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29-4
Catalyst 2960 Switch Software Configuration Guide
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Chapter 29 Configuring QoS
Understanding QoS
Figure 29-2 shows the basic QoS model. Actions at the ingress port include classifying traffic, policing,
marking, queueing, and scheduling:
• Classifying a distinct path for a packet by associating it with a QoS label. The switch maps the CoS
or DSCP in the packet to a QoS label to distinguish one kind of traffic from another. The QoS label
that is generated identifies all future QoS actions to be performed on this packet. For more
information, see the “Classification” section on page 29-5.
• Policing determines whether a packet is in or out of profile by comparing the rate of the incoming
traffic to the configured policer. The policer limits the bandwidth consumed by a flow of traffic. The
result is passed to the marker. For more information, see the “Policing and Marking” section on
page 29-8.
• Marking evaluates the policer and configuration information for the action to be taken when a packet
is out of profile and determines what to do with the packet (pass through a packet without
modification, mark down the QoS label in the packet, or drop the packet). For more information, see
the “Policing and Marking” section on page 29-8.
• Queueing evaluates the QoS label and the corresponding DSCP or CoS value to select into which of
the two ingress queues to place a packet. Queueing is enhanced with the weighted tail-drop (WTD)
algorithm, a congestion-avoidance mechanism. If the threshold is exceeded, the packet is dropped.
For more information, see the “Queueing and Scheduling Overview” section on page 29-11.
• Scheduling services the queues based on their configured shaped round robin (SRR) weights. One
of the ingress queues is the priority queue, and SRR services it for its configured share before
servicing the other queue. For more information, see the “SRR Shaping and Sharing” section on
page 29-12.
Actions at the egress port include queueing and scheduling:
• Queueing evaluates the QoS packet label and the corresponding DSCP or CoS value before selecting
which of the four egress queues to use. Because congestion can occur when multiple receiving ports
simultaneously send data to an egress port, WTD differentiates traffic classes and subjects the
packets to different thresholds based on the QoS label. If the threshold is exceeded, the packet is
dropped. For more information, see the “Queueing and Scheduling Overview” section on
page 29-11.
• Scheduling services the four egress queues based on their configured SRR shared or shaped weights.
One of the queues (queue 1) can be the expedited queue, which is serviced until empty before the
other queues are serviced.
Figure 29-2 Basic QoS Model
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Classification Policing
Generate
QoS label
Actions at ingress Actions at egress
Mark
In profile or
out of profile
Inspect packet
and determine
the QoS label
based on ACLs
or the
configuration.
Compare the
incoming traffic
rate with the
configured policer
and determine if
the packet is in
profile or out of
profile.
Based on whether
the packet is in or
out of profile and
the configured
parameters,
determine whether
to pass through,
mark down, or
drop the packet.
Queueing and
scheduling
Based on the QoS
label, determine
into which of the
egress queues to
place the packet.
Then service the
queues according
to the configured
weights.
Based on the QoS
label, determine
into which of the
ingress queues to
place the packet.
Then service the
queues according
to the configured
weights.
Queueing and
scheduling