Basic Qos Model; Classification - Cisco Catalyst 2360 Software Configuration Manual

Understanding QoS
The classification is carried in the IP packet header, using 6 bits from the deprecated IP type of service
(ToS) field to carry the classification (class) information. Classification can also be carried in the
Layer 2 frame.
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All switches and routers that access the Internet rely on the class information to provide the same
forwarding treatment to packets with the same class information and different treatment to packets with
different class information. Detailed examination of the packet is expected to happen closer to the edge
of the network so that the core switches and routers are not overloaded with this task.
Switches and routers along the path can use the class information to limit the amount of resources
allocated per traffic class. The behavior of an individual device when handling traffic in the DiffServ
architecture is called per-hop behavior. If all devices along a path provide a consistent per-hop behavior,
you can construct an end-to-end QoS solution.
Implementing QoS in your network can be a simple or complex task and depends on the QoS features
offered by your internetworking devices, the traffic types and patterns in your network, and the
granularity of control that you need over incoming and outgoing traffic.

Basic QoS Model

To implement QoS, the switch must distinguish packets or flow from one another (classify), 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 (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).
Actions at the egress port include queueing and scheduling:
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Classification

Classification is the process of distinguishing one kind of traffic from another by examining the fields
in the packet. Classification is enabled only if QoS is globally enabled on the switch. By default, QoS is
globally disabled, so no classification occurs.
During classification, the switch performs a lookup and assigns a QoS label to the packet. The QoS label
identifies all QoS actions to be performed on the packet and from which queue the packet is sent.
The QoS label is based on the CoS value in the packet and decides the queueing and scheduling actions
to perform on the packet. The label is mapped according to the trust setting and the packet type.
For non-IP and IP traffic, you can configure the ports as trusted. Classification is performed on ingress
packets by using the packet CoS value. The default CoS value is 0 which means best-effort traffic. The
default port trust state is untrusted.
Catalyst 2360 Switch Software Configuration Guide
25-2
Prioritization bits in Layer 2 frames:
Layer 2 802.1p frame headers have a 2-byte Tag Control Information field that carries the CoS value
in the three most-significant bits, which are called the User Priority bits. On ports configured as
Layer 2 802.1p trunks, all traffic is in 802.1p frames except for traffic in the native VLAN.
Other frame types cannot carry Layer 2 CoS values.
Layer 2 CoS values range from 0 for low priority to 7 for high priority.
Queueing evaluates the QoS packet label and the corresponding CoS value before selecting which
of the four egress queues to use. For more information, see the
page 25-3.
Chapter 25 Configuring QoS
"Configuring QoS" section on
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