Spanning-Tree UplinkFast

[lagapidis (Lazarus Agapidis)]

Hello Maodo

@kayoutoure

In one of the previous lessons (TCN), it was said about 50s delay for moving from Blocking to Forwarding state. This lesson does only talk about 30s delay. Something important that this lesson has to say is : SW3 does not need to wait for the 20s of “Max Age” (parameter setting on the switch) ; because it detects immediately that a port (belonging to the switch) is not working.

In the TCN lesson, it mentions that there will be a maximum of 50 seconds delay when there is a change in topology. When a TCN is received, a port will go into the blocking state for the Max Age interval which is an interval of 20 seconds by default before beginning the STP recalculation. The maximum of 50 seconds is ONLY for topology changes.

In this lesson, it is mentioned that it takes a maximum of 30 seconds for STP to converge when STP function occurs from scratch, that is, after the switches are rebooted or turned on. This includes ONLY the Listening (15s) and Learning (15s) states. In this case, 30 seconds is only needed.

@kayoutoure

The lesson is saying that the Listening and Learning delay could be as lower as 14s. It would be important to recall that this delay is always twice (2 x 7s) the “Forward Delay” parameter (setting on the switch).

How the result of approximately 14 seconds has been achieved depends on several factors including network diameter. For further information, you can check this Cisco documentation that further explains STP timers in detail.

@kayoutoure

The end of the lesson is very confusing. Once the link between SW3 and SW1 is back ; nothing explains by which process and how long it takes for the ports of those two switches to get back to their former states.

As is stated in the lesson, essentially nothing happens. The root port remains the same and Fa0/14 remains blocked. Since the network is currently working, no changes have been made, BPDUs are successfully exchanged, the network remains in this state until there is a topology change.

I hope this has been helpful!

Laz