  In this work, we investigate the impact of uncertainties due to convective
boundary mixing (CBM), commonly called `overshoot', namely the boundary
location and the amount of mixing at the convective boundary, on stellar
structure and evolution. For this we calculated two grids of stellar evolution
models with the MESA code, each with the Ledoux and the Schwarzschild boundary
criterion, and vary the amount of CBM. We calculate each grid with the initial
masses $15$, $20$ and $25\,\rm{M}_\odot$. We present the stellar structure of
the models during the hydrogen and helium burning phases. In the latter, we
examine the impact on the nucleosynthesis. We find a broadening of the
main-sequence with more CBM, which is more in agreement with observations.
Furthermore during the core hydrogen burning phase there is a convergence of
the convective boundary location due to CBM. The uncertainties of the
intermediate convective zone remove this convergence. The behaviour of this
convective zone strongly affects the surface evolution of the model, i.e. how
fast it evolves red-wards. The amount of CBM impacts the size of the convective
cores and the nucleosynthesis, e.g. the $^{12}$C to $^{16}$O ratio and the weak
s-process. Lastly, we determine the uncertainty that the range of parameter
values investigated introduce and we find differences of up to $70\%$ for the
core masses and the total mass of the star.
