Spanning Tree Protocol Topology Change

1. Topology Change Definition

1) When a port that was forwarding is going down (blocking for instance).
2) When a port transitions to forwarding and the bridge has a designated port. (This means that the bridge is not standalone.)

The process to send a notification to all bridges in the network involves two steps:
1) The bridge notifies the root bridge of the spanning tree.
2) The root bridge "broadcasts" the information into the whole network.

1.1 Notify the Root Bridge:

In normal STP operation, a bridge keeps receiving configuration BPDUs from the root bridge on its root port. However, it never sends out a BPDU toward the root bridge. In order to achieve that, a special BPDU called the topology change notification (TCN) BPDU has been introduced. Therefore, when a bridge needs to signal a topology change, it starts to send TCNs on its root port. The designated bridge receives the TCN, acknowledges it, and generates another one for its own root port. The process continues until the TCN hits the root bridge.

The TCN is a very simple BPDU that contains absolutely no information that a bridge sends out every hello_time seconds (this is locally configured hello_time, not the hello_time specified in configuration BPDUs). The designated bridge acknowledges the TCN by immediately sending back a normal configuration BPDU with the topology change acknowledgement (TCA) bit set. The bridge that notifies the topology change does not stop sending its TCN until the designated bridge has acknowledged it. Therefore, the designated bridge answers the TCN even though it does not receive configuration BPDU from its root.

1.2 Broadcast the Event to the Network
Once the root is aware that there has been a topology change event in the network, it starts to send out its configuration BPDUs with the topology change (TC) bit set. These BPDUs are relayed by every bridge in the network with this bit set. As a result all bridges become aware of the topology change situation and it can reduce its aging time to forward_delay. Bridges receive topology change BPDUs on both forwarding and blocking ports.
The TC bit is set by the root for a period of max_age + forward_delay seconds, which is 20+15=35 seconds by default.