  We analyze spin density wave (SDW) order in iron-based superconductors and
electronic structure in the SDW phase. We consider an itinerant model for
Fe-pnictides with two hole bands centered at $(0,0)$ and two electron bands
centered at $(0,\pi)$ and $(\pi,0)$ in the unfolded BZ. A SDW order in such a
model is generally a combination of two components with momenta $(0,\pi)$ and
$(\pi,0)$, both yield $(\pi,\pi)$ order in the folded zone. Neutron
experiments, however, indicate that only one component is present. We show that
$(0,\pi)$ or $(\pi,0)$ order is selected if we assume that only one hole band
is involved in the SDW mixing with electron bands. A SDW order in such 3-band
model is highly degenerate for a perfect nesting and hole-electron interaction
only, but we show that ellipticity of electron pockets and interactions between
electron bands break the degeneracy and favor the desired $(0,\pi)$ or
$(\pi,0)$ order. We further show that stripe-ordered system remains a metal for
arbitrary coupling. We analyze electronic structure for parameters relevant to
the pnictides and argue that the resulting electronic structure is in good
agreement with ARPES experiments. We discuss the differences between our model
and $J_1-J_2$ model of localized spins.
