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Command Reference Guide
The first three bits of the DSCP specify the class and the last bit is always 0. 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
value for bits 3 and 4, the greater the likelihood that the packets will be dropped.
The following table lists the assured forwarding PHB subclasses and their corresponding DSCP bits and
values.
Class
AF1
AF2
AF3
AF4
Expedited Forwarding PHB
RFC 2598 created a new DiffServ PHB intended to provide the best service possible on an IP network.
Packets using the expedited forwarding PHB markings should provide service to reduce latency, jitter, and
dropped packets, and be guaranteed bandwidth during the entire end-to-end transmission journey through
the network. The DSCP value for the expedited forwarding PHB is 46 (DSCP bits are 101110).
60000CRG0-35E
Bit 3
Bit 4
0
1
1
Subclass
1
2
3
1
2
3
1
2
3
1
2
3
Copyright © 2012 ADTRAN, Inc.
Quality of Service Map Command Set
Drop Precedence
1
Low
0
Medium
1
High
DSCP Bits
DSCP Value
001010
001100
001110
010010
010100
010110
011010
011100
011110
100010
100100
100110
10
12
14
18
20
22
26
28
30
34
36
38
3281
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