9.6.2 Module Quiz - QoS Concepts (Answers)

9.6.2 Module Quiz – QoS Concepts Answers

1. What is the term used to indicate a variation of delay?

speed mismatch
serialization delay
jitter
latency

Explanation: Jitter is a phenomenon caused by a variation in delay. Delay (or latency) is the time it takes for the packet to arrive at its destination. Jitter describes how the voice packets arrive at the destination at varying intervals (not in a steady, consistent stream).

2. A network engineer performs a ping test and receives a value that shows the time it takes for a packet to travel from a source to a destination device and return. Which term describes the value?

jitter
bandwidth
priority
latency

Explanation: Latency refers to the time it takes for a packet to travel from a source device to a destination device. Jitter is the variation in the latency values of received packets. Bandwidth is the measurement of how much traffic is transmitted in a single second. Priority is the preferential treatment of specific classes of traffic.

3. What role do network devices play in the IntServ QoS model?

Network devices provide a best-effort approach to forwarding traffic.
Network devices ensure that resources are available before traffic is allowed to be sent by a host through the network.
Network devices use QoS on a hop-by-hop basis to provide excellent scalability.
Network devices are configured to service multiple classes of traffic and handle traffic as it may arrive.

Explanation: The IntServ QoS model uses resource reservation to guarantee bandwidth and packet-loss rates from end to end. IntServ uses a connection-oriented approach to ensure that available resources are sufficient in the network for the traffic to have a specific level of QoS.

4. Which device would be classified as a trusted endpoint?

firewall
switch
IP phone
router

Explanation: Trusted endpoints are devices that have the capability to mark application traffic at Layer 2 or Layer 3. Trusted endpoints include the following:
– IP phones
– Wireless access points
– Videoconferencing gateways and systems
– IP conferencing stations and more

5. Under which condition does congestion occur on a converged network with voice, video, and data traffic?

if video traffic requests more bandwidth than voice traffic requests
if the request for bandwidth exceeds the amount of bandwidth available
if voice traffic latency begins to decrease across the network
if a user downloads a file that exceeds the file limitation that is set on the server

Explanation: Network congestion is caused by devices requesting more bandwidth than is available on a network. Congestion causes traffic to be temporarily queued or ultimately dropped.

6. Which type of traffic does Cisco recommend be placed in the strict priority queue (PQ) when low latency queuing (LLQ) is being used?

management
video
data
voice

Explanation: Cisco recommends that only voice traffic be placed in the priority queue (PQ) when using the low latency queuing (LLQ) method. All other traffic types are to be placed in other traffic queues and will be forwarded after voice traffic.

7. Which model is the only QoS model with no mechanism to classify packets?

hard QoS
DiffServ
best-effort
IntServ

Explanation: The best-effort model has no mechanism to classify packets and treats all networks packets equally. A voice packet would be treated in the same manner as an email packet.

8. What happens when the memory queue of a device fills up and new network traffic is received?

The network device will drop the arriving packets.
The network device sends the received traffic immediately.
The network device drops all traffic in the queue.
The network device queues the received traffic while sending previously received traffic.

Explanation: Packets received by a network device experiencing congestion cannot be transmitted until previous packets have been processed. If the queue is full, the network device will begin to drop packets as new traffic arrives.

9. What are two characteristics of voice traffic? (Choose two.)

It is bursty.
It consumes few network resources.
It is insensitive to packet loss.
It can tolerate latency up to 400 ms.
It is delay sensitive.

Explanation: Voice traffic does not consume much in the way of network resources, such as bandwidth. However, it is very sensitive to delay and dropped packets. For good voice quality, the amount of latency should be less than 150 milliseconds and packet loss less than 1%.

1o. Which QoS model is very resource intensive and provides the highest guarantee of QoS?

IntServ
DiffServ
soft QoS
best-effort

Explanation: The IntServ QoS model uses resource reservation and admission control mechanisms to schedule network resources.

11. What happens when an edge router using IntServ QoS determines that the data pathway cannot support the level of QoS requested?

Data is not forwarded along the pathway.
Data is forwarded along the pathway using a best-effort approach.
Data is forwarded along the pathway using DiffServ.
Data is forwarded along the pathway using IntServ but not provided preferential treatment.

Explanation: When the IntServ QoS model is being used, if network devices along a data path can reserve the required bandwidth, the originating application can begin transmitting. If the requested resource reservation cannot be allocated along the path, the originating application does not send any data.

12. In QoS models, which type of traffic is commonly provided the most preferential treatment over all other application traffic?

voice traffic
web traffic
email
file transfers

Explanation: Voice traffic from IP phones is commonly provided preferential treatment over all other application traffic such as email, web traffic, and file transfers.

13. Which queuing mechanism supports user-defined traffic classes?

WFQ
CBWFQ
FCFS
FIFO

Explanation: Class-based weighted fair queuing (CBWFQ) goes beyond weighted fair queuing (WFC) by providing support for user-defined traffic classes. With CBWFQ, an administrator can use access control lists to define different traffic classes. Once traffic is classified, it receives is own queue for the egress interface.

14. What mechanism compensates for jitter in an audio stream by buffering packets and then replaying them outbound in a steady stream?

WFQ
voice codec
digital signal processor
playout delay buffer

Explanation: Jitter on the network can cause audio problems that can be heard and which are distracting in voice conversations. To compensate for the effects of jitter, a mechanism called a playout delay buffer is used. The playout delay buffer receives and buffers voice packets and then retransmits them in a steady stream with the jitter removed.

15. What functionality is required on routers to provide remote workers with VoIP and video-conferencing capabilities?

IPsec
PPPoE
QoS
VPN

Explanation: Quality of service (QoS) needs to be enabled on routers to provide support for VoIP and video conferencing. QoS refers to the capability of a network to provide better service to selected network traffic, such as voice and video traffic.

16. What happens when a router interface ingress queue is full and new network traffic is received?

The router sends the received traffic immediately.
The router drops the arriving packets.
The router drops all traffic in the queue.
The router queues the received traffic and sends previously received traffic.

Explanation: When the volume of traffic is greater than what can be transported across the network, devices queue, or hold, the packets in memory until resources become available to transmit them. If the number of packets to be queued continues to increase, the memory in the device fills up, and packets are dropped.

17. Which queuing method provides user-defined traffic classes where each traffic class has a FIFO queue?

CBWFQ
RSVP
WFQ
WRED

Explanation: CBWFQ extends the standard WFQ functionality to provide support for user-defined traffic classes. A FIFO queue is reserved for each class, and traffic belonging to a class is directed to the queue for that class.

18. Which type of traffic does Cisco recommend be placed in the strict priority queue when Low Latency Queuing (LLQ) is being used?

Data
Management
Video
Voice

Explanation: With LLQ, delay-sensitive data is sent first, before packets in other queues are treated. Although it is possible to enqueue various types of real-time traffic to the strict priority queue, Cisco recommends that only voice traffic be directed to the priority queue.

19. What is the default queuing method used on the LAN interfaces of Cisco devices?

CBWFQ
FIFO
LLQ
WFQ

Explanation: When no other queuing strategies are configured, all interfaces except serial interfaces at E1 (2.048 Mbps) and below use FIFO by default. Serial interfaces at E1 and below use WFQ by default.

20. What is the default queuing method used on the slower WAN interfaces of Cisco devices?

CBWFQ
FIFO
LLQ
WFQ

21. Which statement describes the QoS classification and marking tools?

Classification is performed after traffic is marked.
Classification should be done as close to the destination service as possible.
Marking involves adding a value to a packet header.
Marking involves identification of the QoS policy that should be applied to specific packets.

Explanation: Marking means adding a value to the packet header. Devices receiving the packet look at this field to see whether it matches a defined policy. Marking should be done as close to the source device as possible to establish the trust boundary.

22. Which device would be classified as a trusted endpoint?

Firewall
IP conferencing station
Router
Switch

Explanation: Trusted endpoints have the capabilities and intelligence to mark application traffic to the appropriate Layer 2 CoS and/or Layer 3 DSCP values. Examples of trusted endpoints include IP phones, wireless access points, video-conferencing gateways and systems, and IP conferencing stations.

23. How many bits are used to identify the Class of Service (CoS) marking in a frame?

Explanation: The 802.1p standard uses the first 3 bits in the Tag Control Information (TCI) field. Known as the Priority (PRI) field, this 3-bit field identifies the Class of Service (CoS) markings. Three bits means that a Layer 2 Ethernet frame can be marked with one of eight levels of priority (values 0–7).

24. How many levels of priority are possible when using Class of Service (CoS) marking on frames?

Explanation: RFC 2474 redefines the ToS field with a new 6- bit Differentiated Services Code Point (DSCP) QoS field. Six bits offers a maximum of 64 possible classes of service.