How Csuf Works - Cisco Catalyst 3750-X 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 22-5
Switch 1
When certain link loss or spa nning-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-X and 3560-X Switch Software Configuration Guide
22-6
Cross-Stack UplinkFast Topology
Spanning-
tree root
Forward
Forward
Forward
Link 1
(Root link)
100 or 1000 Mbps
Stack-root port
StackWise Plus
port connections
22-7), the Fast Uplink Transition Protocol uses the neighbor list to send fast-transition
Chapter 22
Backbone
Link 2
(Alternate
redundant
link)
100 or 1000 Mbps
Alternate stack-
root port
Switch 2 StackWise Plus
port connections
Switch stack
Configuring Optional Spanning-Tree Features
Figure
Link 3
(Alternate
redundant
link)
100 or 1000 Mbps
Alternate stack-
root port
Switch 3 StackWise Plus
port connections
"Events that Cause Fast Convergence"
22-5, the
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