C H A P T E R
20-1
Catalyst 3550 Multilayer Switch Software Configuration Guide
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20
Configuring QoS
This chapter describes how to configure quality of service (QoS) on your switch. With this feature, you
can provide preferential treatment to certain 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 sends the packets
without any assurance of reliability, delay bounds, or throughput.
Note
For complete syntax and usage information for the commands used in this chapter, refer to the
Catalyst 3550 Multilayer Switch Command Reference for this release.
This chapter consists of these sections:
•
Understanding QoS, page 20-1
•
Configuring QoS, page 20-18
•
Displaying QoS Information, page 20-56
•
QoS Configuration Examples, page 20-56
Note
When you are configuring QoS parameters for the switch, in order to allocate system resources to
maximize the number of possible QoS access control entries (ACEs) allowed, you can use the sdm
prefer access global configuration command to set the Switch Database Management feature to the
access template. For more information on the SDM templates, see the
“Optimizing System Resources
for User-Selected Features” section on page 6-57
.
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 the QoS feature, you can select specific network traffic, prioritize it according to
its relative importance, and use congestion-management and congestion-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 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
