When loop protection is enabled on an interface, what happens when the port stops receiving BPDUs?

A.
The port is placed in a loop-inconsistent role.

B.
The port is placed into listening mode.

C.
The port is transitioned into a forwarding state.

D.
The interface is disabled.

Explanation:
When loop protection is enabled, the spanning-tree topology detects root ports and blocked ports and makes sure both keep receiving BPDUs. If a loop-protection-enabled interface stops receiving BPDUs from its designated port, it reacts as it would react to a problem with the physical connection on this interface. It doesn’t transition the interface to a forwarding state, but instead transitions it to a loop-inconsistent state. The interface recovers and then it transitions back to the spanning-tree blocking state as soon as it receives a BPDU.

We recommend that you enable loop protection on all switch interfaces that have a chance of becoming root or designated ports.
Loop protection is most effective when enabled in the entire switched network. When you enable loop protection, you must configure at least one action (alarm, block, or both).

An interface can be configured for either loop protection or root protection, but not for both.
user@switch> show spanning-tree interface
Spanning tree interface parameters for instance 0

Interface Port ID Designated Designated Port State Role
port ID bridge ID Cost
ge-0/0/0.0 128:513 128:513 32768.0019e2503f00 20000 BLK DIS
ge-0/0/1.0 128:514 128:514 32768.0019e2503f00 20000 BLK DIS
ge-0/0/2.0 128:515 128:515 32768.0019e2503f00 20000 BLK DIS
ge-0/0/3.0 128:516 128:516 32768.0019e2503f00 20000 FWD DESG
ge-0/0/4.0 128:517 128:517 32768.0019e2503f00 20000 FWD DESG
ge-0/0/5.0 128:518 128:2 16384.00aabbcc0348 20000 BLK ALT
ge-0/0/6.0 128:519 128:1 16384.00aabbcc0348 20000 FWD ROOT
ge-0/0/7.0 128:520 128:520 32768.0019e2503f00 20000 BLK DIS (RootIncon)
[output truncated]

Meaning

The operational mode command show spanning-tree interface shows that interface ge-0/0/7.0 has transitioned to a loop inconsistent state. The loop inconsistent state makes the interface block and prevents the interface from becoming a candidate for the root port. When the root bridge no longer receives superior STP BPDUs from the interface, the interface will recover and transition back to a forwarding state. Recovery is automatic.