Using the conventional OMCI-model for handling stacking of additional ONUs to a main ONU (Optical Network Unit) connected to an OLT (Optical Line Terminal) in a PON (Passive Optical Network), when the main ONU is not provided with any chassis with physical slots for insertion of the additional units. Instead, the main ONU is initialized to be provided with a virtual chassis that includes a number of temporary virtual cardholders for assigning to an unknown connected stacked ONU, and a number of non-temporary virtual cardholders for assigning to a known stacked connected ONU. From a management perspective, the virtual cardholder corresponds to a physical cardholder/slot according to the conventional OMCI-model.