Specifications

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Catalyst 2950 and Catalyst 2955 Switch Command Reference
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Chapter 2 Cisco IOS Commands
spanning-tree guard
Do not enable root guard on interfaces that will be used by the UplinkFast feature. With UplinkFast, the
backup interfaces (in the blocked state) replace the root port in the case of a failure. However, if root
guard is also enabled, all the backup interfaces used by the UplinkFast feature are placed in the
root-inconsistent state (blocked) and prevented from reaching the forwarding state. The UplinkFast
feature is not available when the switch is operating in MST mode.
Loop guard is most effective when it is configured on the entire switched network. When the switch is
operating in PVST mode, loop guard prevents alternate and root ports from becoming designated ports,
and spanning tree does not send bridge protocol data units (BPDUs) on root or alternate ports. When the
switch is operating in MST mode, BPDUs are not sent on nonboundary ports only if the port is blocked
by loop guard in all MST instances. On a boundary port, loop guard blocks the port in all MST instances.
To disable root guard or loop guard, use the spanning-tree guard none interface configuration
command. You cannot enable both root guard and loop guard at the same time.
You can override the setting of the spanning-tree loopguard default global configuration command by
using the spanning-tree guard loop interface configuration command.
Examples This example shows how to enable root guard on all the VLANs associated with the specified interface:
Switch(config)# interface fastethernet0/3
Switch(config-if)# spanning-tree guard root
This example shows how to enable loop guard on all the VLANs associated with the specified interface:
Switch(config)# interface fastethernet0/3
Switch(config-if)# spanning-tree guard loop
You can verify your settings by entering the show running-config privileged EXEC command.
Related Commands Command Description
show running-config Displays the current operating configuration. For syntax
information, select Cisco IOS Configuration Fundamentals
Command Reference for Release 12.1 > Cisco IOS File
Management Commands > Configuration File Commands.
spanning-tree cost Sets the path cost for spanning-tree calculations.
spanning-tree loopguard default Prevents alternate or root ports from becoming designated
ports because of a failure that leads to a unidirectional link.
spanning-tree mst cost Configures the path cost for MST calculations.
spanning-tree mst port-priority Configures an interface priority.
spanning-tree mst root Configures the MST root switch priority and timers based on
the network diameter.
spanning-tree port-priority Configures an interface priority.
spanning-tree vlan priority Sets the switch priority for the specified spanning-tree
instance.