Cabletron Systems 9032578-05 manual Configuring ToS Rewrite for IP Packets, Tos-rewrite

Chapter 21: QoS Configuration Guide

For example, setting the ToS field to 0010 specifies that a packet will be routed on the most reliable paths. Setting the ToS field to 1000 specifies that a packet will be routed on the paths with the least delay. (Refer to RFC 1349 for the specification of the ToS field value.)

With the ToS rewrite command, you can access the value in the ToS octet (which includes both the Precedence and ToS fields) in each packet. The upper-layer application can then decide how to handle the packet, based on either the Precedence or the ToS field or both fields. For example, you can configure a router to forward packets using different paths, based on the ToS octet. You can also change the path for specific applications and users by changing the Precedence and/or ToS fields.

Note: In RFC 2574, 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).

7	6	5	4	3	2	1	0
Precedence					ToS		MBZ
<tos-precedence-rewrite>					<tos-rewrite>
	0-7				0-31

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

300 SmartSwitch Router User Reference Manual