This indicates whether this circuit is enabled to run the 3-Way handshake. Enabling RFC 5303's three-way handshake for point-to-point adjacencies fixes a known weakness of the original two-way handshake, where both sides could believe an adjacency was up even when one side's Hello was not actually being received by the other, so leaving it disabled risks silently half-formed adjacencies going undetected. For example, enabling this on a point-to-point WAN circuit lets both routers exchange and confirm each other's extended circuit IDs during the handshake, catching a case where one side thinks the adjacency is up while the other has not actually received a valid Hello yet.