User Manual

ToS Rewrite
438 Enterasys X-Pedition User Reference Manual
Note: In RFC 2474, the IETF redefined the ToS octet as the “DiffServ” byte. You will still be
able to use the ToS rewrite feature to implement DiffServ when this standard is deployed.
Configuring ToS Rewrite for IP Packets
The ToS rewrite for IP packets is set with the qos set command in Configure mode. You can define
the QoS policy based on any of the following IP fields: source IP address, destination IP address,
source port, destination port, ToS, port, or 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 (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 6543210
<tos-precedence-rewrite>
<tos-rewrite>
0-7
0-31
xp(config)# qos set ip tosp6to7 low any any any any 222 any any 224 7