  The radio light curve and spectral evolution of the blazar CTA 102 during its
2006 outburst can be rather well explained by the standard shock-in-jet model.
The results of a pixel-to-pixel spectral analysis of multi-frequency VLBI
images, together with kinematics derived from the MOJAVE survey lead to the
picture of an over-pressured jet with respect to the ambient medium. The
interaction of a traveling shock wave with a standing one is a possible
scenario which could explain the observed spectral behaviour
