Cisco Systems 3750E manual Understanding Cross-Stack UplinkFast, 20-5

Chapter 20 Configuring Optional Spanning-Tree Features

Understanding Optional Spanning-Tree Features

Figure 20-3 UplinkFast Example Before Direct Link Failure

Switch A

(Root)

L1

L2

Switch C

Switch B

L3

Blocked port

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If Switch C detects a link failure on the currently active link L2 on the root port (a direct link failure), UplinkFast unblocks the blocked interface on Switch C and transitions it to the forwarding state without going through the listening and learning states, as shown in Figure 20-4. This change takes approximately 1 to 5 seconds.

Figure 20-4 UplinkFast Example After Direct Link Failure

Switch A
(Root)			Switch B
		L1
	L2		L3
Link failure			UplinkFast transitions port
			directly to forwarding state.
		Switch C

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Understanding Cross-Stack UplinkFast

For Catalyst 3750-E switches, the UplinkFast feature is the cross-stack UplinkFast feature. Cross-stack UplinkFast (CSUF) provides a fast spanning-tree transition (fast convergence in less than 1 second under normal network conditions) across a switch stack. During the fast transition, an alternate redundant link on the switch stack 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. CSUF is automatically enabled when you enable the UplinkFast feature by using the spanning-tree uplinkfast global configuration command.

CSUF might not provide a fast transition all the time; in these cases, the normal spanning-tree transition occurs, completing in 30 to 40 seconds. For more information, see the “Events that Cause Fast Convergence” section on page 20-7.

Catalyst 3750-E and 3560-E Switch Software Configuration Guide

OL-9775-02 20-5