QoS

[WhiteHat]

Hello Friends

        i hope my message finds you well, I have been studying lately QoS from Todd Lammle book and i wanted to test my knowledge after finish the chapter. I serached online questions for practice and found this question with the highlighted answer but referring to to my study it is WFQ.

Weighted Fair Queuing is allows to prioritize the real-Time traffic unlike Round-Robin mechanism. 

PQ Priority Queuing doesn't guarantee packet delivery for low-priority traffic because it process the low-priority traffic when high-priority queue is empty similar to strict priority traffic scheduling.. Why in the question the correct answer is PQ .. so confuses    

Thank you in advance 

[Rich Uline]

WhiteHat,

I think this question is a good example of questions on Cisco exams that have multiple correct answers, but you have to choose the most-correct one. In this case, the question states that the network for which you want to optimize voice traffic primarily passes data traffic. If you were to configure Weighted Fair Queueing, it would allocate an equal share of the bandwidth to each flow. In situations with low congestion (few flows), the VoIP traffic may be allocated more bandwidth than it needs. Conversely, in situations with high congestion (many flows), the VoIP traffic might be edged out by the data traffic and be allocated less bandwidth than it needs. As voice traffic is fairly low bandwidth (even high-bandwidth codecs only use something like 80kbps on the wire) it can be configured for absolute priority under Priority Queueing, while still leaving enough bandwidth for the data traffic.

[WhiteHat]

Rich Uline
Thank you for the provided information, i understood your point and it seems it applies more to practical life work, But referring to the CCNA study frame and books (correct me if i am wrong) the PQ is not good for real-time traffic as it will drop low-priority if the high-priority queue is empty . Please have a closer look with me to the academic reference for both of them and share with me what would choose if you are me.
Todd Lammle Book:
Priority queuing (PQ):
This is not really a good queuing method because lower-priority queues are served only when the higher-priority queues are empty. There are only four queues, and low-priority traffic many never be sent
Weighted fair queuing (WFQ):
This was actually a pretty popular way of queuing for a long time because it divided up the bandwidth by the number of flows, which provided bandwidth for all applications. This was great for real-time traffic, but it doesn’t offer any guarantees for a particular flow.

[Joseph W. Doherty]

The PQ being referenced, in Lammi's book, I believe is an old Cisco QoS technique. I.e. PQ using four queues, each having absolute priority over all queues below it.

The term PQ is also used for current Cisco switch QoS that provides one queue with absolute priority over all other queued traffic. This latter "PQ", for devices that support it, is generally used for traffic like VoIP bearer traffic.

As to why the latter is better, because VoIP is considered more important. I.e. given a choice between poor VoIP quality vs. losing some data traffic, generally the data traffic is lost (especially as data traffic often uses TCP which will re-transmit the lost data traffic - while VoIP traffic that's lost, is not re-transmitted).