Datasheet

Events that Cause Fast Convergence section of this document), the Fast Uplink Transition Protocol uses the
neighbor list to send fast−transition requests on the stack port to stack members.
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 spanning−tree instance by comparing the root, cost, and bridge ID. If the sending switch is the best choice
as the stack root, each switch in the stack returns an acknowledgement; otherwise, it does not respond to the
sending switch (drops the packet). The sending switch then has not received 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 spanning−tree
transitions (blocking, listening, learning, and 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 spanning−tree
instance at a time.
Events that Cause Fast Convergence
Depending on the network event or failure, the CSUF fast convergence may or may not occur. Fast
convergence (less than one second under normal network conditions) occurs under these circumstances:
The stack−root port link fails.•
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 connects the stack root to the spanning−tree root, recovers.•
A network reconfiguration causes a new stack−root switch to be selected.•
A network reconfiguration causes a new port on the current stack−root switch to be chosen as the
stack−root port.
•
Note: The fast transition might not occur if multiple events occur simultaneously. For example, if a stack
member switch is powered off while the link connecting the stack root to the spanning−tree root comes back
up, the normal spanning−tree convergence occurs.
Normal spanning−tree convergence (30 to 40 seconds) occurs under the following conditions:
The stack−root switch is powered off, or the software failed.•
The stack−root switch, which was powered off or failed, is powered on.•
A new switch, which might become the stack root, is added to the stack.•
A switch other than the stack root is powered off or failed.•
A link fails between stack ports on the multidrop backbone.•
Limitations
These limitations apply to CSUF:
Cisco − Catalyst Switch GigaStack Configuration and Implications