Another queuing technique that can be employed on Cisco routers is priority queuing, which uses four different queue levels – high, medium, normal, and low. Traffic can be allocated to these queues based on criteria like protocol or port number. For example, a company might allocate VoIP traffic to the high queue, IPX traffic to the medium queue, telnet traffic to the normal queue, and FTP traffic to the low queue.
Priority queuing works by emptying each queue according to its level of precedence, for example, the high queue will always be emptied first, followed by the medium, normal, and low queues. If the high queue is always full, it will monopolize the network by continually being serviced at the expense of the other queues. Once the high queue is empty, the router will start working on the medium queue, and so forth. However, if more packets were to enter a higher-priority queue, the router will immediately switch back to servicing those packets first. As such, higher-priority traffic has the ability to monopolize access to the network, and packets in lower-priority queues may be delayed or dropped.