37-2
Software Configuration Guide—Release 15.0(2)SG
OL-23818-01
Chapter 37 Configuring Quality of Service
About QoS
Note For complete syntax and usage information for the switch commands used in this chapter, see the Cisco
Catalyst 4500 Series Switch Command Reference and related publications at this location:
http://www.cisco.com/en/US/products/hw/switches/ps4324/index.html
If the command is not found in the Cisco Catalyst 4500 Command Reference, you can locate it in the
larger Cisco IOS library. Refer to the Cisco IOS Command Reference and related publications at this
location:
http://www.cisco.com/en/US/products/ps6350/index.html
About QoS
Typically, networks operate on a best-effort delivery basis, which means that al l traf fic 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 includes the following subsections:
Prioritization, page 37-2
QoS Terminology, page 37-4
Basic QoS Model, page 37-6
Classification, page 37-6
Policing and Marking, page 37-10
Mapping Tables, page 37-14
Queueing and Scheduling, page 37-14
Packet Modification, page 37-16
Per-Port Per-VLAN QoS, page 37-16
QoS and Software Processed Packets, page 37-16

Prioritization

QoS implementation is based on the DiffServ architecture. 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 37-1:
Prioritization values in Layer 2 frames: