We experimentally investigate charge carrier transport in a graphene p-n junction device by using independent p-type and n-type electrostatic gating which allow full characterization of the junction interface in the quantum Hall regime covering awide range of filling factors [-10 <= (v(1), v(2)) <= 10]. Recent charge transportmeasurements across a graphene p-n junction in this quantized regime presume that equilibration of all of the Landauer-Buttiker edge states occurs across the p-n junction interface. Here we show that, in our devices, only the edge state associated with the lowest Landau level fully equilibrate across the p-n junction, while none of the other edge states equilibrate to transmit current across the junction.