Exam PCNSE topic 1 question 409 discussion

B • Priority—Click and select a priority to assign it to a class: ◦ real-time ◦ high ◦ medium ◦ low When contention occurs, traffic that is assigned a lower priority is dropped. Real-time priority uses its own separate queue.

The answer B would be correct if the class 3 and class 5 traffic was under the Egress guranteed troughput. " Class 7 traffic will have the most packets dropped in favor of Classes 3 and 5 maintaining their Egress Guaranteed throughput. " --> it should be because of Egress Max config not guranteed so the first sentence is correct but reason is wrong. Only not false answer is in this case A

It is not A, because the total max egress is defined as 1,000 and the sum of the three kinds of traffic is bigger than that. B is the correct answer, due to the priority they have assigned, 7 being "low" priority.

Leaning heavily towards B. I believe A to be a fact, but the sum of the guaranteed max per class for all classes is already greater than 1000 ( the configured global max) so the fw will start dropping packets and class 7 will lose most of them according to priority. Basically, B is a "better" correct answer for this question than A

I think B is correct

The given image depicts that all the classes have already exceeded their bandwidth which translates to traffic 3,5, and 7 dropping more traffic compared to other classes.

A: Egress Guaranteed—The amount of bandwidth guaranteed for matching traffic. When the egress guaranteed bandwidth is exceeded, the firewall passes traffic on a best-effort basis. Bandwidth that is guaranteed but is unused continues to remain available for all traffic. Depending on your QoS configuration, you can guarantee bandwidth for a single QoS class, for all or some clear text traffic, and for all or some tunneled traffic. Example: Class 1 traffic has 5 Gbps of egress guaranteed bandwidth, which means that 5 Gbps is available but is not reserved for class 1 traffic. If Class 1 traffic does not use or only partially uses the guaranteed bandwidth, the remaining bandwidth can be used by other classes of traffic. However, during high traffic periods, 5 Gbps of bandwidth is absolutely available for class 1 traffic. During these periods of congestion, any Class 1 traffic that exceeds 5 Gbps is best effort https://docs.paloaltonetworks.com/pan-os/9-1/pan-os-admin/quality-of-service/qos-concepts/qos-bandwidth-management

I'm leaning towards B. Here's why: A seems irrelevent. The question is about classes 3,5, and 7. They're exceeding their guaranteed througput, so this doesnt really apply. B is accurate. 7 will have more drops as it's a lower priority. Although I believe this would only be dropped in times of congestion since the traffic above the guaranteed bandwidth is still forwarded as best effort. C isnt accurate. Egress max is the most a class can forward. This makes it sound as if guaranteed or not. D Nothing is hitting the egress max so this seems irrelevant.

Answer B: 325+470+330 > 1000 Egress Max

B as per @confusion

Should be C. All classes should be able to use available unused bandwidth, as far as it doesn't reach their MAX yet.