10.2 Queuing Options
10.2.4 Priority queuing overview
Priority output queuing (PQ) provides a mechanism to use strict priority in selecting which packets to send first on an interface . This technique is useful in environments in which traffic has a hierarchy of importance, and more important traffic should not be delayed by less important traffic. For example, voice or video traffic that is time sensitive would be prioritized over FTP traffic.

PQ categorizes and prioritizes datagrams that travel on an interface. Traffic can be assigned to the various queues according to protocol or Transmission Control Protocol (TCP) port number. PQ controls time-sensitive traffic (such as Digital Equipment Corporation local-area transport such as voice traffic) or mission-critical traffic (such as transaction processing) on low-bandwidth serial links.

With PQ, the high-priority queue is always emptied before the medium-priority queue, and so on (as shown in Figure ). As a result, traffic in lower-priority queues might not get forwarded in a timely manner or not forwarded at all. For this reason, PQ provides the network administrator the most control over deciding which traffic gets forwarded.

An incoming packet is compared with the priority list to select a queue. If there is room, the packet is buffered in memory and waits to be dispatched after the queue is selected. If the queue is full, the packet is dropped. For this reason, controlling queue size is an important configuration task.

The queuing process empties the high-priority queue before the queuing software services the medium-priority queue. The dispatching algorithm controls the queuing process. The dispatching algorithm checks a queue for a packet and then dispatches it. Therefore, the high queue must be empty before the medium queue will get any service. As a result, mission-critical traffic in the high queue is always transmitted before traffic in other queues.

Be very careful when you define the packets that belong in the high queue because these packets are always processed first. If the high queue is always filled, packets in other queues do not have a chance to be transmitted.

You must configure a default queue for traffic that is not identified by the priority list.

WFQ automatically prioritizes traffic to ensure that all traffic is given fair access to bandwidth. Use PQ when you must guarantee that certain types of traffic receive as much of the available bandwidth as needed.