Riverstone Networks WICT1-12 - Manual (Page 573)

Riverstone Networks RS Switch Router User Guide Release 8.0 26-11
QoS Configuration ToS Rewrite
• Source port
• Destination port
• ToS
• Port
• Interface
When an IP packet is received, the ToS field of the packet is ANDed with the <tos-mask> and the resulting value is
compared with the ANDed value of <tos> and <tos-mask> of the QoS policy. If the values are equal, the values of the
<tos-rewrite> and <tos-precedence-rewrite> parameters will be written into the packet.
The <tos> and <tos-mask> parameters use values ranging from 0 to 255. They are used in conjunction with each other
to define which bit in the <tos> field of the packet is significant. The <tos-precedence-rewrite> value ranges from 0
to 7 and is the value that is rewritten in the ToS Precedence field (the first three bits of the ToS octet). The <tos-rewrite>
value ranges from 0 to 31 and is the value that is rewritten in the ToS field.
Figure 26-3 ToS rewrite
The ToS byte rewrite is part of the QoS priority classifier group. The entire ToS byte can be rewritten or only the
precedence part of the ToS byte can be rewritten. If you specify a value for <tos-precedence-rewrite>, then only the
upper three bits of the ToS byte are changed. If you set <tos-precedence-rewrite> to
any
and specify a value for
<tos-rewrite>, then the upper three bits remain unchanged and the lower five bits are rewritten. If you specify values
for both <tos-precedence-rewrite> and <tos-rewrite>, then the upper three bits are rewritten to the
<tos-precedence-rewrite> value and the lower five bits are rewritten to the <tos-rewrite> value.
For example, the following command will rewrite the ToS Precedence field to 7 if the ToS Precedence field of the
incoming packet is 6:
In the above example, the <tos> value of 222 (binary value 1101 1110) and the <tos-mask> value of 224 (binary value
1110 0000) are ANDed together to specify the ToS Precedence field value of 6 (binary value 110). Changing the value
in the <tos-mask> parameter determines the bit in the ToS octet field that will be examined.
qos set ip tosp6to7 low any any any any 222 any any 224 7
MBZ ToS Precedence
7 6 5 4 3 2 1 0
<tos-precedence-rewrite>
<tos-rewrite>
0-7 0-31