User Manual

Configure Quality of Service
511
M6100, M5300, and M7100 Series Managed Switches
QoS Overview
In a typical switch, each physical port consists of 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 cannot be held for transmission and get
dropped by the switch.
QoS is a means of providing consistent, predictable data delivery by distinguishing between
packets with strict timing requirements from those that are more tolerant of delay. Packets
with strict timing requirements are given special treatment in a QoS-capable network. With
this in mind, all elements of the network must be QoS capable. The presence of at least one
node that is not QoS capable creates a deficiency in the network path and the performance of
the entire packet flow is compromised.
Class of Service
The Class of Service (CoS) queueing feature lets you directly configure certain aspects of
switch queueing. This provides 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 that affect queue mapping, such as minimum
guaranteed bandwidth or transmission rate shaping,are user-configurable at the queue (or
port) level.
Eight queues per port are supported.
Use CoS to set the Class of Service trust mode of an interface. Each port in the switch can be
configured to trust one of the packet fields (802.1p or IP DSCP), or to not trust any packet’s
priority designation (untrusted mode). If the port is set to a trusted mode, it uses a mapping
table appropriate for the trusted field being used. This mapping table indicates the CoS
queue to which the packet is forwarded on the appropriate egress ports. Of course, the
trusted field must exist in the packet for the mapping table to be of any use, so there are
default actions performed when this is not the case. These actions involve directing the
packet to a specific CoS level configured for the ingress port as a whole, based on the
existing port default priority as mapped to a traffic class by the current 802.1p mapping table.
Alternatively, when a port is configured as untrusted, it does not trust any incoming packet
priority designation and uses the port default priority value instead. All packets arriving at the
ingress of an untrusted port are directed to a specific CoS queue on the appropriate egress
port(s), in accordance with the configured default priority of the ingress port. This process is
also used for cases where a trusted port mapping cannot be honored, such as when a non-IP
packet arrives at a port configured to trust the IP DSCP value.