Loop Guard is an added logic to receiving BPDUs on Root and Alternate ports with point to point links. These ports could move from root or Alternate to Designated possibly creating a switching loop. Loop Guard presumes that after BPDUs are received on Root and Alternate ports, it is not possible for the ports to suddenly stop receiving BPDUs without them actually going down. A loss of BPDUs on Root and Alternate ports means that a unidirectional link condition might be occurring.
Loop Guard prevents Root and alternate ports from becoming Designated if loss of incoming BPDUs occurs. If this happens and the BPDUs stored on the interfaces expire, Loop Guard will put them into a Loop inconsistent blocking state, they’ll come out of this state once BPDUs begin coming in again.
Loop guard can be activated globally or on a per port basis and doesn’t require other switches to be configured with Loop Guard in order for it to work. When activated globally it automatically protects Root and Alternate Ports on STP Point to point link types. It does not protect shared link types.