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Cisco IE 2000 Switch Software Configuration Guide
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Chapter 38 Configuring Standard QoS
Information About Standard QoS
Information About Standard QoS
This chapter describes how to configure quality of service (QoS) by using automatic QoS (auto-QoS)
commands or by using standard QoS commands on the switch. With QoS, you can provide preferential
treatment to certain types of traffic at the expense of others. Without QoS, the switch offers best-effort
service to each packet, regardless of the packet contents or size. It se nds the packets without any
assurance of reliability, delay bounds, or throughput.
You can configure QoS on physical ports and on switch virtual interfaces (SVIs). Other tha n to apply
policy maps, you configure the QoS settings, such as classification, queueing, and scheduling, the same
way on physical ports and SVIs. When configuring QoS on a physical port, you apply a nonhierarchical
policy map to a port. When configuring QoS on an SVI, you apply a n onhierarchical or a hierarchical
policy map.
The switch supports some of the modular QoS CLI (MQC) commands. For m ore information about the
MQC commands, see the “Modular Quality of Service Command-Line Interface Overview” chapter of
the Cisco IOS Quality of Service Solutions Guide, Release 12.2.
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 the QoS feature, you can select specific network traffic, prioritize it according to
its relative importance, and use congestion-management and conge stion-avoidance techniques to
provide preferential treatment. Implementing QoS in your network makes network performance more
predictable and bandwidth utilization more effective.
The QoS implementation is based on the Differentiated Services (Diff-Serv) 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 a Layer 3 packet are described here and shown
in Figure 38-1:
• Prioritization bits in Layer 2 frames:
Layer 2 IEEE 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 ports configured
as Layer 2 IEEE 802.1Q trunks, all traffic is in IEEE 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 beca use DSCP values are backward-compatible
with IP precedence values.
IP precedence values range from 0 to 7.
DSCP values range from 0 to 63.