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 agent keeps circuit table
entries around for some period after circuit disconnect.