How Csuf Works - Cisco WS-C3750-48PS-S Software Configuration Manual

Understanding Optional Spanning-Tree Features

How CSUF Works

CSUF ensures that one link in the stack is elected as the path to the root. As shown in
stack-root port on Switch 1 provides the path to the root of the spanning tree. The alternate stack-root
ports on Switches 2 and 3 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 1, the root link, is in the spanning-tree forwarding state. Links 2 and 3 are alternate redundant links
that are in the spanning-tree blocking state. If Switch 1 fails, if its stack-root port fails, or if Link 1 fails,
CSUF selects either the alternate stack-root port on Switch 2 or Switch 3 and puts it into the forwarding
state in less than 1 second.
Figure 19-5 Cross-Stack UplinkFast Topology
Switch 1
When certain link loss or spanning-tree events occur (described in
section on page
requests to stack members.
The switch sending the fast-transition request needs to do a fast transition to the forwarding state of a
port that it has chosen as the root port, and it must obtain an acknowledgement from each stack switch
before performing the fast transition.
Catalyst 3750 Switch Software Configuration Guide
19-6
Spanning-
tree root
Forward
Forward
Forward
Link 1
(Root link)
100 or 1000 Mbps
Stack-root port
StackWise
port connections
19-7), the Fast Uplink Transition Protocol uses the neighbor list to send fast-transition
Chapter 19
Backbone
Link 2
(Alternate
redundant
link)
100 or 1000 Mbps
Alternate stack-
root port
Switch 2 StackWise
port connections
Catalyst 3750 switch stack
Configuring Optional Spanning-Tree Features
Figure
Link 3
(Alternate
redundant
link)
100 or 1000 Mbps
Alternate stack-
root port
Switch 3 StackWise
port connections
"Events that Cause Fast Convergence"
19-5, the
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