3Com 4200G 12-Port Configuration Manual page 229

Table 185 Description on DSCP values (Continued)
Key word
af42
af43
cs1
cs2
cs3
cs4
cs5
cs6
cs7
default (be)
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 65 An Ethernet frame with a 802.1Q tag header
As shown in Figure 65, each host supporting 802.1Q protocol adds a 4-bit 802.1Q
tag header after the source address of the former Ethernet frame header when
sending packets.
The 4-bit 802.1Q tag header contains a 2-bit Tag Protocol Identifier (TPID) whose
value is 8100 and a 2-bit Tag Control Information (TCI). TPID is a new class defined by
IEEE to indicate a packet with an 802.1Q tag. Figure 66 describes the detailed
contents of an 802.1Q tag header.
Figure 66 802.1Q tag headers
In Figure 66, the 3-bit priority field in TCI is 802.1p priority in the range of 0 to 7.The
3 bits specify the precedence of the frame.8 classes of precedence are used to
determine which packet is sent preferentially when the switch is congested.
Table 186 Description on 802.1p priority
IP Precedence IP Precedence
(decimal) (binary)
0 000
1 001
2 010
DSCP value (decimal)
36
38
8
16
24
32
40
48
56
0
Description
best-effort
background
spare
Introduction to QoS 215
DSCP value (binary)
100100
100110
001000
010000
011000
100000
101000
110000
111000
000000