844 | Per-VLAN Spanning Tree Plusw w w . d e l l . c o m | s u p p o r t . d e l l . c o mConfigure a Loop GuardThe Loop Guard feature provides protection against Layer 2 forwarding loops (STP loops) caused by ahardware failure, such as a cable failure or an interface fault. When a cable or interface fails, a participatingSTP link may become unidirectional (STP requires links to be bidirectional) and an STP port does notreceive BPDUs. When an STP blocking port does not receive BPDUs, it transitions to a forwarding state.This condition can create a loop in the network.You enable loop guard on a per-port or per-port channel basis.To enable a loop guard on a PVST-enabled port or port-channel interface, enter the spanning-tree mstploopguard command. Refer to STP Loop Guard on page 1064 for more information on how to use the loopguard feature.To disable PVST+ loop guard on a port or port-channel interface, enter the no spanning-tree pvst loopguardcommand in an INTERFACE configuration mode.To verify the PVST+ loop guard configuration on a port or port-channel interface, enter the showspanning-tree pvst [vlan vlan-id] command in global configuration mode.FTOS Behavior: The following conditions apply to a port enabled with loop guard:• Loop guard is supported on any PVST-enabled port or port-channel interface.• Loop guard is supported on a port or port-channel in any Spanning Tree mode:• Spanning Tree Protocol (STP)• Rapid Spanning Tree Protocol (RSTP)• Multiple Spanning Tree Protocol (MSTP)• Per-VLAN Spanning Tree Plus (PVST+)• Root guard and loop guard cannot be enabled at the same time on a PVST+ port. For example, if youconfigure root guard on a port on which loop guard is already configured, the following error message isdisplayed:% Error: LoopGuard is configured. Cannot configure RootGuard.• Enabling Portfast BPDU guard and loop guard at the same time on a port results in a port that remains in ablocking state and prevents traffic from flowing through it. For example, when Portfast BPDU guard andloop guard are both configured:- If a BPDU is received from a remote device, BPDU guard places the port in an err-disabledblocking state and no traffic is forwarded on the port.- If no BPDU is received from a remote device, loop guard places the port in a loop-inconsistentblocking state and no traffic is forwarded on the port.• When used in a PVST+ network, loop guard is performed per-port or per-port channel at a VLAN level. Ifno BPDUs are received on a VLAN interface, the port or port-channel transitions to a loop-inconsistent(blocking) state only for this VLAN.Task Command Syntax Command ModeEnable loop guard on an PVST-enabled port or port-channelinterface.spanning-tree pvst loopguard INTERFACEINTERFACEPORT-CHANNEL
