  We study proximity of the Kepler-25 planetary system to a periodic
configuration, which is known to be the final state of a system that undergoes
smooth migration resulting from the planet-disc interaction. We show that the
system is close to the periodic configuration of 2:1 mean motion resonance
(MMR) what indicates that its past migration was neither disturbed
significantly by turbulence in the disc nor the orbits were perturbed by
planetesimals that left after the disc dispersal. We show that, because of the
TTV model degeneracy, a periodic configuration is difficult to be found when
the standard modelling of the transit timing variations (TTVs) is used. The TTV
signal of a periodic configuration (with anti-aligned apsidal lines) may be
misinterpreted as an aligned non-resonant system. We demonstrate that the
standard MCMC modelling of the Kepler-25 TTVs is very sensitive to an a~priori
information on the eccentricities (prior probability distributions). Wide
priors (of the order of the ones typically used in the literature) result in
favouring the aligned non-resonant configurations with small planets' masses
and moderate eccentricities, while for the narrower priors the most likely are
the anti-aligned resonant systems with larger masses and very low
eccentricities.
