Basic Qos Model - Cisco Catalyst 3750 Software Configuration Manual

Understanding QoS

Basic QoS Model

To implement QoS, the switch must distinguish (classify) packets or flows from one another, assign a
label to indicate the given quality of service as the packets move through the switch, make the packets
comply with the configured resource usage limits (police and mark), and provide different treatment
(queue and schedule) in all situations where resource contention exists. The switch also needs to ensure
that traffic sent from it meets a specific traffic profile (shape).
Figure 26-2
Figure 26-2 Basic QoS Model
Generate
QoS label
Classification
Inspect packet
and determine
the QoS label
based on ACLs
or the
configuration.
Classification
Inspect packet,
and process the
match
commands. The
hierarchical
configuration
determines the
number of queues
to create.
These are the actions when traffic is received by the switch:
•
•
Catalyst 3750 Metro Switch Software Configuration Guide
26-4
shows the basic QoS model.
Actions at ingress
In profile or
out of profile
Policing
Compare the Based on whether
incoming traffic the packet is in or
rate with the out of profile and
configured policer the configured
and determine if parameters,
the packet is in determine whether
profile or out of to pass through,
profile. mark down, or
drop the packet.
In profile or
out of profile
Policing
Compare received Based on whether
traffic rate with the the packet is in or
configured policer out of profile and
and determine if the configured
the packet is in parameters,
profile or out of determine whether
profile. to pass through,
mark down, or
drop the packet.
Classification is the process of generating 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
page 26-6.
Policing decides whether a packet is in or out of profile by comparing the rate of the inbound 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
page 26-9.
Queueing and
Mark
scheduling
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
Mark
scheduling
Based on the
hierarchical QoS
configuration,
create queues for
classes, VLANs,
and physical
interfaces. Place
the packet into the
queue and service
it according to the
configuration.
"Ingress Policing and Marking" section on
Chapter 26
Actions at egress
Traffic sent to
a standard port
Queueing and
scheduling
Based on the QoS
label, determine
into which queue-
set to place the
packet. Then
service the queues
according to the
configured
weights.
Traffic sent to ES port
"Ingress Classification" section on
Configuring QoS
78-15870-01