Reference Guide

996 | Spanning Tree Protocol
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Loop Guard Configuration
You enable STP loop guard on a per-port or per-port channel basis.
To enable the loop guard on an STP-enabled port or port-channel interface, enter the following
spanning-tree loopguard command:
To disable STP loop guard on a port or port-channel interface, enter the
no spanning stp-id 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+)
• Loop guard and root guard cannot be enabled at the same time on an STP port. When you configure loop
guard on a port, an existing root guard configuration is disabled.
• C-Series and E-Series only: Loop guard is supported on a C-Series or E-Series switch configured for
hitless STP (refer to Configuring Spanning Trees as Hitless).
• Be careful if you enable Portfast BPDU guard and loop guard at the same time on a port. Enabling both
features 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 STP loop guard on a port or port-channel interface.
stp-id: Spanning Tree instance. Range: 0 to 63.
cost number: (Optional) Cost value used to select the port
as a forwarding port. Range: 0 to 200000.
priority number: (Optional) Priority used to select the port
as a forwarding port if more than one port have the same
cost. Range: 0 to 240.
portfast [bpduguard [shutdown-on-violation]]: (Optional)
Enable PortFast to start forwarding traffic sooner and
optionally shut down the interface hardware when a BPDU
is received (refer to Enabling PortFast).
spanning-tree stp-id loopguard
[cost number]
[priority number]
[portfast [bpduguard
[shutdown-on-violation]]]
INTERFACE
INTERFACE
PORT-CHANNEL