Both NetScaler HA nodes status NOT_UP after Primary HA-MON enabled interface down

There is one direct cable connectedbetween Primary node and Secondary node, The cable connection is not via switch. Both interfaces are HA MON enabled. Both NetScaler HA nodes status turn to NOTUP after Primary HA-MON enabled interface is down...

NetScaler HA interface report "No HA heartbeats"

The customer saw "No HA heartbeats" on two NetScaler HA nodes' interfaces...