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I was trying to understand Spanning Tree Protocol and I got confused on some points, so I want to ask for some and double check for others:

  1. Do the three timers(including max_age) exist in the bridge per port (i.e. each port have its own three timers) or are there three timers for the whole switch?

  2. If the answer of one is three timers per port (as I think but not sure), Does a Max age expire on any port triggers the whole election process for the whole switch (all ports) to start again ?

  3. If the whole election process starts again does that start by putting each port in a Blocking state for 20 seconds before deciding the final state of each port ?

  4. for designated ports they don't receive any configuration BPDU unless topology change, that means in stability it doesn't receive configuration BPDU so the timer always expires. I feel that contradicts my assumptions in 1,2,3

  5. for root ports and blocked ports when they receive BPDU it only resets the Max age timer if received BPDU like the old one and let it expire otherwise? also if the different received BPDU is superior then replace old, if inferior ignore it, and old is deleted after expiry ?

Please tell me the answer of the five points stating any misconception I may have.

  • 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 and accept your own answer. – Ron Maupin Dec 25 '18 at 8:22
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Do the three timers(including max_age) exist in the bridge per port (i.e. each port have its own three timers) or are there three timers for the whole switch?

Each port has its own timers. On most switches, you can only adjust them globally.

If the answer of one is three timers per port (as I think but not sure), Does a Max age expire on any port triggers the whole election process for the whole switch (all ports) to start again ?

There is no election "process". Ports are continually sending BPDUs.

If the whole election process starts again does that start by putting each port in a Blocking state for 20 seconds before deciding the final state of each port ?

There is no election process, so your question doesn't make sense. From a practical standpoint, you're suggesting that all switches would stop forwarding traffic anytime there was a change in the network -- not a practical idea.

for designated ports they don't receive any configuration BPDU unless topology change, that means in stability it doesn't receive configuration BPDU so the timer always expires. I feel that contradicts my assumptions in 1,2,3

I'm not sure I understand your question, but ports are always sending BPDUs, unless they are down.

for root ports and blocked ports when they receive BPDU it only resets the Max age timer if received BPDU like the old one and let it expire otherwise? also if the different received BPDU is superior then replace old, if inferior ignore it, and old is deleted after expiry ?

May I suggest doing a little more research into STP. It might help you clarify your understanding.

  • "but ports are always sending BPDUs, unless they are down. - not entirely true: bridges are only sending BPDUs "downwards", ie. not out of the root port. – Zac67 Apr 21 '18 at 13:13

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