Topology Changes in Spanning Tree Protocol (STP)

The network topology can happen in a network due to different reasons like a link failure, a Switch (Bridge) failure, or a port transitioning to forwarding state.

The topology change must be notified to all Switches (Bridges) in the network and the process involves two steps:

• The Switch (Bridge) notifies the topology change to Root Bridge

• The Root Switch (Bridge) bridge broadcasts the topology change information into the whole network.

When a Switch (Bridge) discovers topology change, it generates a TCN (Topology Change Notification) BPDU (Bridge Protocol Data Unit) and sends the TCN BPDU on its root port. The upstream Switch (Bridge) responds back the sender with TCA (Topology Change Acknowledgment) BPDU (Bridge Protocol Data Unit) and TCA (Topology Change Acknowledgment) BPDU (Bridge Protocol Data Unit)

 The upstream Switch (Bridge) (bridge which received the TCN BPDU) generates another TCN BPDU and sends out via its Root Port. The process continues until the Root Switch (Root Bridge) receives the TCN BPDU.

When the Root Switch (Root Bridge) is aware that there is a topology change in the network, it starts to send out its Configuration BPDUs with the topology change (TC) bit set. Configuration BPDUs are received by every Switch (Bridge) in the network and all bridges become aware of the network topology change.

Switches (Bridges) keep its MAC address table entries for 300 seconds (5 minutes, known as aging time), by default. When a network topology change happens, the Switch (Bridge) temporarily lowers the aging time to the same as the forward delay time (15 seconds) to relearn the MAC address changes happened because of topology change.

This is important because normally only after five minutes an entry is aged out from the MAC address table of the switch and the network devices could be unreachable for up to 5 minutes.  This is known as a black hole because frames can be forwarded to a device, which is no longer available.