Port congestion notification in a switch

摘要

A congestion notification mechanism provides a congestion status for all destinations in a switch at each ingress port. Data is stored in a memory subsystem queue associated with the destination port at the ingress side of the crossbar. A cell credit manager tracks the amount of data in this memory subsystem for each destination. If the count for any destination exceeds a threshold, the credit manager sends an XOFF signal to the XOFF masks. A lookup table in the XOFF masks maintains the status for every switch destination based on the XOFF signals. An XON history register receives the XOFF signals to allow queuing procedures that do not allow a status change to XON during certain states. Flow control signals directly from the memory subsystem are allowed to flow to each XOFF mask, where they are combined with the lookup table status to provide a congestion status for every destination.