API Guide

Figure 120. 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.
○ 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.
Spanning Tree Protocol (STP)
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