CHAPTER
27-1
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
27
Configuring Quality of Service
This chapter describes how to configure quality of service (QoS) by using automatic QoS (auto-QoS)
commands or by using standard QoS commands on a Catalyst 4500 series switch. It also describes how
to specify different QoS configurations on different VLANs on a given interface (per-port per-VLAN
QoS).
This chapter consists of these sections:
Overview of QoS, page 27-1
Configuring Auto-QoS, page 27-17
Configuring QoS, page 27-23
Note For complete syntax and usage information for the switch commands used in this chapter, refer to the
Catalyst 4500 Series Switch Cisco IOS Command Reference and related publications at
http://www.cisco.com/univercd/cc/td/doc/product/software/ios122/122cgcr/index.htm.

Overview of 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.
QoS selects network traffic (both unicast and multicast), prioritizes it according to its relative
importance, and uses congestion avoidance to provide priority-indexed treatment; QoS can also limit the
bandwidth used by network traffic. QoS can make network performance more predictable and bandwidth
utilization more effective.
This section contains the following subsections:
Prioritization, page 27-2
QoS Terminology, page 27-3
Basic QoS Model, page 27-5
Classification, page 27-6
Policing and Marking, page 27-10
Mapping Tables, page 27-14
Queueing and Scheduling, page 27-14