Cisco Systems 3750E manual How Csuf Works, 20-6

Chapter 20 Configuring Optional Spanning-Tree Features

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 Figure 20-5, the 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 20-5 Cross-Stack UplinkFast Topology

		Backbone
Spanning-
tree root
	Forward
Forward	Forward
Link 1		Link 2	Link 3
(Root link)		(Alternate	(Alternate
		redundant	redundant
		link)	link)
100 or 1000 Mbps		100 or 1000 Mbps	100 or 1000 Mbps
Stack-root port		Alternate stack-	Alternate stack-
		root port	root port

Switch 1

StackWise Plus	Switch 2	StackWise Plus
port connections		port connections
	Catalyst 3750-E switch stack

Switch 3

StackWise Plus port connections

159891

When certain link loss or spanning-tree events occur (described in “Events that Cause Fast Convergence” section on page 20-7), the Fast Uplink Transition Protocol uses the neighbor list to send fast-transition 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-E and 3560-E Switch Software Configuration Guide

20-6 OL-9775-02