The interval at which NCDP configuration PDUs ('Hellos') are to be
generated.
The message interval directly affects the convergence time of
the NCDP algorithm. Convergence time is equal to
the max network diameter * message interval +
transmission delays +
the time a configuration PDU is spent being processed
in each device.
Thus if transmission delays and processing delays are
both close to 0, the convergence time is approximately
( max network diameter * message interval ) milliseconds.
This object is only used if the distribution method is 'ncdp'.