Specifications

CHAPTER 3
Spanning Tree
TCN BPDUs are sent by a downstream switch toward the root when:
n There is a link failure.
n A port starts forwarding, and there is already a designated port.
n 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.
Some of the fields in the BPDU include:
n Root bridge ID: The BID of the current root
n Sender’s root path cost: The cost to the root
n Sender’s bridge ID: Sender’s priority concatenated to MAC
n Sender’s port ID: The port number, transmitted as final tie-breaker
n Hello time: Two seconds by default
n Forward Delay: Fifteen seconds by default
n Max Age: Twenty seconds by default
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CCNP SWITCH 642-813 Quick Reference by Denise Donohue