3 on S1 still exists. Before the MAC address entry ages out, traffic continues to be forwarded to S3. As a result, traffic loss may occur. In this case, the topology change mechanism is required to notify the entire STP network of topology changes.

Figure 1 Traffic is lost because the MAC address entry is not aged out

Figure 2 shows how STP reacts to a topology change.

Figure 2 TCN transmission and TC flooding

1. When the status of the port at point T changes, a downstream device continuously sends TCN BPDUs to the upstream device.
2. The upstream device processes only the TCN BPDUs received on the designated port. The other ports may receive TCN BPDUs but do not process them.
3. The upstream device sets the TCA bits of the Flags field in the configuration BPDUs to 1 and returns the configuration BPDUs to instruct the downstream device to stop sending TCN BPDUs.
4. The upstream device sends a copy of the TCN BPDUs towards the root bridge.
5. Steps 1, 2, 3 and 4 are repeated until the root bridge receives the TCN BPDUs.
6. The root bridge sets the TC and TCA bits of the Flags field in the configuration BPDUs to 1. When set to 1, the TC bit instructs the downstream device to delete MAC address entries, and the TCA bit instructs the downstream device to stop sending TCN BPDUs.

• TCN BPDUs are used to inform the upstream device and root bridge of topology changes.
• Configuration BPDUs with the TCA bit set to 1 are used by the upstream device to inform the downstream device that the topology changes are known and instruct the downstream device to stop sending TCN BPDUs.
• Configuration BPDUs with the TC bit set to 1 are used by the upstream device to inform the downstream device of topology changes and instruct the downstream device to delete MAC address entries. This increases network convergence speed.