Cisco Certified Network Professional 2025 – 400 Free Practice Questions to Pass the Exam

The correct answer is that a port enters the Loop-inconsistent state when it loses all Bridge Protocol Data Units (BPDUs) while using Loop Guard.

Loop Guard is a feature in Cisco's Spanning Tree Protocol (STP) that helps prevent bridge loops by protecting against scenarios where BPDUs are no longer received on a port. When a port configured with Loop Guard stops receiving BPDUs, it assumes that there may be a potential loop situation and therefore transitions into a state that indicates a lack of BPDU receipt. This Loop-inconsistent state prevents the port from transitioning to the forwarding state, which would lead to a potential loop condition.

By entering the Loop-inconsistent state, the port effectively blocks traffic to help ensure network stability and integrity until BPDUs are received again. This is crucial because allowing a port to enter the forwarding state without valid BPDUs could cause bridging loops, which can severely disrupt network operations.

In contrast, the other options do not specifically denote the behavior of a port under these conditions. For instance, the forwarding state would imply normal operation, while the blocking state involves preventing traffic but doesn’t reflect the unique scenario of missing BPDUs in the context of Loop Guard. The errdisable state,