Packet header for cascade or stack which indicates which units have been visited

Network units for use in a cascaded system (fig.2) are organised to forward on the cascade packets having a header field which has portions (BBM) for denoting which of the units in the cascade system have been and have not been visited by the packet. The field is preferably a bit mask having a single respective bit for each unit in the cascade system. A packet may be discarded by a unit when the relevant bit indicated that the packet has already visited that unit. The packet may include a header field (DPK) which indicates whether a destination port within the cascade system is known for the packet and a network unit may perform a lookup (15, fig.1) for a destination port if the packet has not already visited the unit and the destination port is unknown. This header allows for a 'loop back' operation to heal the data path after unit failure.