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CHAPTER 31
Differentiated Services
31.1 Overview
This chapter shows you how to configure Differentiated Services (DiffServ) on the Switch.
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.
31.1.1 What You Can Do
•Use the DiffServ screen (Section 31.2 on page 241) to activate DiffServ to apply marking rules
or IEEE 802.1p priority mapping on the Switch.
•Use the DSCP screen (Section 31.3.1 on page 242) to change the DSCP-IEEE 802.1p mapping.
31.1.2 What You Need to Know
Read on for concepts on Differentiated Services that can help you configure the screens in this
chapter.
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 153 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.
DSCP (6 bits) CU (2 bits)
