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Chapter 69 Policy Rule
XGS2220 Series User’s Guide
518
CHAPTER 69
Policy Rule
69.1 Policy Rules Overview
This chapter shows you how to configure policy rules.
A classifier distinguishes traffic into flows based on the configured criteria (refer to Chapter 68 on page
509 for more information). A policy rule ensures that a traffic flow gets the requested treatment in the
network.
69.1.1 What You Can Do
Use the Policy Rule screen (Section 69.2 on page 519) to enable the policy and display the active
classifiers you configure in the Classifier screen.
69.1.2 DiffServ
DiffServ (Differentiated Services) is a class of service (CoS) model that marks packets so that they
receive specific per-hop treatment at DiffServ-compliant network devices along the route based on the
application types and traffic flow. Packets are marked with DiffServ Code Points (DSCPs) indicating the
level of service desired. This allows the intermediary DiffServ-compliant network devices to handle the
packets differently depending on the code points without the need to negotiate paths or remember
state information for every flow. In addition, applications do not have to request a particular service or
give advanced notice of where the traffic is going.
69.1.3 DSCP and Per-Hop Behavior
DiffServ defines a new DS (Differentiated Services) field to replace the Type of Service (TOS) field in the IP
header. The DS field contains a 2-bit unused field and a 6-bit DSCP field which can define up to 64
service levels. The following figure illustrates the DS field.
DSCP is backward compatible with the three precedence bits in the ToS octet so that non-DiffServ
compliant, ToS-enabled network device will not conflict with the DSCP mapping.
The DSCP value determines the forwarding behavior, the PHB (Per-Hop Behavior), that each packet
gets across the DiffServ network. Based on the marking rule, different kinds of traffic can be marked for
different kinds of forwarding. Resources can then be allocated according to the DSCP values and the
configured policies.
DSCP (6 bits) Unused (2 bits)
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