Per Hop Behavior; Table 14: Assured Forwarding; Table 15: Class Selector - Juniper EX2500 Configuration Manual

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:

Table 14: Assured Forwarding

Drop
Precedence
Low
Medium
High

Table 15: Class Selector

Priority
Highest
Lowest
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 1
AF11 (DSCP 10)
AF12 (DSCP 12)
AF13 (DSCP 14)
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.
Class
Selector
CS7
CS6
CS5
CS4
CS3
CS2
CS1
CS0
Class 2 Class 3
AF21 (DSCP 18) AF31 (DSCP 26)
AF22 (DSCP 20) AF32 (DSCP 28)
AF23 (DSCP 22) AF33 (DSCP 30)
DSCP
56
48
40
32
24
16
8
0
Using DSCP Values to Provide QoS
Chapter 5: Quality of Service
Class 4
AF41 (DSCP 34)
AF42 (DSCP 36)
AF43 (DSCP 38)
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