  We focus on the exceptional flaring activity of 3C 454.3 in November 2010 and
we discuss a theoretical framework addressing all data in their overall
evolution.
  For two weeks the source has shown a plateau of enhanced GeV emission
preceding a sudden major flare lasting about 3 days before decaying. The
gamma-ray flare onset is abrupt (about 6 hours), and is characterized by a
prominent "Compton dominance" with the GeV flux exceeding the pre-flare values
by a factor of 4-5, whereas the optical and X-ray fluxes increased only by a
factor 2.
  We explore two alternatives. Case 1, with high-energy emission originating
within the BLR; and Case 2, with most of it produced outside. We show that Case
1 has considerable problems in explaining the whole set of multifrequency data.
Case 2, instead, leads to a consistent and interesting interpretation based on
the enhanced inverse Compton radiation that is produced as the jet crashes onto
a mirror cloud positioned at few parsec from the BH. This model explains the
gamma-ray vs. optical/X-ray behavior of 3C 454.3, including the otherwise
puzzling phenomena such as the prominent "orphan" optical flare, and the
enhanced line emission with no appreciable gamma-ray counterpart that preceded
the GeV flare. It also accounts for the delayed onset of the latter on top of
the long plateau. Our modelling of the exceptional 3C 454.3 gamma-ray flare
shows that, while emission inside the canonical BLR is problematic, major and
rapid variations can be produced at parsec scales with moderate bulk Lorentz
factors $\Gamma\approx 15$.
