3Com 4500 PWR 26-Port Configuration Manual page 540

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 at Layer 2.
Figure 45-3 An Ethernet frame with an 802.1Q tag header
Source
Destination
Address
Address
6 bytes 6 bytes
As sh own in the figure above, the 4-byte 802.1Q tag header consists of the tag protocol identifier (TPID,
two bytes in length), whose value is 0x8100, and the tag control information (TCI, two bytes in length).
Figure 45-4 describes the detailed contents of an 802.1Q tag header.
Figure 45-4 802.1Q tag headers
In the figure above, the priority field (three bits in length) in TCI is 802.1p priority (also known as CoS
precedence), which ranges from 0 to 7.
Table 45-4 Description on 802.1p
802.1p priority (decimal)
0
1
2
3
4
5
6
7
The precedence is called 802.1p priority becau
defined in detail in the 802.1p specifications.
3) Local precedence
Local precedence is a locally significant precedence that the device assigns to a packet. A local
precedence value corresponds to on
local precedence are processed pr
packet does not carry it after leavin
802.1Q
header
Length/Type
TPID TCI
4 bytes 2 bytes
priorit y
802.1p priority (binary)
000
001
010
011
100
101
110
111
e of the eight hardware output queues. Packets with the highest
eferentially. As local precedence is used only for internal queuing, a
g the queue.
45-6
Data
(CRC - 32)
46 to 1500 bytes 4 bytes
best-effort
background
spare
excellent-effort
controlled-load
video
voice
network-management
se the related applications of this precedence are
FCS
Description