Cisco Exam #642-642 QOS Exam (Cisco Press, set 2)
Check your knowledge of Cisco's Quality of Service protocols with these 15 sample exam questions and answers.
courtesy of Cisco Press
Question:
1. Which of the following are benefits of CQ being enabled on a Cisco router interface?
a. Prioritizes latency-sensitive packets so that they are always scheduled next.
b. Reserves a minimum amount of bandwidth for particular classes of traffic.
c. Provides a place to hold packets in RAM until space becomes available in the hardware queue for the interface.
d. Provides a queue so that as soon as the interface is available to send another packet, the packet can be sent, without requiring an interrupt to the router CPU.
e. Allows configuration of a percentage of the remaining link bandwidth, after allocating bandwidth to the LLQ and the class-default queue.
2. Which of the following are reasons why WFQ might discard a packet instead of putting it into the correct queue?
a. The hold-queue limit for all combined WFQ queues has been exceeded.
b. The queue length for the flow has passed the WRED minimum drop threshold.
c. The WFQ queue length for the queue where the newly-arrived packet should be placed has exceeded the CDT.
d. ECN feedback has been signaled, requesting that the TCP sender slow down.
3. What is the largest number of classes inside a single policy map that can be configured as an LLQ?
a. 1
b. 2
c. 3
d. more than 3
4. Which of the following are true about Policers in general, but not true about Shapers?
a. Monitors traffic rates using concept of token bucket.
b. Can discard traffic that exceeds a defined traffic rate.
c. Can delay packets by queuing in order to avoid exceeding a traffic rate.
d. Can re-mark a packet.
5. If shaping was configured with a rate of 128Kbps, and a Bc of 3200, what value would be calculated for Tc?
a. 125 ms
b. 125 sec
c. 25 ms
d. 25 sec
e. Shaping doesn’t use a Tc
f. Not enough information to tell
6. With single-rate policing, with three possible actions configured, how does the policer replenish buckets into the excess token bucket?
a. By filling Bc * Tc tokens into the first bucket each time interval, with spilled tokens refilling the excess token bucket.
b. By refilling the first bucket, based on a pro-rated amount of Bc, with spilled tokens refilling the excess token bucket.
c. By Be * Tc each time interval.
d. By putting a pro-rated amount of Be into the excess token bucket directly.
e. By Be tokens each second.
7. Which of the following commands, when typed in the correct configuration mode, enables CB policing at 128 kbps, with no excess burst?
a. police 128000 conform-action transmit exceed-action transmit violate-action discard
b. police 128 conform-action transmit exceed-action transmit violate-action discard
c. police 128000 conform-action transmit exceed-action discard
d. police 128 conform-action transmit exceed-action discard
e. police 128k conform-action transmit exceed-action discard
8. For which of the following WRED categories will WRED discard a subset of the packets?
a. Tail Drop
b. Full Drop
c. Random Drop
d. Partial Drop
e. No Drop
9. Imagine that WRED with ECN has been configured for a CBWFQ class. Under which of the following cases could WRED randomly choose to discard a packet, but instead, mark the ECN bits inside the packet header and allowing the packet to pass?
a. Average queue depth between the min and max thresholds, plus the incoming packet ECN field set to 00.
b. Average queue depth between the min and max thresholds, plus the incoming packet ECN field set to 01 or 10.
c. Average queue depth above max threshold, plus the incoming packet ECN field set to 01 or 10.
d. Average queue depth between the min and max thresholds, plus the incoming packet TCP ECE flag must be set to “1”, and ECN field must be set to 01.
10. Which of the following Modular QoS Command-Line Interface (MQC) class subcommands enables CB RTP header compression?
a. compression ip header rtp
b. compression header ip rtp
c. compression ip rtp
d. compression header
e. compression header ip
11. What serialization delay, in milliseconds, would be experienced by a 160 byte fragment on a 64-kbps link?
a. 5
b. 10
c. 15
d. 20
e. 25
f. 30
12. By default, what values will the Catalyst 2950 trust?
a. CoS
b. DSCP
c. CoS and DSCP
d. None of the above
13. How many egress queues are present on each Ethernet interface of the Catalyst 2950?
a. 1
b. 2
c. 4
d. 8
14. Which scheduling method is configured by default on a Catalyst 2950?
a. Strict priority scheduling
b. Custom scheduling
c. Strict priority and WRR scheduling
d. MDRR scheduling
e. WRR scheduling
f. FIFO scheduling
15. Which of the following commands is used to enable strict priority with WRR scheduling on the Catalyst 2950?
a. wrr-queue bandwidth 20 1 80 P
b. wrr-queue bandwidth 0 20 1 80
c. wrr-queue bandwidth 20 1 80 0
d. wrr-queue bandwidth P 20 1 80
1. Answers B and C are correct.
2. Answers A and C are correct.
3. Answer D is correct.
4. Answers B and D are correct.
5. Answer C is correct.
6. Answer B is correct.
7. Answer C is correct.
8. Answer C is correct.
9. Answer B is correct.
10. Answers B and E are correct.
11. Answer D is correct. The answer is derived by dividing the number of bits in the fragment by the link speed, and converting to milliseconds: 160 bytes x 8 bits/byte / 64,000 bits/second = .02 seconds = 20ms.
12. Answer D is correct.
13. Answer C is correct.
14. Answer A is correct.
15. Answer C is correct.
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