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Priority queuing is unique in that it has four pre-configured queues, and while we have some control over those queues, we can't add more of them. Here are the four queues and their default capacity. Each capacity shown here can be changed, but as with all Cisco routing defaults, you should have a very good reason to do so!
High Priority queue, 20-packet capacity.
Medium Priority queue, 40-packet capacity.
Normal Priority queue, 60-packet capacity. (This is the default queue for all traffic when PQ is in use.)
Low queue, 80-packet capacity.
It's up to the network administrator to configure what types of traffic will be placed into each queue, and the key to success with PQ is not defining too many traffic types as high priority. It's vital to remember that PQ is *not* fair, and does *not* work in a round-robin fashion. When packets arrive in the High queue, PQ drops everything it's doing in order to transmit those packets, and packets in other queues are ignored until the High queue is again empty.
If you have too many packet types being placed into the High queue, and even the Medium queue, traffic in the lower-priority Normal and Low queues ends up just sitting there. That's called queue starvation or packet starvation, but whatever you call it, it's a danger with priority queuing – a danger you must avoid!
That's priority queuing theory; now we need to work on some configurations, and we'll do just that in the next installment of this CCNP certification training series!