Cisco WS-C3560E-48PD-SF Command Reference - Page 608

spanning-tree loopguard default Chapter 2 Catalyst 3560 Switch Cisco IOS Commands spanning-tree loopguard default Use the spanning-tree loopguard default global configuration command to prevent alternate or root ports from becoming designated ports because of a failure that leads to a unidirectional link. Use the no form of this command to return to the default setting. spanning-tree loopguard default no spanning-tree loopguard default Syntax Description This command has no arguments or keywords. Defaults Loop guard is disabled. Command Modes Global configuration Command History Release 12.1(19)EA1 Modification This command was introduced. Usage Guidelines You can enable the loop guard feature when the switch is operating in the per-VLAN spanning-tree plus (PVST+), rapid-PVST+, or the multiple spanning-tree (MST) mode. Loop guard is most effective when it is configured on the entire switched network. When the switch is operating in PVST+ or rapid-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 interfaces if the interface is blocked by loop guard in all MST instances. On a boundary interface, loop guard blocks the interface in all MST instances. Loop guard operates only on interfaces that the spanning tree identifies as point-to-point. 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 globally enable loop guard: Switch(config)# spanning-tree loopguard default You can verify your settings by entering the show running-config privileged EXEC command. 2-576 Catalyst 3560 Switch Command Reference 78-16405-05