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Catalyst 2900 Series XL and Catalyst 3500 Series XL Software Configuration Guide
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Chapter 6 Configuring the System
Configuring STP
Configuring Cross-Stack UplinkFast
Cross-stack UplinkFast (CSUF) provides a fast spanning-tree transition (fast
convergence in less than 2 seconds under normal network conditions) across a
stack of switches that use the GigaStack GBICs connected in a shared cascaded
configuration (multidrop backbone). During the fast transition, an alternate
redundant link on the stack of switches is placed into the forwarding state without
causing temporary spanning-tree loops or loss of connectivity to the backbone.
With this feature, you can have a redundant and resilient network in some
configurations.
CSUF might not provide a fast transition all the time; in these cases, the normal
STP transition occurs, which completes in 30 to 40 seconds. For more
information, see the “Events that Cause Fast Convergence” section on page 6-33.
How CSUF Works
CSUF ensures that one link in the stack is elected as the path to the root. As shown
in Figure 6-7, 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 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.
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