Spanning Tree Protocol: Timers

Spanning Tree Protocol consists of the following timers:

  • Hello – This is the time between each BPDU that is transmitted, by default this is set to 2 seconds, this can be tuned between 1 and 10 sec
  • Forward Delay – this is the amount of time defined for a port to be in a listening or learning state.  By default this is 15 seconds, but this can be tuned between 4 and 30 sec.
  • Max Age – this is the timer that controls the max length of time that passes before a bridge port saves its BPDU information.  By default this is 20 seconds but can be tuned between 6 and 40 sec.
  • Message Age – this is not a fixed value this contains the length of time that has passed since the root bridge originated the BPDU.


Other Parameters Spanning Tree uses to determine timers for tuning:

  • Diameter of the STP domain (dia) – this is the value of the maximum number of bridges between any two points in a domain.  IEEE recommends a maximum diameter of 7 bridges with default timers.
  • Bridge Transit Delay (transit delay) – this is the elapsed time between receiving and transmitting of the same frame by the bridge.  This is basically the latency of traffic through the bridge.  IEEE recommends to consider 1 sec as the max bridge transit delay
  • BPDU transmission delay (bpdu_delay) – this is the delay between the time the BPDU is received on a port and the time the config BPDU is transmitted to another port.  IEEE recommends 1 sec as the max BPDU transmission delay
  • Message Age increment overestimate (msg_overestimate) – This is the increment value each bridge adds to the message age before forwarding a BPDU.  Cisco switches add 1 second to the message age before the BPDU is forwarded.
  • Lost Message (lost_msg) – This is the number of BPDUs that can be lost as it moves through the network.  IEEE recommends to us three as the number of BPDUs lost
  • Transmit Halt Delay (Tx_halt_delay) – this is the max amount of time that is needed for a bridge to move a port into blocking after it’s determined that the port must be blocked.   IEEE recommends 1 sec
  • Medium Access Delay (med_access_delay) – This is the value of time that is needed for a device to access media for transmission, Specifically the time between the CPU decision to send a frame and the moment the frame begins to leave the bridge.  IEEE recommends 0.5 sec

From these parameters you can calculate other values:

  • End-To-End BPDU Propagation delay – This is the amount of time that is needed for a BPDU to travel across the network, assuming the diameter of 7 hops, 3 BPDUs lost, and a Hello time of 2 seconds:
    • End-to-end_BPDU_propa_delay
      = ((lost_msg + 1) x hello) + ((BPDU_Delay x (dia – 1)) 
      = ((3 + 1) x hello) + ((1 x (dia – 1)) 
      = 4 x hello + dia – 1 
      = 4 x 2 + 6 
      = 14 sec
  • Message Age Overestimate – this parameter is used to account the age of the BPDU since the origination, assume each bridge increases the message age by 1 sec
    • Message_age_overestimate 
      = (dia – 1) x overestimate_per_bridge 
      = dia – 1 
      = 6
  • Maximum Fram Lifetime – This value is the max time that a frame previously sent remains in the network before it reaches the destination.
    • Maximum_frame_lifetime
      = dia x transit_delay + med_access_delay 
      = dia + 0.5 
      = 7.5 
      = 8  (rounded)
  • Max Transmission Halt Delay – This is the value of time that is needed for effectively blocking a port after the decision to block is made.  IEEE uses 1 sec as the max for that event:
    • Maximum_transmission_halt_delay
      = 1

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