This value sets the clock mode for this HDLC interface, applicable only when layered directly over a High Speed Serial Interface (HSSI); it defaults to hdlcClockUnsupported(0) for non-HSSI lower interfaces or hdlcClockLine(2) for HSSI ones if not explicitly configured. Admins set it to control whether the interface derives timing from the line or generates it internally, relevant when connecting DTE/DCE equipment over HSSI. Choosing a clock mode other than the HSSI default would be necessary when this device needs to act as the clock source rather than follow the line's timing.