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Spanning Tree Protocol (STP) Failover Scenario

 Today we are  going to discuss on the how failover in the STP will comes when the links between the switches are down and how path is re-selected for the frames to be floated from source to destination.

Let's start with the example or the scenario as defined below 

Now as per the above picture you can see that we have the failure of the link between the Switch or Catalyst A and Catalyst C. Please don't misunderstood regarding the catalyst. Catalyst is same as switch.

Fig 1.1- STP Failover Scenario

Now Catalyst A is the root bridge here, When link between switch A and Switch C fails. FE1/2 of switch C will transition to Forwarding status. But there is change in the topology and Root Bridge or Catalyst A will send the TCN BPDU to all switches or catalyst ( as there is change in the topology now ) 15 seconds is for CAM table to time out and start process of  re-fresh or  re-flooding from time of TCN BPDU arriving at switch. 

As Root Bridge says to clear your CAM tables so that you can get the TCN BPDU further. So Switch C waits for TCN BPDU from Root after the failure.  Root SW A sends TCN to B, B sends it to SW C who will change its port upon receiving TCN. I guess you are able to understand now... No ?

Let me explain in another way , Link that is Root port on C goes down, C still has other port getting BPDUs from B so it does not have to wait for timer to expire (no need for 20 sec max age) and moves the other port to listening, learning , and forwarding.

This is layer 1 failure on same switch that has root and blocking port. Switch knows that one of its port failed so starts STP process on the other port skipping 20 sec. Total 30 sec

Note: STP only Root switch will send TCN when it hears about topology change. When non-root switch gets it, it starts 15 sec timer and sends TCN ACK back to Root.

Hope it clarify the failure scenario in the STP which causes TCN BPDU to comes in to the picture. We will come with another failure scenario where the link between the Catalyst A and Catalyst B fails and then we will discuss what happen and how TCN BPDU will generate and how much time it takes in STP to converges.


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