Chapter 5: Quality of Service

Per Hop Behavior

The DSCP value determines the Per Hop Behavior (PHB) of each packet. The PHB is the forwarding treatment given to packets at each hop. QoS policies are built by the application of a set of rules to packets, based on the DSCP value, as they hop through the network.

The default settings are based on the following standard PHBs, as defined in the

IEEE standards:

„Expedited Forwarding (EF)—This PHB has the highest egress priority and lowest drop precedence level. EF traffic is forwarded ahead of all other traffic. EF PHB is described in RFC 2598.

„Assured Forwarding (AF)—This PHB contains four service levels, each with a different drop precedence, as shown in Table 14. Routers use drop precedence to determine which packets to discard last when the network becomes congested. AF PHB is described in RFC 2597.

Table 14: Assured Forwarding

Drop

 

 

 

 

Precedence

Class 1

Class 2

Class 3

Class 4

 

 

 

 

 

Low

AF11 (DSCP 10)

AF21 (DSCP 18)

AF31 (DSCP 26)

AF41 (DSCP 34)

 

 

 

 

 

Medium

AF12 (DSCP 12)

AF22 (DSCP 20)

AF32 (DSCP 28)

AF42 (DSCP 36)

 

 

 

 

 

High

AF13 (DSCP 14)

AF23 (DSCP 22)

AF33 (DSCP 30)

AF43 (DSCP 38)

 

 

 

 

 

„Class Selector (CS)—This PHB has eight priority classes, with CS7 representing the highest priority, and CS0 representing the lowest priority, as shown in Table 15. CS PHB is described in RFC 2474.

Table 15: Class Selector

 

Class

 

Priority

Selector

DSCP

 

 

 

Highest

CS7

56

 

 

 

 

CS6

48

 

 

 

 

CS5

40

 

 

 

 

CS4

32

 

 

 

 

CS3

24

 

 

 

 

CS2

16

 

 

 

 

CS1

8

 

 

 

Lowest

CS0

0

 

 

 

Using DSCP Values to Provide QoS „ 61

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Juniper Networks EX2500 manual Per Hop Behavior, Assured Forwarding Drop Precedence Class