Chapter 14: Quiz – QoS (Answers) CCNPv8 ENCOR

13. Which statement describes a network that supports QoS?

  • The fewest possible devices are affected by a failure.
  • The network should be able to expand to keep up with user demand.
  • The network provides predictable levels of service to different types of traffic.
  • Data sent over the network is not altered in transmission.

Explanation: QoS means that the network can deliver services in a predictable, measurable, and, if necessary, guaranteed manner. QoS uses classification and queuing to manage the delivery of data, minimizing delay and packet loss.

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

  • DiffServ
  • best-effort
  • IntServ
  • soft QoS

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

15. Which DSCP value should be applied to voice traffic?

  • AF11
  • AF31
  • CS1
  • EF

Explanation: Voice traffic is extremely sensitive to bandwidth delays and jitter and should be marked with Expedited Forwarding (EF) DSCP value to ensure QoS. This marking would give voice traffic a iP equivalent precedence level of 5.

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

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

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.

17. In network design, which technology can be implemented to prioritize traffic based on its importance and technical requirements?

  • STP
  • QoS
  • RTP
  • TCP
  • VPN

Explanation: Quality of service (QoS) is used in a packet switched environment to provide traffic shaping and policing to achieve service quality in a given network.

18. What functionality is required on routers to provide remote workers with VoIP and videoconferencing capabilities?

  • IPsec
  • QoS
  • VPN
  • PPPoE

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

19. What are the three categories of tools that can be used in IP networks to implement QoS? (Choose three.)

  • classification and marking
  • congestion management
  • congestion avoidance
  • integrated services
  • differentiated services
  • best effort

Explanation: Best effort is the default packet forwarding design and provides no QoS. Differentiated services and integrated services are models of applying QoS mechanisms. Classification and marking, congestion management, and congestion avoidance are the three categories of QoS tools that are applied to IP networks.

20. When using the IP phones, users complain that sound quality is poor and sometimes the voice heard is broken. Which section of the router configuration should the network administrator check in order to find the possible cause?

  • access control lists
  • interface bandwidth
  • traffic queuing parameters
  • routing protocol update settings

Explanation: If priority queuing is not configured properly in the router configuration, it will cause voice packets to drop because of congestion in the queues. The result will be delay and jitter. The voice traffic needs to be prioritized above application and data traffic.

21. What type of queuing provides the best quality for voice applications?

  • custom queuing
  • FIFO queuing
  • priority queuing
  • weighted round robin (WRR) queuing

Explanation: Voice applications are very sensitive to bandwidth delays, latency, and jitter and would be most suited to the priority queuing method. Priority queuing (PQ) is a set of four queues (high,medium,normal and low). Because all the queues are served in strict priority order, this makes it suitable for voice applications that need high priority.

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

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

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.

23. Which QoS model uses the DSCP bits to mark packets and provides 64 possible classes of service?

  • IntServ
  • best-effort
  • DiffServ
  • FIFO

Explanation: The DiffServ model uses 6-bits known as the DiffServ Code Point (DSCP) bits to mark traffic and offers a maximum of 64 possible classes of service. Diffserv-aware routers can then implement per-hop behaviors (PHBs) that can control packet forwarding based on the specified class of service.

24. Which two statements are true about WRED? (Choose two.)

  • WRED will use the average queue depth of an interface when determining if a packet should be dropped.
  • WRED uses the IP precedence or DSCP marking of a packet when determining if a packet should be dropped.
  • WRED does not support ECN.
  • WRED uses a tail drop system for its queue buffers.
  • WRED will drop packets marked AFx1 more aggressively than it will drop packets marked AFx3 from the queue.

Explanation: Weighted random early detection (WRED) is a congestion avoidance mechanism. Different IP precedence and DSCP values each have RED profiles, which determine the probability of packet discard based on the average queue depth of an interface.

“Do I Know This Already?” Quiz Answers:

1. Which of the following are the leading causes of quality of service issues? (Choose all that apply.)

  • Bad hardware
  • Lack of bandwidth
  • Latency and jitter
  • Copper cables
  • Packet loss

Explanation: The leading causes of quality of service issues are lack of bandwidth, latency and jitter, and packet loss.

2. Network latency can be broken down into which of the following types? (Choose all that apply.)

  • Propagation delay (fixed)
  • Time delay (variable)
  • Serialization delay (fixed)
  • Processing delay (fixed)
  • Packet delay (fixed)
  • Delay variation (variable)

Explanation: Network latency can be broken down into propagation delay, serialization delay, processing delay, and delay variation.

3. Which of the following is not a QoS implementation model?

  • IntServ
  • Expedited forwarding
  • Best effort
  • DiffServ

Explanation: Best effort, IntServ, and DiffServ are the three QoS implementation models.

4. Which of the following is the QoS implementation model that requires a signaling protocol?

  • IntServ
  • Best Effort
  • DiffServ
  • RSVP

Explanation: IntServ uses Resource Reservation Protocol (RSVP) to reserve resources throughout a network for a specific application and to provide call admission control (CAC) to guarantee that no other IP traffic can use the reserved bandwidth.

5. Which of the following is the most popular QoS implementation model?

  • IntServ
  • Best effort
  • DiffServ
  • RSVP

Explanation: DiffServ is the most popular and most widely deployed QoS model. It was designed to address the limitations of the best effort and IntServ.

6. True or false: Traffic classification should always be performed in the core of the network.

  • True
  • False

Explanation: Packet classification should take place at the network edge, as close to the source of the traffic as possible, in an effort to provide an end-to-end QoS experience.

7. The 16-bit TCI field is composed of which fields? (Choose three.)

  • Priority Code Point (PCP)
  • Canonical Format Identifier (CFI)
  • User Priority (PRI)
  • Drop Eligible Indicator (DEI)
  • VLAN Identifier (VLAN ID)

Explanation: The TCI field is a 16-bit field composed of the 3-bit Priority Code Point (PCP) field (formerly PRI), the 1-bit Drop Eligible Indicator (DEI) field (formerly CFI), and the 12-bit VLAN Identifier (VLAN ID) field.

8. True or false: The DiffServ field is an 8-bit Differentiated Services Code Point (DSCP) field that allows for classification of up to 64 values (0 to 63).

  • True
  • False

Explanation: The IPv4 ToS field and the IPV6 traffic class field were redefined as an 8-bit Differentiated Services (DiffServ) field. The DiffServ field is composed of a 6-bit Differentiated Services Code Point (DSCP) field that allows for classification of up to 64 values (0 to 63) and a 2-bit Explicit Congestion Notification (ECN) field.

9. Which of the following is not a QoS PHB?

  • Best Effort (BE)
  • Class Selector (CS)
  • Default Forwarding (DF)
  • Assured Forwarding (AF)
  • Expedited Forwarding (EF)

Explanation: Four PHBs have been defined and characterized for general use:

  • Class Selector (CS) PHB: The first 3 bits of the DSCP field are used as CS bits; the class selector bits make DSCP backward compatible with IP Precedence because IP Precedence uses the same 3 bits to determine class.
  • Default Forwarding (DF) PHB: Used for best-effort service.
  • Assured Forwarding (AF) PHB: Used for guaranteed bandwidth service.
  • Expedited Forwarding (EF) PHB: Used for low-delay service.

10. Which traffic conditioning tool can be used to drop or mark down traffic that goes beyond a desired traffic rate?

  • Policers
  • Shapers
  • WRR
  • None of the above

Explanation: Policers drop or re-mark incoming or outgoing traffic that goes beyond a desired traffic rate.

11. What does Tc stand for? (Choose two.)

  • Committed time interval
  • Token credits
  • Bc bucket token count
  • Traffic control

Explanation: The Committed Time Interval (Tc) is the time interval in milliseconds (ms) over which the Committed Burst (Bc) is sent. Tc can be calculated with the formula Tc = (Bc [bits] / CIR [bps]) × 1000. For single-rate three-color markers/policers (srTCMs) and two-rate three-color markers/policers (trTCMs), Tc can also refer to the Bc Bucket Token Count (Tc), which is the number of tokens in the Bc bucket.

12. Which of the following are the recommended congestion management mechanisms for modern rich-media networks? (Choose two.)

  • Class-based weighted fair queuing (CBWFQ)
  • Priority queuing (PQ)
  • Weighted RED (WRED)
  • Low-latency queuing (LLQ)

Explanation: CBWFQ and LLQ provide real-time, delay-sensitive traffic bandwidth and delay guarantees while not starving other types of traffic.

13. Which of the following is a recommended congestion-avoidance mechanism for modern rich-media networks?

  • Weighted RED (WRED)
  • Tail drop
  • FIFO
  • RED

Explanation: WRED provides congestion avoidance by selectively dropping packets before the queue buffers are full. Packet drops can be manipulated by traffic weights denoted by either IP Precedence (IPP) or DSCP. Packets with lower IPP values are dropped more aggressively than are those with higher IPP values; for example, IPP 3 would be dropped more aggressively than IPP 5 or DSCP, and AFx3 would be dropped more aggressively than AFx2, and AFx2 would be dropped more aggressively than AFx1.

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