Understanding Cross-Stack Uplinkfast - Cisco Catalyst 3120 Software Manual

Chapter 19 Configuring Optional Spanning-Tree Features
Figure 19-3
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 19-4
Switch A
(Root)

Understanding Cross-Stack UplinkFast

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-12247-01
UplinkFast Example Before Direct Link Failure
L1
L2
Blocked port
Switch C
UplinkFast Example After Direct Link Failure
L1
L2
Link failure
Switch C
19-7.
Switch B
L3
Figure
Switch B
L3
UplinkFast transitions port
directly to forwarding state.
Cisco Catalyst Blade Switch 3120 for HP Software Configuration Guide
Understanding Optional Spanning-Tree Features
19-4. This change takes
"Events that Cause Fast
19-5