By means of Monte-Carlo modeling of thermonuclear (TN) burn wave propagation in spherical laser deuterium-tritium targets criteria of fast ignition are elaborated and corresponding energy gain is evaluated. The critical ignitor parameters are calculated both for homogeneous and inhomogeneous targets with different parameters of main fuel. It is shown that in strong inhomogeneous target plasma the minimum values of required ignition energy could increase twice. Besides it is shown that critical values of ignitor dimension and energy are dependent on different distribution of energy between the electrons and ions of ignitor plasma. If all the additional thermal energy is coupled to ignitor electrons the value of corresponding ignition energy is 3+4 times as many as in the case of equal initial temperature of ignitor ions and electrons. The overcritical burn efficiency and target gain is practically independent on ignition origin and may be evaluated with a good accuracy by the simple asymptotic expression.