ZyXEL Communications MES3500-24 User Manual

MES3500-24/24F User’s Guide

This chapter shows you how to configure policy rules. 

A classifier distinguishes traffic into flows based on the configured criteria (refer to 

 for more information). A policy rule ensures that a traffic flow gets the requested 

treatment in the network. 

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.

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.

You must first configure a classifier in the Classifier screen. Refer to 

 for 

more information. 

DSCP (6 bits)

Unused (2 bits)