How Cross-Stack Uplinkfast Works - Cisco Catalyst 2960-XR Configuration Manual

Cross-Stack UplinkFast
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 95
Events That Cause Fast Convergence, on page 86

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.
Catalyst 2960-XR Switch Layer 2 Configuration Guide, Cisco IOS Release 15.0(2)EX1
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Configuring Optional Spanning-Tree Features
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