ZyXEL Communications XGS4700 Series, Manual

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XGS4700-48F User’s Guide
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Differentiated Services
This chapter shows you how to configure Differentiated Services (DiffServ) on the
Switch.
39.1 DiffServ Overview
Quality of Service (QoS) is used to prioritize source-to-destination traffic flows. All
packets in the flow are given the same priority. You can use CoS (class of service)
to give different priorities to different packet types.
DiffServ 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.
39.1.1 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 6-bit DSCP field which
can define up to 64 service levels and the remaining 2 bits are defined as currently
unused (CU). The following figure illustrates the DS field.
Figure 181 DiffServ: Differentiated Service 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 PHB (Per-Hop Behavior), that each packet gets as
it is forwarded across the DiffServ network. Based on the marking rule different
DSCP (6 bits) CU (2 bits)