New CCNA QOS Questions & Answers With Explanation

By | November 24, 2017

Question 1.

Where should QoS marking be performed?

A. Closest to the source of the traffic
B. Closest to the Internet router
C. On every device in the network
D. On the core router in the network

Answer A.

Explanation

It’s highly recommended that QoS marking should always be performed closest to the source of the traffic. All
switches and routers in the network should be configured to properly prioritize markings of traffic in queues.

Question 2.

Which measurement describes the time a packet takes from source to
destination?
A. Bandwidth
B. Delay
C. Jitter
D. Loss

Answer B.

Explanation

Delay is the time it takes for a packet to travel from source to destination,
which is a description of one-way delay.
Round-trip delay is the time it takes for the packet to travel from source to destination (one-way delay) plus the time it takes for the destination computer to send the packet back to the originating node to form a round-trip.

Question 3.

How do routers classify traffic for QoS?

A. Access control lists
B. Layer 2 ASICs
C. Route tables
D. Frame filters

Answer A.

Explanation

QoS classifies traffic with access control lists and applies markings to the
packets.




Question 4.

Which measurement describes the variation of consecutive packet time from
source to destination?
A. Bandwidth
B. Delay
C. Jitter
D. Loss

Answer C.

Explanation

Jitter is the measurement of variation between consecutive packet times from source to destination (one-way delay). For example, if the first packet takes 10 ms, and the second, third, and fourth take 10 ms, the jitter or variance is 0 ms. The simple calculation is an average of packet times. However, data size has an influence on jitter, so the more accurate calculation is J = J + (D (I – 1 , I) – J) / 16.

Question 5

Which is a true statement about the Class of Service field?

A. The Class of Service field is a layer 3 field.
B. The Class of Service field is only present in 802.1Q frames.
C. The Class of Service field is present from end to end of a transmission.
D. The Class of Service field is 6 bits.

Answer B.

Explanation

The Class of Service field is only found in layer 2 transmissions, specifically only across trunks due to the dependency of 802.1Q. The CoS field is a 3-bit field in the 802.1Q frame type.

Question 6.

What is the measurement of packets discarded due to congestion of queues?
A. Bandwidth
B. Delay
C. Jitter
D. Loss

Answer D.

Explanation

A loss of QOS traffic is the measurement of discarded packets. The measurement is a percentage of transmitted packets. For example, if 100 packets are transmitted and 3 packets are dropped, then the loss is 3%. Loss can be attributed to congestions, faulty wiring, EMI, or device queue congestion.

Question 7

Which technology is used to identify and mark applications such as BitTorrent and Netflix for QoS?
A. ACLs
B. Routing tables
C. NBAR2
D. Tail drop

Answer C.

Explanation

Network-Based Application Recognition version 2 is used to classify applications based on a Protocol Description Language Module (PDLM).

Question 8.

Which Layer 3 protocol is used for marking packets with QoS?
A. DSCP
B. 802.1Q
C. CoS
D. QoE

Answer A.

Explanation

The Differentiated Services Code Point (DSCP) is a 6-bit value in the Type of Service (ToS) field of the IP header. The DSCP value defines the importance of packets at layer 3.




Question 9

Which is a correct statement about device trust boundary for QoS?

A. The trust boundary should always be an asset IT controls.
B. The switch should always be set as the trust boundary.
C. Only routers can create trust boundaries.
D. Only IP phones can become trust boundaries.

Answer A.

Explanation

A malicious user can mark all of his or her traffic as high priority. Therefore, a
trust boundary must be established by the network administrator. A common trust
boundary device is the IP phone, but it is any device that is in control of the network
administrator.

Question 10.

Which DSCP marking has the highest priority?
A. DSCP AF 43
B. DSCP EF 46
C. DSCP AF 11
D. DSCP AF 00

Answer. B.

Explanation

The standardized marking of DSCP EF, or Expedite Forwarding, is a decimal
equivalent of 46. This marking has the highest priority and should be used for VoIP
traffic and video