1

From my understanding so far, to estimate forward delay, the following formula should be used.

(end-to-end_bpdu_propagation_delay + message_age_overestimate +
tx_halt_delay + max_frame_lifetime)/2
(14 + 6 + 1 + 8) / 2 (With Default values)
15 (Round Figure)

I understood the formula except why the Message Age Overestimate needs to be taken into account for this calculation.

  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you can provide your own answer and accept it. – Ron Maupin Aug 7 '17 at 16:42
1

Cisco has a document which describes this: Understanding and Tuning Spanning Tree Protocol Timers

Message age overestimate—The purpose of this parameter is to account for the age of the BPDU since origination. Assume that each bridge increases the BPDU message age by 1 sec. The formula is:

Message_age_overestimate 
= (dia – 1) x overestimate_per_bridge 
= dia – 1 
= 6

Default Values of Spanning Tree Protocol Timers

forward delay

The movement of a port into the listening state indicates that there is a change in the active STP topology and that a port will go from blocking to forwarding. So the listening and learning periods during which the forward delay runs must cover this consecutive period:

  • Time from when the first bridge port enters the listening state (and stays there through the subsequent reconfiguration) to when the last bridge in the bridged LAN hears of the change in the active topology

    In addition, you need to count the same delay that you use to calculate max age (message age overestimate and BPDU propagation delay).

  • Time for the last bridge to stop forwarding frames that are received on the previous topology (maximum transmission halt delay), until the last frame that is forwarded on the previous topology disappears (maximum frame lifetime)

    This amount of time is necessary in order to be sure that you do not get duplicated frames.

Therefore, twice the time of the forward delay (listening time + learning time) contains all these parameters. The formula is:

2 x forward delay 
= end-to-end_BPDU_propagation_delay + Message_age_overestimate + 
    Maximum_frame_lifetime + Maximum_transmission_halt_delay 
= 14 + 6 + 7.5 + 1 = 28.5 

forward_delay 
= 28.5 /2 
= 15 (rounded)

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