Per-Hop Behavior - Lenovo Flex System Fabric CN4093 Application Manual

Per-Hop Behavior

190
CN4093 Application Guide for N/OS 8.2
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
applying a set of rules to packets, based on the DSCP value, as they hop through
the network.
The CN4093 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 below. Routers use drop precedence to
determine which packets to discard last when the network becomes congested.
AF PHB is described in RFC 2597.
Drop
Precedence Class 1
Low AF11 (DSCP
10)
Medium AF12 (DSCP
12)
High 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 below.
CS PHB is described in RFC 2474.
Priority Class Selector
Highest CS7
CS6
CS5
CS4
CS3
CS2
CS1
Lowest CS0
Class 2 Class 3
AF21 (DSCP AF31 (DSCP
18) 26)
AF22 (DSCP AF32 (DSCP
20) 28)
AF23 (DSCP AF33 (DSCP
22) 30)
DSCP
56
48
40
32
24
16
8
0
Class 4
AF41 (DSCP
34)
AF42 (DSCP
36)
AF43 (DSCP
38)