As shown in b, 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). c presents the format of the 802.1Q tag header.

c.802.1Q tag header

 

 

 

Byte 1

 

 

 

 

 

 

Byte 2

 

 

 

 

 

 

Byte 3

 

 

 

 

 

 

Byte 4

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

TPID (Tag protocol identifier)

 

 

 

 

 

 

TCI (Tag control information)

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

CFI

 

 

 

 

 

 

 

 

 

 

1

0

0

0

0

0

0

1

0

0

0

0

0

0

0

0

Priority

 

 

 

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

The priority in the 802.1Q tag header is called 802.1p precedence, because its use is defined in IEEE 802.1p. 4 presents the values for 802.1p precedence.

4.Description on 802.1p precedence

802.1p precedence (decimal)

802.1p precedence (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

 

 

 

Queue scheduling

In general, congestion management adopts queuing technology. The system uses a certain queuing algorithm for traffic classification, and then uses a certain precedence algorithm to send the traffic. Each queuing algorithm is used to handle a particular network traffic problem and has significant impacts on bandwidth resource assignment, delay, and jitter.

In this section, two common hardware queue scheduling algorithms Strict Priority (SP) queuing and Weighted Round Robin (WRR) queuing are introduced.

SP queuing

SP queuing is specially designed for mission-critical applications, which require preferential service to reduce response delay when congestion occurs.

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