Understanding Cross-Stack Uplinkfast - Cisco Catalyst 3750-X Software Configuration Manual

Chapter 22 Configuring Optional Spanning-Tree Features
Figure 22-3
Switch A
(Root)
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
approximately 1 to 5 seconds.
Figure 22-4
Switch A
(Root)

Understanding Cross-Stack UplinkFast

For Catalyst 3750-X 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
Convergence" section on page
OL-21521-01
UplinkFast Example Before Direct Link Failure
L1
L2
Blocked port
Switch C
UplinkFast Example After Direct Link Failure
L1
L2
Link failure
Switch C
22-7.
Understanding Optional Spanning-Tree Features
Switch B
L3
Figure
Switch B
L3
UplinkFast transitions port
directly to forwarding state.
Catalyst 3750-X and 3560-X Switch Software Configuration Guide
22-4. This change takes
"Events that Cause Fast
22-5