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EX2500 Ethernet Switch Configuration Guide
60 Using DSCP Values to Provide QoS
Using DSCP Values to Provide QoS
The switch uses the Differentiated Services (DiffServ) architecture to provide QoS
functions. DiffServ is described in IETF RFCs 2474 and 2475.
The six most significant bits in the ToS byte of the IP header are defined as DiffServ
Code Points (DSCP). Packets are marked with a certain value depending on the type
of treatment the packet must receive in the network device. DSCP is a measure of
the Quality of Service (QoS) level of the packet.
The switch can classify traffic by reading the DiffServ Code Point (DSCP) or IEEE
802.1p priority value, or by using filters to match specific criteria. When network
traffic attributes match those specified in a traffic pattern, the policy instructs the
switch to perform specified actions on each packet that passes through it. The
packets are assigned to different Class of Service (COS) queues and scheduled for
transmission.
Differentiated Services Concepts
To differentiate between traffic flows, packets can be classified by their DSCP
value. As shown in Figure 12, the Differentiated Services (DS) field in the IP header
is an octet, and the first six bits, called the DS Code Point (DSCP), can provide QoS
functions. Each packet carries its own QoS state in the DSCP. There are 64 possible
DSCP values (0 through 63).
Figure 12: Layer 3 IPv4 Packet
The switch can perform the following actions to the DSCP:
Read the DSCP value of ingress packets.
Map the DSCP value to a Class of Service queue (COSq).
The switch can use the DSCP value to direct traffic prioritization.
With DiffServ, you can establish policies to direct traffic. A policy is a
traffic-controlling mechanism that monitors the characteristics of the traffic (for
example, its source, destination, and protocol), and performs a controlling action
on the traffic when certain characteristics are matched.
7 6 5 4 3 2 1 0
Differentiated Services Code Point (DSCP)
unused
Version
Length
ToS
Length
ID
Offset TTL
Proto
FCS
DIP
SIP
Data