  We present a geometric multigrid solver for the Compact Discontinuous
Galerkin method through building a hierarchy of coarser meshes using a simple
agglomeration method which handles arbitrary element shapes and dimensions. The
method is easily extendable to other discontinuous Galerkin discretizations,
including the Local DG method and the Interior Penalty method. We demonstrate
excellent solver performance for Poisson's equation, provided a flux
formulation is used for the operator coarsening and a suitable switch function
chosen for the numerical fluxes.
