A loop-inconsistent state is a spanning-tree port condition triggered when Loop Guard stops receiving expected BPDUs. The port is disabled instead of moving from discarding to forwarding, preventing a Layer 2 loop. section 15.4.2
When BPDUs resume, the port automatically returns to its normal discarding state. Loop Guard and Root Guard cannot be enabled together on the same port because they respond to different BPDU conditions. section 15.4.2
Cisco status example
After enabling Loop Guard with spanning-tree guard loop, a port that stops receiving BPDUs reports LOOPGUARD_BLOCK. show spanning-tree displays status BKN and the marker LOOP_Inc. section 15.4.2
Figure 15.9 A loop is caused when SW3 stops receiving BPDUs from SW2. (1) A software malfunction on SW2 prevents it from sending BPDUs. (2) After the max age timer counts down, SW3’s port becomes a designated port and transitions to the forwarding state. (3) All three links between the switches are active, resulting in a Layer 2 loop.
Trace how a failed BPDU source, the max-age timer, and an unintended forwarding transition could activate all links and create a Layer 2 loop. section 15.4.2
