16-5
Catalyst2950 and Catalyst2955 Switch Software Configuration Guide
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Chapter16 Configuring Optiona l Spa nning-Tree Features Understanding Optional Spanning-Tree Features
Figure16-4 UplinkFast Example After Direct Link Failure
Understanding Cross-Stack UplinkFast
Cross-stack UplinkFast (CSUF) provides a fast spanning-tree transition (fast convergence in less than
1 s econd unde r normal network condi tions) across a st ack of swit ches that use th e GigaStack G BIC
modules connected in a shared cascaded configuration (multidrop backbone). During the fast tr ansition,
an alternate redundant link on the stack of switches is pl aced i n the forwa rd ing stat e w ith out causi n g
temporary spanning-tree loops or loss of connectivity to the ba ck bone. With this fea ture , yo u c an h ave
a redundant and resilient network in some configurations. Y ou enable CSUF b y using the spanning-tree
stack-port interface 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 page16-7.

How CSUF Works

CSUF ensures that one link in the stack is elected as the path to the root. As shown in Figure16-5,
Switches A, B, and C are cascaded through the GigaStack GBIC module to form a multidrop backbone,
which communicates control and data traffic across the switches at the access layer. The switches in the
stack use their stack ports to communicate with each other and to connect to the stack back bone; stac k
ports are always in the spanning-tree forwarding state. The s tack-root p ort on Switch A provid es the path
to the root of the spanning tree; the alternate stack-root ports on Switches B and C can provide an
alternate path to the spanning-tree root if the current stack-root switch fails or if its link to the
spanning-tree root fails.
Link A, the root link, is in the spanning-tree forwarding sta te; L inks B an d C are al ter nate re dun dan t
links that are in the spanning-tree blocking state. If Switch A fails, i f its stack-root port fails, or if Link
A fails, CSUF selects either the Switch B or Switch C alternate stack-root port and puts it into the
forwarding state in less than 1 second.
L1
L2 L3
Switch C
Switch A
(Root) Switch B
UplinkFast transitions port
directly to forwarding state.
Link failure
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