Understanding Cross-Stack Uplinkfast - Cisco Catalyst 2950 Software Manual

Understanding Advanced STP Features
Figure 9-6
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 port 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 9-7
Switch A
(Root)

Understanding Cross-Stack UplinkFast

Cross-stack UplinkFast (CSUF) provides a fast spanning-tree transition (fast convergence in less than 1
second under normal network conditions) across a stack of switches that use the GigaStack GBICs
connected in a shared cascaded configuration (multidrop backbone). During the fast transition, an
alternate redundant link on the stack of switches 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 might not provide a fast transition all the time; in these cases, the normal STP transition occurs,
completing in 30 to 40 seconds. For more information, see the
section on page
Catalyst 2950 Desktop Switch Software Configuration Guide
9-12
UplinkFast Example Before Direct Link Failure
L1
L2
Blocked port
Switch C
UplinkFast Example After Direct Link Failure
L1
L2
Link failure
Switch C
9-14.
Switch B
L3
Figure
Switch B
L3
UplinkFast transitions port
directly to forwarding state.
Chapter 9 Configuring STP
9-7. This change takes
"Events that Cause Fast Convergence"
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