  We show that scattering-based interaction among nearly degenerate optical
modes is the key factor in low threshold generation of Kerr frequency combs in
nonlinear optical resonators possessing small group velocity dispersion (GVD).
The mode interaction is capable of producing drastic change in the local GVD,
resulting in either a significant reduction or increase of the oscillation
threshold. It is also responsible for the majority of observed combs in
resonators characterized with large normal GVD. We present results of our
numerical simulations as well as supporting experimental data.
