The PTP clock in the router monitors the Sync, Follow_Up (if present), and Delay_Resp messages received from external neighbor ports. If a high variation is detected in the network path between the external neighbor port and the local port, that neighbor port is considered unusable (PTSF-unusable as defined in the ITU-T G.8275.1 recommendation). When a neighbor is unusable, all Announce messages from that neighbor are discarded on reception and excluded from the BMCA. If the neighbor is the parent clock to the local clock, the local clock must either select a new parent clock or go into holdover. In addition, any neighbor clock marked as unusable cannot act as the parent to the local PTP clock until underlying condition is investigated and resolved, and the unusable state is cleared. The unusable state is cleared when PTP, PTSF-unusable monitoring, or the local PTP port is administratively disabled, the PTP port is deleted, or the external neighbor port stops sending messages to the node. It can also be cleared by using the appropriate clear command.