The generic reason code why this circuit was last disconnected,
as reported by the DLSw partner in a HALT_DL or HALT_DL_NOACK.
If the partner does not send a reason code in these messages, or
the DLSw implementation does not report receiving one, the value
-unknown- is returned.
This object is present only if the implementation keeps circuit
table entries around for some period after circuit disconnect.