Administrator Guide

As shown in STP topology 3 (bottom middle), after you enable loop guard on an STP port or port-channel on Switch C, if no BPDUs are
received and the max-age timer expires, the port transitions from a blocked state to a Loop-Inconsistent state (instead of to a
Forwarding state). Loop guard blocks the STP port so that no traffic is transmitted and no loop is created.
As soon as a BPDU is received on an STP port in a Loop-Inconsistent state, the port returns to a blocking state. If you disable STP loop
guard on a port in a Loop-Inconsistent state, the port transitions to an STP blocking state and restarts the max-age timer.
Figure 142. 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.
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Spanning Tree Protocol (STP)