Indicates the current loopback state of this interface.
The interface can be in one the following states:
Line Loopback
Payload Loopback
no loopback
Loopback modes can be initiated by either of two methods:
Manager requested Loopback is initiated by the Manager of
this device.
Network requested Loopback is initiated by this interface
receiving an in-band request on the network from the far
end device.