Congestion Management and Queuing - Priority Queuing

PQ - Priority Queuing uses four queues, high/medium/low/normal. Packets must be assigned to each queue, if packets are not assigned they will be put into the normal queue. Access-lists are commonly used to identify the traffic to be queued.

1. If packets exist in the high priority queue then packets in other queues will not be processed until it is empty.
2. If the high priority queue is empy then one packet will be processed from the medium priority queue.
3. If the medium priority queue is empty then a packet from the normal queue is processed.
4. If the normal priority queue is empy then a packet from the low priority queue is processed.

After processing one packet the packet schedular will always start from the beginning again, checking the priority queues in the order described above. The problem with this is that low priority queues may be starved if higher priority queues have traffic waiting constantly.

Cisco IOS command priority-list can be used to define the queue that packets should be sent to.