3Com WX3000 Priority Trust Mode, 802.1p priority decimal 802.1p priority binary Description

As shown in the figure above, each host supporting 802.1Q protocol adds a 4-byte 802.1Q tag header after the source address of the former Ethernet frame header when sending packets.

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 1-3 describes the detailed contents of an 802.1Q tag header.

Figure 1-3802.1Q tag headers

			Byte 1								Byte 2							Byte 3									Byte 4
			TPID (Tag Protocol Identifier)																TCI (Tag Control Information)
1	0	0	0	0	0	0	1	0	0	0	0	0	0	0	0		Priority			cfi					VLAN ID
7	6	5	4	3	2	1	0	7	6	5	4	3	2	1	0	7	6	5	4	3	2	1	0	7	6	5	4	3	2	1	0

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 1-3Description on 802.1p priority

802.1p priority (decimal)	802.1p priority (binary)	Description
0	000	best-effort
1	001	background
2	010	spare
3	011	excellent-effort
4	100	controlled-load
5	101	video
6	110	voice
7	111	network-management

The precedence is called 802.1p priority because the related applications of this precedence are defined in detail in the 802.1p specifications.

Priority Trust Mode

A device can assign different types of precedence to the packets it receives as configured, such as 802.1p precedence, DSCP precedence, local precedence, and drop precedence.

Among the above-mentioned precedence types:

z

z

The local precedence is only of local significance. A local precedence corresponds to a specific output queue. Packets with higher local precedence values take precedence over those with lower precedence values and will be processed preferentially.

The drop precedence determines which packets are dropped preferentially. The higher the drop precedence, the more likely a packet is dropped.

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