Cisco WS-C3560E-48PD-SF Command Reference - Page 604
spanning-tree guard
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spanning-tree guard Chapter 2 Catalyst 3560 Switch Cisco IOS Commands spanning-tree guard Use the spanning-tree guard interface configuration command to enable root guard or loop guard on all the VLANs associated with the selected interface. Root guard restricts which interface is allowed to be the spanning-tree root port or the path-to-the root for the switch. Loop guard prevents alternate or root ports from becoming designated ports when a failure creates a unidirectional link. Use the no form of this command to return to the default setting. spanning-tree guard {loop | none | root} no spanning-tree guard Syntax Description loop none root Enable loop guard. Disable root guard or loop guard. Enable root guard. Defaults Root guard is disabled. Loop guard is configured according to the spanning-tree loopguard default global configuration command (globally disabled). Command Modes Interface configuration Command History Release 12.1(19)EA1 Modification This command was introduced. Usage Guidelines You can enable root guard or loop guard when the switch is operating in the per-VLAN spanning-tree plus (PVST+), rapid-PVST+, or the multiple spanning-tree (MST) mode. When root guard is enabled, if spanning-tree calculations cause an interface to be selected as the root port, the interface transitions to the root-inconsistent (blocked) state to prevent the customer's switch from becoming the root switch or being in the path to the root. The root port provides the best path from the switch to the root switch. When the no spanning-tree guard or the no spanning-tree guard none command is entered, root guard is disabled for all VLANs on the selected interface. If this interface is in the root-inconsistent (blocked) state, it automatically transitions to the listening state. 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 the rapid-PVST+ or MST mode. 2-572 Catalyst 3560 Switch Command Reference 78-16405-05