By means of Monte-Carlo modeling of thermonuclear (TN) burn wave propagation in isochoric spherical laser deuterium-tritium targets criteria of fast ignition are elaborated and corresponding energy gain is evaluated. It is shown that the target thermonuclear (TN) burning is independent on ignitor presence if the ignitor areal density and temperature are lower than the critical ones. In the opposite case TN flash results in effective burning with the gain G∼10³, and the TN energy release is practically independent on the further increase of ignitor parameters. The critical ignitor parameters are calculated for targets with different parameters of main fuel The overcritical target gain is practically independent on ignition origin and may be evaluated with a good accuracy by the simple asymptotic expression. It is shown that the energy coupled to ignitor with critical ignitor parameters tends to some minimum value in the limit of small ignitor mass. The corresponding values of energy are obtained as a function of the temperature of the main fuel. Under these optimum ignition conditions the parameters of extra laser pulse are evaluated taking into account the fast ions energy transport mechanism.