24 - Questions - Quality of Service (QoS)

How many bits constitute the DSCP field of the IP header?

6 bits

Where within an enterprise network should classification and marking, and policing be performed?

Access layer

What is one way that QoS achieves a better and more predictable network service?

By providing tools for managing network congestion.

Services are provided to which entities in the DiffServ model?

Classes of traffic

Which QoS tools analyze the traffic and decide which category the packet belongs in?

Classification and marking tools

Which option is a Layer 2 QoS marking?

CoS

Your boss has asked you to analyze the network traffic of your company. Which two types of traffic are considered delay-sensitive? (Choose two.)

Interactive database application live streaming video

Which three steps need to be taken to identify network traffic? (Choose three.)

Network audit Business audit Understand service levels

Which QoS mechanism will drop the traffic if the flow uses more than the allotted bandwidth?

Policing

Which term is used to describe the time between a packet being transmitted and a packet being received by an endpoint?

The correct answer is "Delay." Delay is the time that it takes for a packet to reach the receiving endpoint after being transmitted by the sender.

Which PHB would be indicated if the DSCP was equal to decimal 46 (binary 101110)?

The correct answer is "EF PHB". Packets requiring EF PHB should be marked with a DSCP binary value of 101110, or 46.

Which queuing mechanism has a single queue and sends packets in the exact order that they arrived?

The correct answer is "FIFO." FIFO is a single queue with packets that are sent in the exact order that they arrived. PQ is a set of four queues that are served in strict-priority order. CQ is a set of 16 queues with a round-robin scheduler. WFQ divides interface bandwidth by the number of flows, thus ensuring proper distribution of the bandwidth for all applications. Lastly, LLQ consists out of PQ and CBWFQ.

Which statement regarding QoS mechanisms is correct?

The correct answer is "Packets are marked after classification." Classification should happen as few times as possible because it takes time and uses up resources. For that reason, packets are marked after classification, usually at the ingress edge of a network. Lastly, when there is a congestion on a link, the traffic can be policed (dropped), or shaped (slowed down).

Which statement about QoS is correct?

The correct answer is "QoS does not treat all traffic as equal." Quality of service manages contention for network resources to maximize the end-user experience. QoS gives priority to some traffic over other traffic types. For example, when there is a congestion on a link, less important traffic is delayed or dropped in favor of delay-sensitive, business-important traffic.

Which option is a congestion avoidance mechanism?

The correct answer is "WRED." Cisco uses WRED, which is a congestion avoidance mechanism that drops packets randomly if congestion occurs, which results in avoiding TCP global synchronization. The randomness is skewed by traffic weights.

Which QoS mechanism will delay traffic if a session uses more than the allotted bandwidth?

The correct answer is "shaping." A classifier is a tool that inspects packets within a field to identify the type of traffic that the packet is carrying. Traffic is then directed to a policy enforcement mechanism, which includes marking, queuing, policing, and shaping. The aim of marking is to preserve the classification decision that the classifier tool reached. When traffic exceeds the allocated rate, the policers drop traffic, while the shapers slow down the traffic by delaying it.

Which statement is a general guideline for implementing campus QoS?

The correct answers are: "Classify and mark applications as close to their sources as technically and administratively feasible." Enterprise Campus Guidelines are: classify and mark applications as close to their sources as technically and administratively feasible; police unwanted traffic flows as close to their sources as possible; always perform QoS in hardware rather than software when a choice exists; enable queuing policies at every node where the potential for congestion exists; protect the control plane and the data plane.

Which three parameters are used to define QoS policy for a class of traffic? (Choose three.)

The correct answers are: "minimum or maximum bandwidth," "congestion-management technique," and "traffic priority." Defining the QoS policy for each traffic class requires setting the minimum and maximum bandwidth limits, assigning priority to the class, and using QoS technologies to manage congestion.

Imagine that you are the sole network engineer for your company. Suddenly, your users complain that the network is very slow. After further enquiry you found out that there are several users that consume a lot of bandwidth by using torrent/peer-to-peer applications. What are the two use cases that you could use QoS to solve this problem? (Choose two.)

The offending traffic can be dropped. The offending traffic can be throttled to minimal bandwidth.

Which type of traffic is smooth and predictable?

Voice