STP convergence time

[santo.suy]

I have learned during CISCO trainning course (the BCMSN) that the STP convergence time is by default 50 seconds which corresponds to the sum of :

- Blocking phase (Max age time = 20s)

- Listening phase (Forwarding delay = 15 s)

- Learning phase (Forwarding delay = 15 s)

On, the other hand, I saw on the CISCO web site some paper "Understanding STP" for example, where it was described a special BPDU message, the TCN (Topology Change Notification). In this paper, it seems that in certain circumstancies, when the switch detect that the topology has changed, it it not necessary to go through al the three phases.

Can you tell me the exact situations where the convergence time is smaller that 50 seconds and why ( which phases is applicable, which one is not) ?

[r.srivastava]

TCN's are nothing but a notification mesage indicating that port state has been changed .This has got servity level 5.

For the ports which are not connected to any other HUB,Switch,Routers ( any device which can cause STP parameter to rebuilt ) ,you can use the portfast feature by issuing the command " set spantree portfast 3/12 enable " for the 12th port of the 3rd module.by enabling this you can make sure that whenever this port comes online switch will not take its time (50 Sec) to bring the port up.it will assume that this is a port directlly connected to the server/computer and will bring the port up Imdt.