3Com 2928 User Guide - Page 466

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 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 76543210765432107654321076543210 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) 0 000 1 001 2 010 3 011 4 100 5 101 6 110 7 111 Description best-effort background spare excellent-effort controlled-load video voice 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. 2-6