27-4
Software Configuration Guide—Release 12.2(25)SG
OL-7659-03
Chapter27 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 Layer2 CoS values.
Note On interfaces configured as Layer2 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. See the “Configuring DSCP Maps” section on page 27-51.
Note Layer3 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 27 -1.
Table27-1 IP Precedence and DSCP Values
3-bit IP
Precedence
6 MSb1 of ToS 6-bit
DSCP
3-bit IP
Precedence
6 MSb1 of ToS 6-bit
DSCP
876 543 876 543
00
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
41
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
10
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
51
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