Spanning tree loop guard

Spanning tree loop guard feature says that when a switch is sending but not receiving BPDUs on the interface, LoopGuard will place the interface in the loop-inconsistent state and block all traffic.

Sometimes this description may be confusing because we know that in classic spanning tree, only root bridge can generate BPDUs and the other bridge can only relay these BPDUs, in another words, root ports and blocked ports will not send BPDUs. Blocked ports will not send BPDUs in rapid spanning tree neither, although in RSTP all bridges will generate and send its own BPDU to peer switches via designated or root port.

To understand this we must realize during spanning tree convergence period ‘root bridge, selection, root port selection, etc’, each switch DOES send out and receive BPDUs from peer side. Therefore loop-guard will be in action during convergence period. In some cases convergence does not necessarily need reselect root bridge or root port on the switch, but it is convergence process on this specific blocked port, it will go through the whole convergence period (blocking,listening, learning, forwarding)