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Multiple Spanning Tree Protocol | 653
Configure a Loop Guard
The Loop Guard feature provides protection against Layer 2 forwarding loops (STP loops) caused by a
hardware failure, such as a cable failure or an interface fault. When a cable or interface fails, a participating
STP link may become unidirectional (STP requires links to be bidirectional) and an STP port does not
receive 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 an MSTP-enabled port or port-channel interface, enter the
spanning-tree mstp
loopguard
command. Refer to STP Loop Guard on page 1064 for more information on how to use the loop
guard feature.
To disable MSTP loop guard on a port or port-channel interface, enter the
no spanning-tree mstp loopguard
command in an INTERFACE configuration mode.
To verify the MSTP loop guard configuration on a port or port-channel interface, enter the
show
spanning-tree msti [instance-number] guard
command in global configuration mode.
FTOS Behavior: The following conditions apply to a port enabled with loop guard:
• Loop guard is supported on any MSTP-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 MSTP port. For example, if you
configure root guard on a port on which loop guard is already configured, the following error message is
displayed:
% 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 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.
Task Command Syntax Command Mode
Enable loop guard on an MSTP-enabled port or port-channel
interface.
spanning-tree mstp loopguard
INTERFACE
INTERFACE
PORT-CHANNEL