1020 | Spanning Tree Protocol (STP)w w w . d e l l . c o m | s u p p o r t . d e l l . c o m Loop Guard ConfigurationYou enable STP loop guard on a per-port or per-port channel basis.To enable a loop guard on an STP-enabled port or port-channel interface, enter the spanning-tree 0loopguard command:To disable STP loop guard on a port or port-channel interface, enter 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, enter the showspanning-tree 0 guard [interface interface] command in global configuration mode.FTOS Behavior: The following conditions apply to a port enabled with loop guard:• Loop guard is supported on any STP-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 an STP port. For example, if you configure loop guard ona port on which root guard is already configured, the following error message is displayed:% Error: RootGuard is configured. Cannot configure LoopGuard.• C-Series and E-Series only: Loop guard is supported on a C-Series or E-Series switch configured for hitless STP (seeConfiguring Spanning Trees as Hitless).• Enabling Portfast BPDU guard and loop guard at the same time on a port results in a port that remains in a blocking stateand prevents traffic from flowing through it. For example, when Portfast BPDU guard and loop guard are both configured:- If a BPDU is received from a remote device, BPDU guard places the port in an err-disabled blocking state andno traffic is forwarded on the port.- If no BPDU is received from a remote device, loop guard places the port in a loop-inconsistent blocking stateand 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 VLAN level. If no BPDUsare received on a VLAN interface, the port or port-channel transitions to a loop-inconsistent (blocking) state only for thisVLAN.Task Command Syntax Command ModeEnable loop guard on a port or port-channel interface.0: Enables loop guard on an STP-enabled port assigned toinstance 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.spanning-tree {0 | mstp | rstp |pvst} loopguardINTERFACEINTERFACEPORT-CHANNEL
