Cisco Catalyst 4500 Series, Software Configuration Manual

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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
Layer 2 802.1Q frame headers have a 2-byte Tag Control Information field that carries the CoS
value in the three most significant bits, which are called the User Priority bits.
Other frame types cannot carry Layer 2 CoS values.
Note On interfaces configured as Layer 2 ISL trunks, all traffic is in ISL frames. On interfaces
configured as Layer 2 802.1Q trunks, all traffic is in 802.1Q frames except for traffic in the
native VLAN.
– Layer 3 IP precedence values—The IP version 4 specification defines the three most significant
bits of the 1-byte ToS field as IP precedence. IP precedence values range between zero for low
priority and seven for high priority.
–
Layer 3 differentiated services code point (DSCP) values—The Internet Engineering Task
Force (IETF) has defined the six most significant bits of the 1-byte IP ToS field as the DSCP.
The per-hop behavior represented by a particular DSCP value is configurable. DSCP values
range between 0 and 63.
Note
Layer 3 IP packets can carry either an IP precedence value or a DSCP value. QoS supports
the use of either value, since DSCP values are backwards compatible with IP precedence
values. See
Table 44-1 .
Table 44-1 IP Precedence and DSCP Values
3-bit IP
Precedence 6 MSb
1
of ToS 6-bit
DSCP 3-bit IP
Precedence 6 MSb
1
of ToS 6-bit
DSCP
8 7 6 5 4 3 8 7 6 5 4 3
0 0
0
0
0
0
0
0
0 0
0
0
0
0
0
0
0 0
0
0
0
0
0
0
0 0
0
0
0
1
1
1
1 0
0
1
1
0
0
1
1 0
1
0
1
0
1
0
1 0
1
2
3
4
5
6
7 4 1
1
1
1
1
1
1
1 0
0
0
0
0
0
0
0 0
0
0
0
0
0
0
0 0
0
0
0
1
1
1
1 0
0
1
1
0
0
1
1 0
1
0
1
0
1
0
1 32
33
34
35
36
37
38
39
1 0
0
0
0
0
0
0
0 0
0
0
0
0
0
0
0 1
1
1
1
1
1
1
1 0
0
0
0
1
1
1
1 0
0
1
1
0
0
1
1 0
1
0
1
0
1
0
1 8
9
10
11
12
13
14
15
5 1
1
1
1
1
1
1
1 0
0
0
0
0
0
0
0 1
1
1
1
1
1
1
1 0
0
0
0
1
1
1
1 0
0
1
1
0
0
1
1 0
1
0
1
0
1
0
1 40
41
42
43
44
45
46
47