Spanning Tree Topology Change Notification (TCN)

Hello Rene,

I am getting confused with this concepts in STP, The more i read the more i am getting confused
Please help me upnderstand this concepts, I 've wriiten my understanding and questions.

In a Steady state Network assuming the packet between computer A and Computer B takes (Switch B --> Switch A --> Switch C --> Switch D)

**Question related to interface states and convergence time**

Switch C (Root Bridge) generates configuration BPDU’s and sends it to Switch A and Switch D and in turn they relay it to switch B
When we have an indirect link failure (shutting down the interface between Switch A and Switch C i.e fa0/16)
Switch A will start advertising itself as Root switch to switch B (downstream)
Since the BPDU received at switch B is inferior to the BPDU which it had received earlier, it will start the max-age timer ignoring BPDU’s till 20 secs
Once the max-age is expired the interface fa0/19 of switch B will move into Listening state, eventually move out to learning and forwarding
Totally it takes 50 sec for the interface (fa0/19) to move from blocking to Forwarding state
what exactly happens in the listening state ? it will start electing the root switch, root port, dp etc ? is my other understanding are correct ?

**Question related to Topology Change**

When ever we have a topology change the switch with interface going up/down, will generate a BPDU with TCN flag set and sends it towards the root switch
This is the only time where a non-root switch will generate a BPDU towards the root switch
The switches which receive the BPDU will acknowledge back to the sender switch which generated the TCN and also relay the BPDU towards the root switch
once it reaches the root switch, it will propagate the changes to the entire switch network in that segment
upon receiving on this BPDU every switch in that segment will reduce mag-aging timer to 15 secs
so my question is TCN is more of to do with aging out the mac-table and it has no significance in the interface state changes ?

I’ve written my understaning and questions. Pleae help me with this questions.

Regards!
Shayan