STP Convergence Time

                   Later on this Module, we gonna get into something called Rapid Spanning Tree Protocol and that’s going to allow Convergence happen really really fast, but before we get there, I wanna take a look at Traditional Spanning Tree Protocol. Let’s take a look at

Ø  Traditionally how long is to take Convergence?

                 If we lose the link over, which we had been forwarding traffic. How long, is it take that Blocking Port to Transition into a Forwarding State? If you take a look at the topology on Picture.

               You will see that fastethernet 1/0/2 on Switch A is currently Blocking. But let say, we had a link failure. Let say there was a failure of that Top Network Segment.       

 The Question is? How long is gonna take for that Blocking Port fastethernet 1/0/2 on Switch A to Transition into Forwarding. Well with Traditional Spanning Tree Protocol, we gonna remain in that Blocking State for 20 Seconds.

And during that 20 seconds Period of time. We waiting to receive a BPDU.

“A Bridge Protocol Data Unit coming into fastethernet1/0/1, that’s going to tell the Switch that, it still has a path back to the Root Bridge, that sent us that BPDU”.

                                But if BPDU is not seen on fastethernet 1/0/1 for that 20 seconds Period of time.The Switch starts to realize that something must be wrong and its starts to Transition its Blocking Port fastethernet 1/0/2, its start to Transition that port to the Forwarding State.

The Next Transitional State we have is the Listening State.    

                                 And we stay in the Listening State for 15 Seconds, and during the Listening State this Switch is Listening, it’s examining BPDU’s that are being received over fastethernet 1/0/2, even though we are not Forwarding traffic over that Port, we can still receive BPDU’s is coming into that port.

And After 15 Seconds of Listening, we Transition into the Learning State.

                          And we stay there for 15 Seconds, and the Switches on Learning State is gonna start the Populate it’s Mac-Address Table, the Mac-Addresses that are being seen of a Fastethernet 1/0/2. And after remaining in that Learning State for 15 Seconds.

Finally we transition that Port to the Forwarding State

                          And it becomes the Root Port of Switch A in this case, and it starts Send and Receive traffic, and if we add upon this Different Times, we see that it took 50 Seconds to Transition from a Blocking State to a Forwarding State

          And Please realize, here we talking about the Traditional IEEE 802.1D Spanning Tree Protocol, and there is a Point of confusion that many people have, when we talk about Timers, and that Point of Confusion in this

                 What if I Plugin a Laptop for example into a Switchport, that was not currently in use, another words it was not in the Blocking State, it would just not connected with anything.

                What if I just Plug in a Laptop to a Port with its default configuration on a Switch, is it gonna take 50 Seconds to Transition to Forwarding.

Ø  Actually no, it’s gonna take 30 seconds, do you know why?

  
Because This Port is not initially in the Blocking State

Ø  it doesn’t have to stay in the Blocking State for 20 seconds.

Ø  It can immediately go into the Listening State for the 15 Seconds,

Ø  Then into the Learning State for 15 Seconds,

Ø  And the Transition to Forwarding.

However even that 30 Seconds Period of time might be a little long for us.

          The good news is, we gonna talk about a feature coming up called “Port Fast” that can make that Transition happen much Quicker.

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