The influence of reflections on the operation of the 2MW, CW 170 GHz coaxial cavity gyrotron oscillating in the TE34, 19- mode is studied. In particular, frequency-independent power reflections, e.g. from plasma or some elements of the transmission line, larger than 1% and occurring in a poor phase may lead to oscillation breakdown and should be avoided. Frequency-dependent reflections from a specially designed diamond window influence mode competition, but contract the oscillation region of the operating mode noticeably only under the unrealistic assumption that all reflected power returns to the cavity. The conclusion is that the gyrotron is sufficiently robust against possible negative effects related to reflections.