The shock is considered that develops, when two plasma clouds collide at the speed 0.9 c. Initially, an almost flow-aligned magnetic field is introduced, which decreases the growth rates of the oblique mixed mode instability and of the filamentation instability. A 2D PIC simulation demonstrates that a planar, electromagnetic wave structure is growing that amplifies the magnetic field component orthogonal to the flow velocity vector. The formation of the forward and reverse shocks is followed with a 1D PIC simulation and it is shown that an energy equi-partition is established downstream between the ions, the electrons and the magnetic field.