Cisco CCNP Certification Training / ONT Exam Tutorial:
Introduction To Priority Queuing
By Chris Bryant, CCIE #12933
To earn your Cisco CCNP certification, you’ve got to pass the ONT exam – and there’s no more important topic on the ONT exam than queuing. In this series of tutorials, we’ll take a look at the theory and configuration of different queuing strategies, and we’ll start with Priority Queuing (PQ).
PQ has four and only four queues. All four are predefined as to priority and capacity. Note that the Normal queue is the default queue; packets that have not had a priority explicitly assigned to them are placed into that queue.
As you'll see in just a moment, changing the capacity of any of these queues is easy. The difficult part of working with PQ is resisting the temptation to configure a lot of traffic as high priority (yeah, I know, everybody's traffic is high priority - just like their email, right?).
Why is this a problem? PQ does not work in a round-robin format, as some other queuing strategies do. Regardless of how much traffic is waiting in the lower queues, the High-priority queue is always going to be given first priority, and that means traffic in the lower-priority queues can sit there for a long time. Let's take a look at one basic scenario that illustrates this.
As the network admin, you decide that FTP packets should be given the highest priority possible. You configure FTP packets to be placed in the High-priority queue, and they're transmitted before any other traffic. Since you've defined no other priorities, all other traffic is placed into the Normal queue.
This in itself would probably not cause a problem, but the temptation is to give too many other traffic types a higher priority as well. If too many traffic types are placed into the higher queues, packets can end up sitting in the lower queues for too long.
With PQ, anytime a packet enters the High queue, the router will stop transmitting any other queue's traffic and transmit the high-priority traffic. The router can be transmitting traffic normally, in this case from the Normal queue...
... but it only takes a single packet to arrive on a higher-priority queue for the router to stop all other transmissions in favor of the newly-arrived packet.
The packets in the other queues now have to wait their turn – and if too many packets enter those higher-priority queues, that turn may not come for quite a while!
This is referred to as packet starvation and queue starvation, but no matter what you call it, it's bad! The first step toward preventing this scenario is resisting the temptation to define too many different traffic types as high or medium priority.
And how do you define traffic with PQ? I'll show you how in Part 2 of this Cisco CCNP ONT Exam Training tutorial!
Get ready for the ultimate in CCNP ONT exam certification training - with The Ultimate ONT Study Package!
To your success,
Chris Bryant
CCIE #12933
chris@thebryantadvantage.com
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