1020 | Spanning Tree Protocol (STP)
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Loop Guard Configuration
You 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 0
loopguard
command:
To disable STP loop guard on a port or port-channel interface, enter the
no spanning-tree 0 loopguard
command in an INTERFACE configuration mode.
To verify the STP loop guard configuration on a port or port-channel interface, enter the
show
spanning-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 on
a 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 (see
Configuring 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 state
and 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 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-inconsistent blocking 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 VLAN level. 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.
Task Command Syntax Command Mode
Enable loop guard on a port or port-channel interface.
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.
spanning-tree {0 | mstp | rstp |
pvst} loopguard
INTERFACE
INTERFACE
PORT-CHANNEL
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