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Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
78-6511-08
Chapter 6 Configuring the System
Configuring STP
Figure 6-9
Cross-Stack UplinkFast Topology
CSUF implements the Stack Membership Discovery Protocol and the Fast Uplink Transition Protocol.
Using the Stack Membership Discovery Protocol, all stack switches build a neighbor list of stack
members through the receipt of discovery hello packets. When certain link loss or STP events occur
(described in the
“Events that Cause Fast Convergence” section on page 6-39
), 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 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.
Switch A
Spanning
tree root
Backbone
Multidrop backbone
(GigaStack GBIC connections)
Stack port
47572
Switch B
Stack port
Fwd
Link A
(Root link)
Link B
(Alternate
redundant
link)
Link C
(Alternate
redundant
link)
100 or 1000 Mbps
100 or 1000 Mbps
100 or 1000 Mbps
Fwd
Fwd
Switch C Stack port
Stack root port
Alternate stack
root port
Alternate stack
root port
