Timers

STP Timer	Purpose	Duration
Hello	How often the root bridge sends hello BPDUs	2sec
Forward delay	How long the switch will stay in the Listening and Learning states (each state is 15 seconds = total 30 seconds)	15sec
Max Age	How long an interface will wait after ceasing to receive Hello BPDUs to change the STP topology	20sec (10* hello)

Hello

Switches do not forward the BPDUss out of their root ports and non-designated ports, only their designated ports.

Max Age

Counts down the 2 seconds

The timer is reset and it will start counting again

If a connection fails, the timer won't be reset

If another BPDU is received before the max age timer counts down to 0, the time will reset to 20 seconds and no changes will occur.

If another BPUD is not received, the max age timer counts down to 0 and the switch will reevaluate its STP choices, including root bridge, and local root, designated, and non-designated ports.

If a non-designated port is selected to become a designated or root port, it will transition from the blocking state to the listening state (15 seconds), then learning state (15 seconds), and then finally the forwarding state. So, it can take a total of 50 seconds for a blocking interface to transition to forwarding.

These timers and transitional states are to make sure that loops aren't accidentally created by an interface moving to forwarding state too soon.

A forwarding interface can move directly to a blocking state (there is no worry about creating a loop by blocking an interface).

A blocking interface cannot move directly to forwarding state. It must go through the listening and learning state.