44-5
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
Chapter 44 Configuring Quality of Service
Overview of QoS
• Classification is the selection of traffic to be marked.
• Marking, according to RFC 2475, is the process of setting a Layer 3 DSCP value in a packet; in this
publication, the definition of marking is extended to include setting Layer 2 CoS values.
• Policing is limiting bandwidth used by a flow of traffic. Policing can mark or drop traffic.
Basic QoS Model
Figure 44-2 illustrates a high-level flow of QoS function.
Figure 44-2 QoS Packet Processing
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0
0
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0
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1
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1. MSb = most significant bit
Table 44-1 IP Precedence and DSCP Values (continued)
3-bit IP
Precedence
6 MSb
1
of ToS 6-bit
DSCP
3-bit IP
Precedence
6 MSb
1
of ToS 6-bit
DSCP
876 543 876 543
Packet
Reception
Input
Output
Input QoS
Classification
Input
Policing
and Marking
Forwarding
Lookup
Output QoS
Classification
Output
Policing
and Marking
Active Queue
Management
Port/Queue
Scheduling
(Sharing/
Shaping)
Packet
Transmission
203973
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