Chapter 19: QoS Configuration Guide
270
DIGITAL GIGAswitch/Router User Reference Manual
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 (the last five bits
of the ToS octet, which includes both the ToS field and the MBZ bit).
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.
MBZ
ToS
Precedence
7
6
5
4
3
2
1
0
<
tos-precedence-rewrite
>
<
tos-rewrite
>
0-7
0-31
gs/r(config)#
qos set ip tosp6to7 low any any any any 222 any any 224 7
