SROS Command Line Interface Reference Guide
Quality of Service (QoS) Map Commands
5991-2114
© Copyright 2007 Hewlett-Packard Development Company, L.P.
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Default PHB
All DS-compliant nodes must provide a Default PHB to offer best-effort forwarding service. For Default
PHBs, the DSCP value is 0. Any packet that does not contain a standardized DSCP should be mapped to
the Default PHB and handled accordingly.
Class Selector PHB
In the Class Selector PHB, the first three bits in the DSCP value are used for backwards compatibility to
systems implementing IP precedence. In this scenario, all but the first three bits of the DS field are set to 0.
This compatibility requires DS-compliant nodes to provide the same data services as are provided by
nodes implementing IP precedence. The following table is a comparison of IP precedence values to their
corresponding DSCP values.
Assured Forwarding PHB
The flexibility of DiffServ allows for more developed sub-classes of service within each main class using
the last three bits of the DSCP. As defined in RFC2597, the Assured Forwarding PHB creates four main
classes of service:
The first three bits of the DSCP specify the class and the last bit is always zero. Each class is separated
into subclasses using the two remaining bits in the DSCP (bits 3 and 4). The subclasses are divided based
on the likelihood that packets in the class are dropped in the event of network congestion. The higher the
IP Precedence
Value (bits)
DSCP Value (bits)
0 (000)
0 (000000)
1 (001)
8 (001000)
2 (010)
16 (010000)
3 (011)
24 (011000)
4 (100)
32 (100000)
5 (101)
40 (101000)
6 (110)
48 (110000)
7 (111)
56 (111000)
Class
DSCP Bits
AF1
001XX0
AF2
010XX0
AF3
011XX0
AF4
100XX0
X indicates a do not care value
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