1-7
DSCP value (decimal)
DSCP value (binary)
Description
16 010000
cs2
24 011000
cs3
32 100000
cs4
40 101000
cs5
48 110000
cs6
56 111000
cs7
0 000000
be
(default)
2) 802.1p
precedence
802.1p precedence lies in Layer 2 packet headers and is applicable to occasions where
Layer 3 packet analysis is not needed and QoS must be assured at Layer 2.
Figure 1-3
An Ethernet frame with an 802.1q tag header
As shown in
Figure 1-3
, each host supporting the 802.1q protocol adds a 4-byte 802.1q
tag header after the source address field of the former Ethernet frame header when
sending packets.
The 4-byte 802.1q tag header consists of a two-byte tag protocol identifier (TPID) field,
whose value is 0x8100, and a two-byte tag control information (TCI) field.
Figure 1-4
presents the format of the 802.1q tag header.
Figure 1-4
802.1q tag header
In
Figure 1-4
, the three-bit priority field in TCI is 802.1p precedence (also known as
CoS precedence), which ranges from 0 to 7.