Administrator Guide
Figure 121. STP Loop Guard Prevents Forwarding Loops
Configuring Loop Guard
Enable STP loop guard on a per-port or per-port channel basis.
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+)
• You cannot enable root guard and loop guard 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.
• 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.
Spanning Tree Protocol (STP)
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