Basic Qos Model - Cisco Catalyst 4500 Series Software Configuration Manual

Chapter 44 Configuring Quality of Service
Table 44-1
3-bit IP
Precedence
2
3
1. MSb = most significant bit
•
•
•

Basic QoS Model

Figure 44-2 illustrates a high-level flow of QoS function.
Figure 44-2 QoS Packet Processing
Input
Output
IP Precedence and DSCP Values (continued)
1
6 MSb of ToS
8 7 6 5 4 3
0 1 0 0 0 0
0 1 0 0 0 1
0 1 0 0 1 0
0 1 0 0 1 1
0 1 0 1 0 0
0 1 0 1 0 1
0 1 0 1 1 0
0 1 0 1 1 1
0 1 1 0 0 0
0 1 1 0 0 1
0
0 1 1 0 1
0 1 1 0 1 1
0 1 1 1 0 0
0 1 1 1 0 1
0 1 1 1 1 0
0 1 1 1 1 1
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.
Packet
Reception
Output QoS
Classification
Catalyst 4500 Series Switch, Cisco IOS Software Configuration Guide - Cisco IOS XE 3.9.xE and IOS 15.2(5)Ex
6-bit 3-bit IP
DSCP Precedence
16 6
17
18
19
20
21
22
23
24 7
25
26
27
28
29
30
31
Input QoS
Classification
Output
Active Queue
Policing
Management
and Marking
1
6 MSb of ToS 6-bit
DSCP
8 7 6 5 4 3
1 1 0 0 0 0 48
1 1 0 0 0 1 49
1 1 0 0 1 0 50
1 1 0 0 1 1 51
1 1 0 1 0 0 52
1 1 0 1 0 1 53
1 1 0 1 1 0 54
1 1 0 1 1 1 55
1 1 1 0 0 0 56
1 1 1 0 0 1 57
1 1 1 0 1 0 58
1 1 1 0 1 1 59
1 1 1 1 0 0 60
1 1 1 1 0 1 61
1 1 1 1 1 0 62
1 1 1 1 1 1 63
Input
Policing
and Marking
Port/Queue
Scheduling
(Sharing/
Shaping)
Overview of QoS
Forwarding
Lookup
Packet
Transmission
44-5