Dell PowerConnect 8100 CLI Reference Guide - Page 870

Enhanced Transmission Selection NOTE: Enhanced Transmission Selection commands are only supported on PC81xx series switches. CLI commands and Dell OpenManage Switch Administrator pages are not available for other switch models. Overview In a typical switch or router, each physical port supports one or more queues for transmitting packets on the attached network. Multiple queues per port are often provided to give preference to certain packets over others based on user-defined criteria. When a packet is queued for transmission in a port, the rate at which it is serviced depends on how the queue is configured and possibly the amount of traffic present in the other queues of the port. If a delay is necessary, packets get held in the queue until the scheduler authorizes the queue for transmission. As queues become full, packets have no place to be held for transmission and get dropped by the device. The drop precedence of a packet is an indication of whether the packet is more or less likely to be dropped during times of queue congestion. Often referred to as packet coloring, a low drop precedence (green) allows the packet to be transmitted under most circumstances, a higher drop precedence (yellow) subjects the packet to dropping when bursts become excessive, while the highest drop precedence (red) discards the packet whenever the queue is congested. In some hardware implementations, the queue depth can be managed using tail dropping or a weighted random early discard, or a weighted random early discard (WRED), technique. These methods often use customizable threshold parameters that are specified on a per-dropprecedence basis. The PowerConnect QoS implementation contains Differentiated Services (DiffServ) support that allows traffic to be classified into streams and given certain QOS treatment in accordance with defined per-hop behaviors. However, the DiffServ feature does not offer direct configuration of the hardware CoS queue resources. The CoS Queuing feature offers a new capability for the user to directly configure certain aspects of device queuing to provide the desired QOS behavior for different types of network traffic when the complexities of DiffServ are not required. The priority of a packet arriving at an interface can be used to steer the packet to the appropriate outbound CoS queue through a 870 Data Center Bridging Commands