RackSwitch G8000 Application Guide
108
Chapter 6: Quality of Service BMD00041, November 2008
Per Hop Behavior
The DSCP value determines the Per Hop Behavior (PHB) of each packet. The PHB is the for-
warding treatment given to packets at each hop. QoS policies are built by applying 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 stan-
dards:
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 below. 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 high-
est priority, and CS0 representing the lowest priority, as shown below. CS PHB is
described in RFC 2474.
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)
Priority Class Selector DSCP
Highest CS7 56
CS6 48
CS5 40
CS4 32
CS3 24
CS2 16
CS1 8
Lowest CS0 0
