The Active Topology After Spanning Tree Is Complete
A
B C
D
E
Bridge Protocol Data Units (BPDU)
Switches exchange BPDUs. There are two types of BPDUs: Configuration
and Topology Change (TCN).
Configuration BPDUs are sent every two seconds from the root toward the
downstream switches. They:
■ Are used during an election.
■ Maintain connectivity between switches.
■ Send timer information from the root.
TCN BPDUs are sent toward the root when:
■ There is a link failure.
■ A port starts forwarding, and there is already a designated port.
■ The switch receives a TCN from a neighbor.
When a switch receives a TCN BPDU, it acknowledges that with a configuration
BPDU that has the TCN Acknowledgment bit set.
When the root bridge receives a TCN, it starts sending configuration BPDUs
with the TCN bit set for a period of time equal to max age plus forward delay.
Switches that receive this change their MAC table aging time to the Forward
Delay time, causing MAC addresses to age faster. The topology change also
causes an election of the root bridge, root ports, and designated ports.