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Configuring Optional Spanning-Tree Features
Information About Configuring the Optional Spanning-Tree Features
Figure 46
UplinkFast Example Before Direct Link Failure
Switch A
(Root)
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 47
UplinkFast Example After Direct Link Failure
Switch A
(Root)
Enabling UplinkFast for Use with Redundant Links
UplinkFast cannot be enabled on VLANs that have been configured with a switch priority. To enable UplinkFast on a VLAN
with switch priority configured, first restore the switch priority on the VLAN to the default value by using the no
spanning-tree vlan vlan-id priority global configuration command.
Note:
When you enable UplinkFast, it affects all VLANs on the switch. You cannot configure UplinkFast on an individual
VLAN.
You can configure the UplinkFast feature for rapid PVST+ or for the MSTP, but the feature remains disabled (inactive) until
you change the spanning-tree mode to PVST+.
When UplinkFast is enabled, the switch priority of all VLANs is set to 49152. If you change the path cost to a value less
than 3000 and you enable UplinkFast or UplinkFast is already enabled, the path cost of all interfaces and VLAN trunks is
increased by 3000 (if you change the path cost to 3000 or above, the path cost is not altered). The changes to the switch
priority and the path cost reduce the chance that a switch will become the root switch.
When UplinkFast is disabled, the switch priorities of all VLANs and path costs of all interfaces are set to default values if
you did not modify them from their defaults.
L1
L2
Blocked port
Switch C
Figure 47 on page
357. This change takes approximately 1 to 5 seconds.
L1
L2
Link failure
Switch C
Switch B
L3
Switch B
L3
UplinkFast transitions port
directly to forwarding state.
357
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