This controls what happens when the peer hasn't negotiated a magic number yet but sends a null or invalid one in an LCP Echo packet: ignore the mismatch and keep the connection, or treat it as an error and terminate the link. Admins enable it for interoperability with peer implementations that handle magic numbers loosely during early negotiation. For example, turning this on can stop a link from being torn down against an older client that briefly echoes a zero magic number right after coming up.