  The properties of collisionless shocks are frequently assessed in the
magnetohydrodynamics (MHD) model. Yet, in a collisionless plasma, an ambient
magnetic field can sustain a stable anisotropy in the upstream or the
downstream, resulting in a departure from the MHD predicted behavior. We
present a model allowing to derive the downstream anisotropy, hence the shock
density jump, in terms of the upstream quantities. For simplicity, the case of
a parallel shock in pair plasma is considered. Contrary to previous works where
the upstream was assumed isotropic, here the upstream anisotropy
$A=T_\perp/T_\parallel$ is a free parameter. The strong sonic shock regime is
formally identical to the isotropic upstream case. Yet, for intermediate sonic
Mach numbers, a variety of behaviors appear as a result of the anisotropy of
the upstream.
