Spanning-tree convergence

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Nov 26th, 2007
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regarding STP convergence times, from what i always had as certain was that on a STP/PVSTP environment after a failure it will take one switch 1xMAX-AGE + 2xFORWARDING-TIME to put one of it's BLOCKING ports forwarding traffic. I also know that when the root bridge receives a TCN will generate a BPDU with TC bit set wich in turn will decrease the mac-address table aging time to the value of the forwarding-time.

Is this correct like the way i've described?

I've labbed one scenario in which let's say i want to have on a particular VLAN every new port forwarding in 12s. So i set for this vlan a Forwarding-time of 6s and a Max-age of 6s. When i configure on any switch one port as access port for this vlan it takes the 12s to go to forwarding mode (doesn't need to wait "max-age" as the port is starting new). Now if i simulate a failure (on the diagram below i've done it in the two places marked with an "x" (not at the same time), it takes around 16s to have the STP converged and the ports on the new topology on "forwarding" status. I would expect this time to be 18s.

I did the same test with forwarding time 4s and max age 6s, and i got 12s convergence time (instead of 14s).

Does anyone has an idea about the reason for this



/ | \

/ | \ x

/ | SW2

/ | \

SW1 | x \

| SW3


Thanks in advance

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