Cisco Catalyst 2960 Series, Configuration Manual

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of connectivity to the backbone. With this feature, you can have a redundant and resilient network in some
configurations. CSUF is automatically enabled when you enable the UplinkFast feature.
CSUF might not provide a fast transition all the time; in these cases, the normal spanning-tree transition
occurs, completing in 30 to 40 seconds. For more information, see Related Topics.
Related Topics
Enabling UplinkFast for Use with Redundant Links , on page 307
Events That Cause Fast Convergence, on page 294
How Cross-Stack UplinkFast Works
Cross-Stack UplinkFast (CSUF) ensures that one link in the stack is elected as the path to the root.
The stack-root port on Switch 1 provides the path to the root of the spanning tree. The alternate stack-root
ports on Switches 2 and 3 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 1, the root link, is in the spanning-tree forwarding state. Links 2 and 3 are alternate redundant links that
are in the spanning-tree blocking state. If Switch 1 fails, if its stack-root port fails, or if Link 1 fails, CSUF
selects either the alternate stack-root port on Switch 2 or Switch 3 and puts it into the forwarding state in less
than 1 second.
Consolidated Platform Configuration Guide, Cisco IOS Release 15.2(4)E (Catalyst 2960-X Switches)
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Information About Optional Spanning-Tree Features