Cisco CCNP Certification Training Tutorial: Priority Queuing For The ONT Exam
Priority queuing is a vital topic for your Cisco CCNP ONT exam studies, and it seems simple enough - but there are dangers you must avoid!† Learn the ins and outs of priority queuing from Chris Bryant, CCIE #12933.
You canít 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 Ė youíll see them all and more on your ONT exam.†† Today, weíll 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 canít 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.
Itís 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.† Itís 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 itís 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.† Thatís called queue starvation or packet starvation, but whatever you call it, itís a danger with priority queuing Ė a danger you must avoid!
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ABOUT THE AUTHOR
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, ďHow To Pass The CCNAĒ, is also available. Earn your Microsoft Server 2008 certification with The Bryant Advantage!