Cascade system for network units

A network unit which is capable of use in a cascade stack of network units and includes:

• a multiplicity of ports for receiving and forwarding addressed data packets;
• a switching engine for forwarding received packets to at least one port in accordance with address data in the packets;
• and
• at least two cascade ports for connection to other network units in the stack,
• wherein said switching engine provides a normal mode wherein packets received at any of the cascade ports are forwarded from the same port in the same direction of progress and a loop-back mode wherein a packet received at a cascade port is forwarded from a different cascade port in a different direction of progress.

A network unit which is capable of use in a cascaded stack of network units and includes:

• a multiplicity of ports for receiving and forwarding addressed data packets, a switching engine for directing received packets to at least one port in accordance with address data in the packets;
• at least two cascade ports for receiving packets from and sending packets to adjacent network units in the cascaded stack;
• and
• means for detecting an operational failure between this network unit and an adjacent network unit in the stack and for controlling the switching engine to redirect packets which would otherwise be sent from a particular port to that adjacent network unit to be forwarded from another port whereby to be sent to a different network unit in the stack.

A network unit capable of use within a cascaded stack of network units, and including:

• a multiplicity of ports for receiving addressed data packets from and sending packets to an external network;
• a look-up database for relating address data in packets to forwarding data which includes an identification of at least one port and a switching engine which responds to forwarding data to direct packets to at least one corresponding port;
• and
• at least one cascade port for transmission of data packets between the unit and other units in the cascade stack;
• wherein the unit provides for each packet that it receives at one of the said multiplicity of ports and forwards from a cascade port a header field that indicates whether a destination port in the stack is known for the packet and a header field for identifying the destination port and the unit on which the destination port is located.

A network unit capable of use within a cascaded stack of network units, and including:

• a multiplicity of ports for receiving addressed data packets from and sending packets to an external network;
• a look-up database for relating address data in packets to forwarding data which includes an identification of at least one port;
• a switching engine which responds to forwarding data to direct packets to at least one corresponding port;
• and
• at least one cascade port for transmission of data packets between the unit and other units in the cascade stack;
• wherein the unit provides for each packet that it forwards from a cascade port a header field which has portions for denoting which of the network units of the stack have been and have not been traversed by the packet.