How Csuf Works - Cisco Catalyst 3550 series 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
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 Switch A fails, if 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.
Figure 18-5 Cross-Stack UplinkFast Topology
Switch A
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 "Events that Cause Fast Convergence" section on page
the neighbor list to send fast-transition requests on the stack port to stack members.
Catalyst 3550 Multilayer Switch Software Configuration Guide
18-6
Spanning-
tree root
Forward
Forward
Forward
Link A
(Root link)
100 or 1000 Mbps
Stack-root port
Stack port
Chapter 18
Backbone
Link B
(Alternate
redundant
link)
100 or 1000 Mbps
Alternate stack-
root port
Switch B Stack port
Multidrop backbone
(GigaStack GBIC connections)
18-7), the Fast Uplink Transition Protocol uses
Configuring Optional Spanning-Tree Features
Figure
Link C
(Alternate
redundant
link)
100 or 1000 Mbps
Alternate stack-
root port
Switch C Stack port
18-5,
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