EX2500 Ethernet Switch Configuration Guide
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
Version | ToS | Length | ID | Offset | TTL | Proto | FCS | SIP | DIP | Data | |
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Differentiated Services Code Point (DSCP)
unused
7
6
5
4
3
2
1
0
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
60 Using DSCP Values to Provide QoS
