Network unit for use in a cascade system

A multi-port network unit for use in a cascade system of network units sends from a cascade port a packet including a special header having a source port ID, a destination port ID and a 'destination port known' field. The port IDs identify both a unit and a port within a unit. A routing database (25) is set up, optionally by a discovery protocol, in terms of ports and either destination units (within the cascade) or source units (within the cascade). The database includes a mesh table (25a), indicating from which cascade port a packet with a known destination port ID should be forwarded, without needing a fresh look-up. The database also includes a multicast exclusion table (25b) which allows ingress of packets with source unit IDs matched to an ingress port. The scheme allows a general mesh type of cascade while dynamically preventing closed loops.