3Com 2928 - Baseline Plus Switch PWR, Руководство пользователя

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2-6
Figure 2-4
An Ethernet frame with an 802.1Q tag header
As shown in
Figure 2-4 , 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 2-5 presents the format of the 802.1Q tag header.
Figure 2-5
802.1Q tag header
1 0 0 0 0 0 0 1 0 0 0 0 0 0 0 Priority VLAN ID
TPID (Tag protocol identifier) TCI (Tag control information)
Byte 1 Byte 2
0
Byte 3 Byte 4
CFI
7 5 4 3 2 1 0 7 5 4 3 2 1 0 6 6 7 5 4 3 2 1 0 7 5 4 3 2 1 0 6 6
The priority in the 802.1Q tag header is called 802.1p precedence, because its use is defined in IEEE
802.1p.
Table 2-3 presents the values for 802.1p precedence.
Table 2-3
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.