Cisco WS-C2950-24 Configuration Manual page 170

Configuring STP
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The fast transition might not occur if multiple events occur simultaneously.
Note
For example, if a stack member switch is powered down, and at the same time,
a link connecting the stack root to the STP root comes back up, the normal STP
convergence occurs.
Normal STP convergence (30 to 40 seconds) occurs under these conditions:
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The fast transition of CSUF depends on the amount of network traffic and how
Note
you connect the GigaStack GBICs across the stack switches. Because the Fast
Uplink Transition Protocol only waits 2 seconds to receive acknowledgements
from all stack switches, heavy network traffic might prevent the fast transition
from occurring within this time frame. Instead of a fast transition, the normal
STP convergence then occurs.
Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
6-34
A network reconfiguration causes a new port on the current stack root switch
to be chosen as the stack root port.
The stack root switch is powered down or the software failed.
The stack root switch, which was powered down or failed, is powered up.
A new switch, which might become the stack root, is added to the stack.
A switch other than the stack root is powered down or failed.
A link fails between stack ports on the multidrop backbone.
Chapter 6 Configuring the System
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