  Thermalization of a longitudinally expanding color glass condensate with
Bjorken boost invariant geometry is investigated within microscopical parton
cascade BAMPS. Our main focus lies on the detailed comparison of
thermalization, observed in BAMPS with that suggested in the \BUp scenario. We
demonstrate that the tremendous production of soft gluons via $gg \to ggg$,
which is shown in the \BUp picture as the dominant process during the early
preequilibration, will not occur in heavy ion collisions at RHIC and LHC
energies, because the back reaction $ggg\to gg$ hinders the absolute particle
multiplication. Moreover, different from the \BUp scenario, soft and hard
gluons thermalize at the same time. The time scale of thermal equilibration
obtained from BAMPS calculations is of order $\as^{-2} (\ln \as)^{-2}
Q_s^{-1}$. After this time the gluon system exhibits nearly hydrodynamical
behavior. The shear viscosity to entropy density ratio has weak dependence on
$Q_s$ and lies close to the lower bound from the AdS/CFT conjecture.
