  Context: The blazar CTA 102 (z=1.037) underwent a historical radio outburst
in April 2006. This event offered a unique chance to study the physical
properties of the jet. Aims: We used multifrequency radio and mm observations
to analyze the evolution of the spectral parameters during the flare as a test
of the shock-in-jet model under these extreme conditions. Methods: For the
analysis of the flare we took into account that the flaring spectrum is
superimposed on a quiescent spectrum. We reconstructed the latter from archival
data and fitted a synchrotron self-absorbed distribution of emission. The
uncertainties of the derived spectral parameters were calculated using Monte
Carlo simulations. The spectral evolution is modeled by the shock-in-jet model,
and the derived results are discussed in the context of a geometrical model
(varying viewing angle) and shock-shock interaction. Results: The evolution of
the flare in the turnover frequency-turnover flux density plane shows a double
peak structure. The nature of this evolution is dicussed in the frame of
shock-in-jet models. We discard the generation of the double peak structure in
the turnover frequency-turnover flux density plane purely based on geometrical
changes (variation of the Doppler factor). The detailed modeling of the
spectral evolution favors a shock-shock interaction as a possible physical
mechanism behind the deviations from the standard shock-in-jet model.
