Ccna certification Practice Tests

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Appendix 

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  Answers to Practice Test Questions

 77.  B.  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. DSCP AF 43 is an incorrect answer. DSCP AF 11 is an incorrect answer. DSCP 

AF 00 is an incorrect answer.

 78.  C.  The maximum delay that VoIP traffic should not exceed is 150 ms. At 150 ms, you will 

have call disruption. 10 ms can normally only be achieved on the same LAN; therefore, 

it is not a recommended maximum. 90 ms is the far end of the scale and sometimes 

seen in WAN communications. 300 ms is roughly one-third of a second and traffic will 

experience echoes and drops.

 79.  B.  Low Latency Queuing (LLQ) overrides Class Based Weighted Fair Queuing (CBWFQ). 

CBWFQ uses a weighted round-robin scheduling of packets. LLQ has priority override 

when packets come in matching the classification for LLQ. FIFO queues work on a first in, 

first out system but do not have a concept of priority. Committed information rate (CIR) is 

a term used with Frame Relay.

 80.  B.  QoS queue starvation occurs when the Low Latency Queuing (LLQ) is given priority 

over the Class-Based Weighted Fair Queuing (CBWFQ). Therefore, policing of the 

LLQ will help limit queue starvation and allow those queues an equal share of the total 

output bandwidth. Class-Based Weighted Fair Queuing is not a method to combat queue 

starvation. First in, first out (FIFO) is not a method to combat queue starvation.

 81.  A.  Shaping monitors the bit rate of packets. If the bit rate is exceeded for a configured 

queue, then shaping holds packets over the configured bit rate, causing a delay. Shaping of 

packets does not drop packets when the bandwidth is over the configured bit rate. Shaping 

will not use jitter as a control method when the bandwidth is over the configure bit rate. 

Shaping has no mechanism to control speed, only the rate at which packets are released.

 82.  C.  Class-Based Weighted Fair Queuing, or CBWFQ, is driven by a round-robin scheduler. 

The queues are weighted for priority in the scheduler and the packets are put into the 

queues upon classification. Low Latency Queueing (LLQ) does not use a round-robin 

scheduler. First in, first out (FIFO) does not use a round-robin scheduler. Priority 

Queueing (PQ) does not use a round-robin scheduler.

 83.  B.  Policing monitors the bit rate of packets. If the bit rate is exceeded for a configured 

queue, then policing drops packets over the configured bit rate, causing loss. In some cases 

it can be configured to remark the packets. Policing does not hold packets in the queue 

over the configured bit rate to cause delay. Policing does not hold packets in the queue over 

the configured bit rate to cause jitter. Policing will not slow packets in the queue over the 

configured bit rate to adhere to the bit rate.

 84.  B.  QoS policing should be implemented to adhere network traffic to a contracted 

committed information rate (CIR). As an example, if your enterprise contracted a Metro 

Ethernet connection with an access link of 1 Gb/s and a CIR of 400 Mb/s, you would 

need to make sure that traffic does not exceed the CIR except for occasional bursts. QoS 

policing is not used to police LAN applications. QoS policing is not used to police WAN 

applications. QoS will not help with maintaining a contracted burst rate.

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Chapter 4: IP Services (Domain 4)