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with higher priority, and only when the queue with higher priority is empty, packets in the queue

with lower priority are sent.

2. 802.1P Based

Figure 8-2 802.1Q 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. The 802.1p priority value determines how the switch

maps the ingress packets to the priority queues. The mapping relationship between eight 802.1p

priority value and priority queues is shown as follows:

Figure 8-3 Map 802.1P priority

 Priority 1 and 2 are assigned to the 1 (Lowest) priority queue.

 Priority 0 and 3 are assigned to the 2 (Normal) priority queue.

 Priority 4 and 5 are assigned to the 3 (Medium) priority queue.

 Priority 6 and 7 are assigned to the 4 (Highest) priority queue.

When 802.1P QoS mode is enabled, the switch will automatically map the ingress packets to

priority queues based on the 802.1p priority and the above mapping relationship. After that, the

switch will preferentially send packets in the queue with higher priority, and only when the queue

with higher priority is empty, packets in the queue with lower priority are sent. As for the untagged

packets, the switch will forward it according to the default port-based QoS mode.

3. DSCP based

Figure 8-4 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