54Priority queuingFigure 12 Priority queuing (PQ)Priority queuing is designed for mission-critical applications. The key feature ofmission-critical applications is that they require preferential service to reduce theresponse delay when congestion occurs. Priority queuing can flexibly determine theorder of forwarding packets by network protocol (for example, IP), incoming interface,packet length, source/destination address, and so on. Priority queuing classifies packetsinto four queues: top, middle, normal, and bottom, in descending priority order. Bydefault, packets are assigned to the normal queue. Each of the four queues is a FIFOqueue.Priority queuing schedules the four queues strictly according to the descending order ofpriority, 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 thesecond highest priority. In this way, you can assign the mission-critical packets to thehigh priority queue to ensure that they are always served first. The common servicepackets are assigned to the low priority queues and transmitted when the high priorityqueues are empty.The disadvantage of priority queuing is that packets in the lower priority queues cannotbe transmitted if there are packets in the higher queues for a long time.
