Cisco Systems OL-8550-09 Understanding Cross-Stack UplinkFast

20-5
Catalyst 3750 SwitchSoftware Configuration Guide
OL-8550-09
Chapter20 Configuring Optional Spanning-Tree Features
Understanding Optional Spanning-Tree Features
Figure20-3 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 SwitchC and transitions it to the forwarding state without
going through the listening and learning states, as shown in Figure20-4. This change takes
approximately 1 to 5 seconds.
Figure20-4 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 1second 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 page20-7.
L1
L2 L3
Switch C
Switch A
(Root) Switch B
Blocked port
43575
L1
L2 L3
Switch C
Switch A
(Root) Switch B
UplinkFast transitions port
directly to forwarding state.
Link failure
43576