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10-14
Catalyst 3550 Multilayer Switch Software Configuration Guide
78-11194-03
Chapter 10 Configuring STP
Understanding Advanced STP Features
How CSUF Works
CSUF ensures that one link in the stack is elected as the path to the root. As shown in Figure 10-8,
Switches A, B, and C are cascaded through the GigaStack GBIC to form a multidrop backbone, which
communicates control and data traffic across the switches at the access layer. The switches in the stack
use their stack ports to communicate with each other and to connect to the stack backbone; stack ports
are always in the STP forwarding state. The stack-root port on Switch A provides the path to the root of
the spanning tree; the alternate stack-root ports on Switches B and C can provide an alternate path to the
spanning-tree root if the current stack-root switch fails or if its link to the spanning-tree root fails.
Link A, the root link, is in the STP forwarding state; Links B and C are alternate redundant links that are
in the STP blocking state. If Switch A fails, if its stack-root port fails, or if Link A fails, CSUF selects
either the Switch B or Switch C alternate stack-root port and puts it into the forwarding state in less
than 1 second.
Figure 10-8 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 “Events that Cause Fast Convergence” section on page 10-15), the Fast Uplink Transition
Protocol uses the neighbor list to send fast-transition requests on the stack port to stack members.
Switch A
Spanning-
tree root
Backbone
Multidrop backbone
(GigaStack GBIC connections)
Stack port
49067
Switch B
Stack port
Forward
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
Forward
Forward
Switch C
Stack port
Stack-root port
Alternate stack-
root port
Alternate stack-
root port