Chapter 29 Configuring Qo - Cisco Catalyst 3550 series Software Configuration Manual

Understanding QoS
Understanding QoS
Typically, networks operate on a best-effort delivery basis, which means that all traffic has equal priority
and an equal chance of being delivered in a timely manner. When congestion occurs, all traffic has an
equal chance of being dropped.
When you configure QoS, you can select specific network traffic, prioritize it according to its relative
importance, and use congestion-management and congestion-avoidance techniques to give preferential
treatment. Implementing QoS in your network makes network performance more predictable and
bandwidth utilization more effective.
The QoS implementation is based on the DiffServ architecture, an emerging standard from the Internet
Engineering Task Force (IETF). This architecture specifies that each packet is classified upon entry into
the network. 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. These special bits in the Layer 2 frame or in the Layer 3 packet are described here
and shown in
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Catalyst 3550 Multilayer Switch Software Configuration Guide
29-2
Figure 29-1:
Prioritization bits in Layer 2 frames:
Layer 2 Inter-Switch Link (ISL) frame headers have a 1-byte User field that carries an IEEE 802.1P
class of service (CoS) value in the three least-significant bits. On interfaces configured as Layer 2
ISL trunks, all traffic is in ISL frames.
Layer 2 802.1Q 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 interfaces configured
as Layer 2 802.1Q trunks, all traffic is in 802.1Q 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.
Prioritization bits in Layer 3 packets:
Layer 3 IP packets can carry either an IP precedence value or a Differentiated Services Code Point
(DSCP) value. QoS supports the use of either value because DSCP values are backward-compatible
with IP precedence values.
IP precedence values range from 0 to 7.
DSCP values range from 0 to 63.
Chapter 29 Configuring QoS
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