Of the choices provided, the %STP-2-BRIDGE_ASSURANCE_BLOCK error message or the %STP-2-DISPUTE_DETECTED error message might appear when a unidirectional link failure exists between two switches. A unidirectional link failure exists when a defective cable causes one device to not receive what the other device sends.

The %STP-2-BRIDGE_ASSURANCE_BLOCK error message appears when a Spanning Tree Protocol (STP) bridge protocol data unit (BPDU) has not been received on an interface with Bridge Assurance enabled. Bridge Assurance ensures that BPDUs are sent bidirectionally onall network interfaces. If an interface with Bridge Assurance does not receive a BPDU, or if the connected interface does not have Bridge Assurance enabled, the interface is put into an inconsistent state and is blocked. Bridge Assurance is supported only with Rapid PerVLAN Spanning Tree Plus (RPVST+) and Multiple Spanning Tree (MST) and only on point-to-point links.

The %STP-2-DISPUTE_DETECTED error message appears when a switch receives inferior BPDUs that are marked as designated and learning or forwarding. This indicates that the connected switch is not receiving superior BPDUs from the local switch. As a result, the local switch will record the %STP-2-DISPUTE_DETECTED error and shut down the interface to prevent a bridging loop.

The %STP-2-BLOCK_BPDUGUARD error message appears not when a unidirectional link failure exists, but when a BPDU has been received on an interface with BPDU guard enabled. When an interface that is configured with BPDU guard receives a BPDU, BPDU guard immediately puts the interface into the errdisable state and shuts down the interface. Afterward, the interface must be manually reenabled, or it can be recovered automatically through the errdisable timeout function.

The %STP-2-BLOCK_PVID_LOCAL and %STP-2-BLOCK_PVID_PEER error messages appear not when a unidirectional link failure exists, but when an interface has received a BPDU that is tagged with the same virtual LAN (VLAN) ID as the interface's native VLAN.Native VLAN BPDUs are sent untagged, so if a switch receives BPDUs that are tagged with the native VLAN for that interface, the local switch will record the % STP-2-BLOCK_PVID_LOCAL error message and block the interface? the remote switch will record the % STP2BLOCK_PVID_PEER error message.