3Com Switch 4800G 24-Port - Page 863

Traffic Classification Overview 863
■ Class selector (CS) class: This class comes from the IP ToS field and includes
eight subclasses;
■ 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 precedence
802.1p precedence 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
at Layer 2.
Figure 257 An Ethernet frame with an 802.1Q tag header
As shown in the figure above, the 4-byte 802.1Q tag header contains a 2-byte Tag
Protocol Identifier (TPID) whose value is 8100 and a 2-byte Tag Control
Information (TCI). TPID is a new class defined by IEEE to indicate a packet with an
802.1Q tag. Figure 258 describes the detailed contents of an 802.1Q tag header.
Table 66 Description on DSCP precedence values
DSCP value (decimal) DSCP value (binary) Description
46 101110 ef
10 001010 af11
12 001100 af12
14 001110 af13
18 010010 af21
20 010100 af22
22 010110 af23
26 011010 af31
28 011100 af32
30 011110 af33
34 100010 af41
36 100100 af42
38 100110 af43
8 001000 cs1
16 010000 cs2
24 011000 cs3
32 100000 cs4
40 101000 cs5
48 110000 cs6
56 111000 cs7
0 000000 be (default)
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)