TL-SL3428_V3_User_Guide - TP-Link

Figure 9-2 802.1Q frameAs shown in the figure above, each 802.1Q Tag has a Pri field, comprising 3 bits. The 3-bit priorityfield is 802.1p priority in the range of 0 to 7. 802.1P priority determines the priority of the packetsbased on the Pri value. On the Web management page of the switch, you can configure differentpriority tags mapping to the corresponding priority levels, and then the switch determine whichpacket is sent preferentially when forwarding packets. The switch processes untagged packetsbased on the default priority mode.3. DSCP PriorityFigure 9-3 IP datagramAs shown in the figure above, the ToS (Type of Service) in an IP header contains 8 bits. The firstthree bits indicate IP precedence in the range of 0 to 7. RFC2474 re-defines the ToS field in the IPpacket header, which is called the DS field. The first six bits (bit 0-bit 5) of the DS field indicateDSCP precedence in the range of 0 to 63. The last 2 bits (bit 6 and bit 7) are reserved. On the Webmanagement page, you can configure different DS field mapping to the corresponding prioritylevels. Non-IP datagram with 802.1Q tag are mapped to different priority levels based on 802.1Ppriority mode if 8021.1P Priority mode is enabled; the untagged non-IP datagram are mappedbased on port priority mode.‣ Schedule ModeWhen the network is congested, the problem that many packets compete for resources must besolved, usually in the way of queue scheduling. The switch implements four scheduling queues,TC0, TC1, TC2 and TC3. TC0 has the lowest priority while TC3 has the highest priority. The switchprovides four schedule modes: SP, WRR, SP+WRR and Equ.1. SP-Mode: Strict-Priority Mode. In this mode, the queue with higher priority will occupy thewhole bandwidth. Packets in the queue with lower priority are sent only when the queue withhigher priority is empty. The switch has four egress queues labeled as TC0, TC1, TC2 andTC3. In SP mode, their priorities increase in order. TC3 has the highest priority. Thedisadvantage of SP queue is that: if there are packets in the queues with higher priority for along time in congestion, the packets in the queues with lower priority will be “starved to death”because they are not served.109