Method for multi-priority, multicast flow control in a packet switch

In a data communications system having a switch fabric and one or more port cards, each port card having associated therewith at least one logical address and a physical address, and having, or coupled to, a buffer, the buffer adapted for coupling to, or being integrated into, the switch fabric, the buffer having a buffer occupancy defined as the number of memory locations in the buffer is used for storing incoming data as compared to the total number of memory locations in the buffer, a method of controlling multipriority data flow through the buffer, the multipriority data comprising a plurality of data types including a first data type having a first priority and a second data type having a second priority lower than the first priority, the first data type including one or more first-priority bit streams, and the second data type including one or more second-priority bit streams, the method comprising the steps of:

• (a) monitoring the occupancy of the buffer on the port card;
• (b) if the occupancy exceeds a second threshold greater than a first threshold, then disabling transmission of all second-priority bit streams destined for at least one corresponding logical address of the port card until the buffer occupancy drops below the first threshold;
• (c) if the buffer occupancy drops below the first threshold, enabling transmission of all second-priority bit streams destined for the logical address of the port card previously under flow control;
• (d) if the buffer occupancy exceeds a fourth threshold greater than a third threshold, then disabling transmission of all first-priority bit streams destined for at least one corresponding logical address of the port card until the buffer occupancy drops below the third threshold;
• (e) if the buffer occupancy drops below the third threshold, enabling transmission of all first-priority bit streams destined to the logical address of the port card previously under flow control;
• (f) determining whether the destination logical address of a received bit stream received by the port card is a multicast address;
• (g) maintaining a list at the port card of all flow controlled bit streams whose destination logical address is a multicast address;
• (h) disabling the transmission of all bit streams destined to the physical address of a port card having a buffer occupancy greater than a fifth threshold;
• (i) if the buffer occupancy drops below the third threshold, enabling transmission of all first-priority bit streams destined to the physical address of the port card;
• and
• (j) the buffer occupancy drops below the first threshold, enabling transmission of all second-priority bit streams destined to the physical address of the port card.