3Com Switch 4200G Configuration Guide

Introduction to QoS 215
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.
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
Table 185 Description on DSCP values (Continued)
Key word DSCP value (decimal) DSCP value (binary)
Table 186 Description on 802.1p priority
IP Precedence
(decimal)
IP Precedence
(binary)
Description
0 000 best-effort
1 001 background
2 010 spare