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XGS-4526/4528F/4728F User’s Guide
207
CHAPTER 21
Policy Rule
This chapter shows you how to configure policy rules.
21.1 Policy Rules Overview
A classifier distinguishes traffic into flows based on the configured criteria (refer to
Chapter 20 on page 201 for more information). A policy rule ensures that a traffic
flow gets the requested treatment in the network.
21.1.1 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.
21.1.2 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.
DSCP (6 bits) Unused (2 bits)