Cisco Systems 2960

29-4

Catalyst 2960 Switch SoftwareConfiguration Guide

78-16881-01

Chapter29 Co nfiguring 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 a nother. 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 page29-5.

•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 shap ed 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 O verview” 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

86682

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.