• Spanning Tree Protocol (STP)• Rapid Spanning Tree Protocol (RSTP)• Multiple Spanning Tree Protocol (MSTP)• Per-VLAN Spanning Tree Plus (PVST+)• You cannot enable root guard and loop guard at the same time on an STP port. For example, if youconfigure loop guard on a port on which root guard is already configured, the following error messageis displayed: % Error: RootGuard is configured. Cannot configure LoopGuard.• Enabling Portfast BPDU guard and loop guard at the same time on a port results in a port that remains ina blocking state and prevents traffic from flowing through it. For example, when Portfast BPDU guardand loop 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, STP loop guard is performed per-port or per-port channel at a VLANlevel. If no BPDUs are received on a VLAN interface, the port or port-channel transitions to a Loop-Inconsistent (Blocking) state only for this VLAN.To enable a loop guard on an STP-enabled port or port-channel interface, use the following command.• Enable loop guard on a port or port-channel interface.INTERFACE mode or INTERFACE PORT-CHANNEL modespanning-tree {0 | mstp | rstp | pvst} loopguard• 0: enables loop guard on an STP-enabled port assigned to instance 0.• mstp: enables loop guard on an MSTP-enabled port.• rstp: enables loop guard on an RSTP-enabled port.• pvst: enables loop guard on a PVST-enabled port.To disable STP loop guard on a port or port-channel interface, use the no spanning-tree 0 loopguardcommand in an INTERFACE configuration mode.To verify the STP loop guard configuration on a port or port-channel interface, use the show spanning-tree 0 guard [interface interface] command in a global configuration mode.Displaying STP Guard ConfigurationTo display the STP guard configuration, use the following command.The following example shows an STP network (instance 0) in which:• Root guard is enabled on a port that is in a root-inconsistent state.• Loop guard is enabled on a port that is in a listening state.• BPDU guard is enabled on a port that is shut down (Error Disabled state) after receiving a BPDU.• Verify the STP guard configured on port or port-channel interfaces.show spanning-tree 0 guard [interface interface]Example of Viewing STP Guard ConfigurationDell#show spanning-tree 0 guardInterfaceSpanning Tree Protocol (STP) 1090
