3Com Switch 4200G Configuration Guide

214 CHAPTER 27: QOS CONFIGURATION
■ RFC2474 re-defines the ToS field in the IP packet header, which is called the DS
field. The first six (bit 0-bit 5) bits of the DS field indicate DSCP precedence in the
range of 0 to 63.The first three bits in DSCP precedence are class selector
codepoints, bit 4 and bit 5 indicate drop precedence, and bit 6 is zero indicating
that the device sets the service class with the DS model.
■ The last two bits (bit 6 and bit 7) are reserved bits.
The precedence values of the IP packet indicate 8 different service classes.
The DiffServ network defines four traffic classes:
■ Expedited Forwarding (EF) class: In this class, packets can be forwarded regardless
of link share of other traffic. The class is suitable for preferential services with low
delay, low packet loss ratio, low variation and assured bandwidth (such as virtual
leased line);
■ Assured forwarding (AF) class: This class is further divided into four subclasses
(AF1/2/3/4) and a subclass is further divided into three drop priorities, so the AF
service level can be segmented. The QoS rank of the AF class is lower than that of
the EF class;
■ Class selector (CS) class: This class comes from the IP TOS field and includes 8
classes;
■ Best Effort (BE) class: This class is a special class without any assurance in the CS
class. The AF class can be degraded to the BE class if it exceeds the limit. Current IP
network traffic belongs to this class by default.
Table 184 Description on IP Precedence
IP Precedence
(decimal)
IP Precedence
(binary)
Description
0 000 routine
1 001 priority
2 010 immediate
3 011 flash
4 100 flash-override
5 101 critical
6 110 internet
7 111 network
Table 185 Description on DSCP values
Key word DSCP value (decimal) DSCP value (binary)
ef 46 101110
af11 10 001010
af12 12 001100
af13 14 001110
af21 18 010010
af22 20 010100
af23 22 010110
af31 26 011010
af32 28 011100
af33 30 011110
af41 34 100010