Cisco Catalyst 4500 series Administration Manual page 996

Overview of QoS
Note
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–
Note
Table 42-1
3-bit IP
Precedence
0
1
Software Configuration Guide—Release IOS XE 3.6.0E and IOS 15.2(2)E
42-4
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.
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.
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 42-1.
IP Precedence and DSCP Values
1
6 MSb of ToS 6-bit
DSCP
8 7 6 5 4 3
0 0 0 0 0 0 0
0 0 0 0 0 1 1
0 0 0 0 1 0 2
0 0 0 0 1 1 3
0 0 0 1 0 0 4
0 0 0 1 0 1 5
0 0 0 1 1 0 6
0 0 0 1 1 1 7
0 0 1 0 0 0 8
0 0 1 0 0 1 9
0 0 1 0 1 0 10
0 1 1 11
0 0 1
0 0 1 1 0 0 12
0 0 1 1 0 1 13
0 0 1 1 1 0 14
0 0 1 1 1 1 15
Chapter 42
1
3-bit IP 6 MSb of ToS
Precedence
8 7 6 5 4 3
4 1 0 0 0 0
1 0 0 0 0
1 0 0 0 1
1 0 0 0 1
1 0 0 1 0
1 0 0 1 0
1 0 0 1 1
1 0 0 1 1
5 1 0 1 0 0
1 0 1 0 0
1 0 1 0 1
1 0 1 0 1
1 0 1 1 0
1 0 1 1 0
1 0 1 1 1
1 0 1 1 1
Configuring Quality of Service
6-bit
DSCP
0 32
1 33
0 34
1 35
0 36
1 37
0 38
1 39
0 40
1 41
0 42
1 43
0 44
1 45
0 46
1 47
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