Datasheet

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. On the catalyst 2900/3500XL, 2950, and 3550 series switches you enable CSUF by first
entering the spanning−tree uplinkfast command in global configuration mode, and then using the
spanning−tree stack−port interface configuration command on the Gigastack interface desired. On catalyst
switches 3750 CSUF is automatically enabled when you enable the UplinkFast feature by using the
spanning−tree uplinkfast global configuration command. The CSUF feature is supported only when the
switch is running PVST.
Note: The 2900xl and 3500xl switches require code version 12.0(5)XW or higher to support the Cross−stack
uplinkfast feature.
How CSUF Works
CSUF ensures that one link in the stack is elected as the path to the root. As shown in the figure below,
Switches A, B, and C are cascaded through the GigaStack GBIC 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 backbone; stack ports are always
in the spanning−tree forwarding state. The stack−root port on Switch A provides 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 state; Links B and C are alternate redundant links
that are in the spanning−tree blocking state. If Link A, Switch A or its stack−root port fails, CSUF selects
either the Switch B or Switch C alternate stack−root port and puts it into the forwarding state in less than one
second.
Cross−Stack UplinkFast Topology
CSUF uses the Stack Membership Discovery Protocol to build a neighbor list of stack members through the
receipt of discovery hello packets. When certain link loss or spanning−tree events occur (described in the
Cisco − Catalyst Switch GigaStack Configuration and Implications