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2. 802.1

P Priority

Figure 9-2 802.1Q frame

As shown in the figure above, each 802.1Q Tag has a Pri field, comprising 3 bits. The 3-bit priority

field is 802.1p priority in the range of 0 to 7. 802.1P priority determines the priority of the packets

based on the Pri value. On the Web management page of the switch, you can configure different

priority tags mapping to the corresponding priority levels, and then the switch determine which

packet is sent preferentially when forwarding packets. The switch processes untagged packets

based on the default priority mode.

3. DSCP Priority

Figure 9-3 IP datagram

As shown in the figure above, the ToS (Type of Service) in an IP header contains 8 bits. The first

three bits indicate IP precedence in the range of 0 to 7. RFC2474 re-defines the ToS field in the IP

packet header, which is called the DS field. The first six bits (bit 0-bit 5) of the DS field indicate

DSCP precedence in the range of 0 to 63. The last 2 bits (bit 6 and bit 7) are reserved. On the Web

management page, you can configure different DS field mapping to the corresponding priority

levels. Non-IP datagram with 802.1Q tag are mapped to different priority levels based on 802.1P

priority mode; the untagged non-IP datagram are mapped based on port priority mode.

 Schedule Mode

When the network is congested, the problem that many packets compete for resources must be

solved, 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 switch

provides 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 the

whole bandwidth. Packets in the queue with lower priority are sent only when the queue with

higher priority is empty. The switch has four egress queues labeled as TC0, TC1, TC2 and

TC3. In SP mode, their priorities increase in order. TC3 has the highest priority. The

disadvantage of SP queue is that: if there are packets in the queues with higher priority for a

long time in congestion, the packets in the queues with lower priority will be “starved to death”

because they are not served.