The invention relates to a method to discover Optical Network Units (ONU) with different speed capabilities in an Ethernet Passive Optical Network (EPON) by an Optical Line Termination (OLT), where logical links in the network are identified by Logical Link IDs, and the OLT can transmit and receive data with 1 Gbit/s-speed (1 G) and 10 Gbit/s-speed (10 G), and the discovery process is handled in the OLT by a Multi Point Control Process (MPCP), which exchanges messages as Data Units (MPCPDU) with the ONUs. This is achieved by extending the standard MPCPDUs in a way, that backward compatability is assured.