  A broadband study of the high redshift blazar S5 0836+71 (z = 2.172) is
presented. Multi-frequency light curves show multiple episodes of X-ray and
$\gamma$-ray flares, while optical-UV fluxes show little variations. During the
GeV outburst, the highest $\gamma$-ray flux measured is (5.22 $\pm$ 1.10)
$\times$ 10$^{-6}$ ph cm$^{-2}$ s$^{-1}$ in the range of 0.1-300 GeV, which
corresponds to an isotropic $\gamma$-ray luminosity of (1.62 $\pm$ 0.44)
$\times$ 10$^{50}$ erg s$^{-1}$, thereby making this as one of the most
luminous $\gamma$-ray flare ever observed from any blazar. A fast $\gamma$-ray
flux rising time of $\sim$3 hours is also noticed which is probably the first
measurement of hour scale variability detected from a high redshift (z > 2)
blazar. The various activity states of S5 0836+71 are reproduced under the
assumption of single zone leptonic emission model. In all the states, the
emission region is located inside the broad line region, and the optical-UV
radiation is dominated by the accretion disk emission. The modeling parameters
suggests the enhancement in bulk Lorentz factor as a primary cause of the
$\gamma$-ray flare. The high X-ray activity with less variable $\gamma$-ray
counterpart can be due to emission region to be located relatively closer to
the black hole where the dominating energy density of the disk emission results
in higher X-ray flux due to inverse-Compton scattering of disk photons.
