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Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
78-6511-05
Chapter 6 Configuring the System
Configuring STP
The switch sending the fast-transition request needs to do a fast transition to the
forwarding state of a port that it has chosen as the root port, and it must obtain an
acknowledgement from each stack switch before performing the fast transition.
Each switch in the stack determines if the sending switch is a better choice than
itself to be the stack root of this STP instance by comparing STP root, cost, and
bridge ID. If the sending switch is the best choice as the stack root, the switch in
the stack returns an acknowledgement; otherwise, it does not respond to the
sending switch (drops the packet) and prevents the sending switch from receiving
acknowledgements from all stack switches.
When acknowledgements are received from all stack switches, the Fast Uplink
Transition Protocol on the sending switch immediately transitions its alternate
stack root port to the forwarding state. If acknowledgements from all stack
switches are not obtained by the sending switch, the normal STP transitions
(blocking, listening, learning, forwarding) take place, and the spanning-tree
topology converges at its normal rate (2 * forward-delay time + max-age time).
The Fast Uplink Transition Protocol is implemented on a per-VLAN basis and
affects only one STP instance at a time.
Events that Cause Fast Convergence
Depending on the network event or failure, fast convergence provided by CSUF
might or might not occur.
Fast convergence (within 2 seconds under normal network conditions) occurs
under these circumstances:
• The stack root port link goes down.
If two switches in the stack have alternate paths to the root, only one of the
switches performs the fast transition.
• The failed link, which connected the stack root to the STP root, comes back
up.
• A network reconfiguration causes a new stack root switch to be selected.
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