Juniper Networks EX2500 - Manual (Page 77)

Using DSCP Values to Provide QoS „ 63
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.
„ 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 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)
Table 15: Class Selector
Priority
Class
Selector
DSCP
Highest CS7 56
CS6 48
CS5 40
CS4 32
CS3 24
CS2 16
CS1 8
Lowest CS0 0