Indicates whether the particular virtual circuit is
operational. In the absence of a Data Link Connection
Management Interface, virtual circuit entries (rows)
may be created by setting virtual circuit state to
'active', or deleted by changing Circuit state to
'invalid'.
Whether or not the row actually disappears is left to
the implementation, so this object may actually read as
'invalid' for some arbitrary length of time. It is
also legal to set the state of a virtual circuit to
'inactive' to temporarily disable a given circuit.
The use of 'invalid' is deprecated in this SNMP Version
2 MIB, in favor of usdFrCircuitRowStatus.