Dell Networking 2048, Reference Manual

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Data Center Bridging Commands
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-drop-
precedence basis.
The Dell Networking 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
mapping table. CoS queue characteristics such as minimum guaranteed
bandwidth, transmission rate shaping, etc. are now user configurable at the
queue (or port) level.
The CoS queue feature provides a method to configure Traffic Class Groups
(TCGs) to extend the CoS queue management. Multiple CoS queues can be
mapped to a single TCG. Each TCG can have a configured minimum
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