3Com Switch 7754, Configuration Manual

Page: 659 / 940

Overview 659
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.
2 802.1p priority
802.1p priority lies in Layer 2 packet headers and is applicable to occasions where
the Layer 3 packet header does not need analysis but QoS must be assured in
Layer 2.
Figure 171 An Ethernet frame with a 802.1Q tag header
Table 529 Description on DSCP values
DSCP 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
af42 36 100100
af43 38 100110
cs1 8 001000
cs2 16 010000
cs3 24 011000
cs4 32 100000
cs5 40 101000
cs6 48 110000
cs7 56 111000
default (be) 0 000000
Length/Type Data
6 bytes 6 bytes 4 bytes 2 bytes 46 to 1500 bytes 4 bytes
TPID TCI
Source
Address
Destination
Address
802.1Q
header
FCS
(CRC - 32)