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20-5
Catalyst 3750 Switch Software Configuration Guide
OL-8550-09
Chapter 20 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Figure 20-3 UplinkFast Example Before Direct Link Failure
If Switch C detects a link failure on the currently active link L2 on the root port (a direct link failure),
UplinkFast unblocks the blocked interface on Switch C and transitions it to the forwarding state without
going through the listening and learning states, as shown in Figure 20-4. This change takes
approximately 1 to 5 seconds.
Figure 20-4 UplinkFast Example After Direct Link Failure
Understanding Cross-Stack UplinkFast
For Catalyst 3750 switches, the UplinkFast feature is the cross-stack UplinkFast feature. Cross-stack
UplinkFast (CSUF) provides a fast spanning-tree transition (fast convergence in less than 1 second under
normal network conditions) across a switch stack. During the fast transition, an alternate redundant link
on the switch stack is placed in 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 is automatically enabled when you enable the UplinkFast feature by using
the spanning-tree uplinkfast global configuration command.
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 the “Events that Cause Fast
Convergence” section on page 20-7.
L1
L2 L3
Switch C
Switch A
(Root)
Switch B
Blocked port
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L1
L2 L3
Switch C
Switch A
(Root)
Switch B
UplinkFast transitions port
directly to forwarding state.
Link failure
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