The topology change (TC) bit will be set for a duration of 35 seconds on configuration bridge protocol data units (BPDUs) that are sent by the root bridge after it receives a topology change notification (TCN) BPDU. TCN BPDUs are a Spanning Tree Protocol (STP) mechanism sent from a designated bridge back to the root bridge to inform the root bridge of a change in the network topology. TCN BPDUs are sent under the following circumstances: when a switch receives a TCN from another non-root bridge, when a link has failed, or when a port begins to forward packets despite the bridge already having a designated port. Ports that have PortFast enabled will not send TCN BPDUs when entering or leaving the forwarding state.

After a bridge receives a TCN BPDU, it will send a BPDU with the topology change acknowledgment (TCA) bit set back to the bridge that sent the TCN BPDU. When the root bridge receives a TCN BPDU, it begins sending configuration BPDUs to the downstream bridge devices. Configuration BPDUs contain a TC bit indicating that a change has occurred. This bit is set for a period of the max_age timer plus the forward_delay timer. STP defaults the max_age timer to 20 seconds and the forward_delay timer to 15 seconds. Therefore, the TC bit will be set for 35 seconds. TC BPDUs will cause a switch to shorten the aging time for Media Access Control (MAC) addresses from 300 seconds to the forward_delay value, which is 15 seconds by default. Excessive flooding can occur as the switch receives packets and forwards the traffic out all ports.

The TC bit will not be set for a duration of two seconds in this scenario. The default hello timer is two seconds. The hello_time is the interval at which BPDUs are sent.