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Priority queuing
Figure 12
Priority queuing (PQ)
Priority queuing is designed for mission-critical applications. The key feature of
mission-critical applications is that they require preferential service to reduce the
response delay when congestion occurs. Priority queuing can flexibly determine the
order of forwarding packets by network protocol (for example, IP), incoming interface,
packet length, source/destination address, and so on. Priority queuing classifies packets
into four queues: top, middle, normal, and bottom, in descending priority order. By
default, packets are assigned to the normal queue. Each of the four queues is a FIFO
queue.
Priority queuing schedules the four queues strictly according to the descending order of
priority, as shown in
Figure 12
. It sends packets in the queue with the highest priority first.
When the queue with the highest priority is empty, it sends packets in the queue with the
second highest priority. In this way, you can assign the mission-critical packets to the
high priority queue to ensure that they are always served first. The common service
packets are assigned to the low priority queues and transmitted when the high priority
queues are empty.
The disadvantage of priority queuing is that packets in the lower priority queues cannot
be transmitted if there are packets in the higher queues for a long time.
