You can’t earn your CCNP certification without passing the Cisco ONT exam, and queuing is a huge topic on that exam! Priority queuing, LLQ, weighted fair queuing – you’ll see them all and more on your ONT exam. Today, we’ll take a close look at priority queuing theory.
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.
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!
Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free Cisco CCNP Training and CCNA Exam Training tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages.
You can also visit his blog, which is updated several times daily with new Cisco certification articles, free tutorials, and daily CCNA / CCNP exam questions!
Visit his blog and sign up for Certification Central, a daily newsletter packed with CCNA, Network+, A+, and CCNP certification exam practice questions. A free 7-part course, “How To Pass The CCNA", is also available.
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Article added to SearchWarp.com on Monday, July 23, 2007 View other articles written by Chris Bryant CCIE 12933(13,715)
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