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Cisco Systems IntelligentGigabit Ethernet Switch Modules for the IBMBladeCenter, Software Configuration Guide
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Chapter23 Co nfiguring QoS
Understanding QoS
Displaying Standard QoS Information, page 23-36
Standard QoS Configuration Examples, page 23-36
Understanding QoS
This section describes how QoS is implemented on the switch. For a list of available features, see
Table23-1 on pag e 23-1.
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 give
preferential treatment. Implementing QoS in your network makes network performance mor e
predictable and bandwidth utilization more effective.
The QoS implementation is based on the DiffServ archit ecture, 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 23-1:
Prioritization values in Layer 2 frames
Layer 2 IEEE 802.1Q frame headers have a 2-byte Tag Control Information field that carries the
class of service (CoS) value in the three most-significant bits, which are called the User Priority bits.
On interfaces configured as Layer 2 IEEE 802.1Q trunks, all tra ffic 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 a Differentiated Services Code Point (DSCP) value. The supported
DSCP values are 0, 8, 10, 16, 18, 24, 26, 32, 34, 40, 46, 48, and 56.