Cisco Systems 3550 Understanding Cross-Stack UplinkFast

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Catalyst 3550 Multilayer Switch Software Configuration Guide
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Chapter10 Configuring STP Understanding Advanced STP Features
Figure10-6 UplinkFast Example Before Direct Link Failure
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 port on Switch C and transitions it to the forwarding state without
going through the listening and learning states, as sho wn i n Figur e 10-7. This change takes
approximately1 to 5 seconds. For more information, see the “Configuring UplinkFast for Use with
Redundant Links” section on page 10-34.
Figure10-7 UplinkFast Example After Direct Link Failure
Understanding Cross-Stack UplinkFast
Cross-stack UplinkFast (CSUF) provides a fast spanning-tree transition (fast convergence in less than 1
second under normal network conditions) across a stack of switches that use the GigaStack GBICs
connected in a shared cascaded configuration (multidrop backbone). During the fast transition, an
alternate redundant link on the stack of switches is placed in the forwarding state without causing
temporary spanning-tree loops or loss of connectivity to the b ac kbone . With this feat ur e, y ou can ha ve
a redundant and resilient network in some configurations.
CSUF might not provide a fast transition all the time; in these cases, the normal STP transition occurs,
completing in 30 to 40 seconds. For more information, see the “Events that Cause Fast Convergence”
section on page 10-15. For configuration information, see the “Configuring Cross-Stack UplinkFast”
section on page 10-35.
L1
L2 L3
Switch C
Switch A
(Root) Switch B
Blocked port
43575
L1
L2 L3
Switch C
Switch A
(Root) Switch B
UplinkFast transitions port
directly to forwarding state.
Link failure
43576