Cisco Exam #642-642: QOS (Cisco Press, set 1)
Check your knowledge of Cisco's Quality of Service protocols with these 15 sample exam questions and answers.
courtesy of Cisco Press
Question:
1. TCP Slow Start controls the rate a TCP sender sends data by controlling:
a. Growth of the Advertised Window
b. Growth of the Congestion Window
c. Calculation of the Average Queue Depth
d. The wait-for-acknowledgement timer
2. Which of the following is the most likely reason for packet loss in a typical network?
a. Bit errors during transmission
b. Jitter thresholds being exceeded
c. Tail drops when queues fill
d. TCP flush messages as a result of Round-Trip Times varying wildly
3. Which of the following are not one of the major planning steps when implementing QoS Policies?
a. Divide traffic into classes
b. Define QoS policies for each class
c. Mark traffic as close to the source as possible
d. Identify traffic and its requirements
4. Which of the following tools monitors the rate at which bits are sent out an interface?
a. LLQ
b. CB Shaping
c. WRED
d. CB Policing
e. MLP LFI
5. According to the DiffServ RFCs, which PHB defines a set of three DSCPs in each service class, with different drop characteristics for each of the three DSCP values?
a. Expedited Forwarding
b. Class Selector
c. Assured Forwarding
d. Multi-class-multi-drop
6. Imagine a packet is marked with AF31. Later, a QoS tool classifies the packet. Which of the following classification criteria would match the packet, assuming the marking had not been changed from the original AF31 marking?
a. Match of DSCP CS3
b. Match of precedence 3
c. Match on DSCP AF32
d. Match on DSCP AF31
e. Match on DSCP 24
7. Which of the following MQC commands is most related to the process of classifying packets into service classes?
a. service-policy
b. Route-map
c. map-policy
d. policy-map
e. map-class
f. class-map
8. Which of the following is not true about the mechanics of MQC?
a. Packets are classified inside a class map
b. PHBs are defined inside a service policy
c. Matching multiple DSCPs requires multiple match commands
d. One command is used to enable a QoS policy on an interface for packets both entering and exiting an interface
9. Which of the following router commands displays the configuration that results from enabling AutoQos VoIP on a router’s S0/0 interface, including the details of any class maps or policy maps? (Link: AutoQos VoIP Default Configuration)
a. show autoqos
b. show auto qos
c. show auto qos interface s0/0
d. show running-config
10. Which of the following tools can be used to classify packets generated on behalf of an application that dynamically allocates the TCP ports numbers used by the application?
a. CB Marking
b. ECN
c. NBAR
d. Pre-classify
11. Imagine a router configuration with several class-map commands, with a policy map referring to the service classes defined in the class-map commands. The policy map has been enabled for incoming packets on interface Fa0/1. What command would you look for in order to tell if Class Based Marking was in use, as opposed to some other MQC command?
a. match
b. match-all
c. priority
d. mark
e. set
12. Examine the following configuration snippet, and assume that all commands related to the class-map and all interface commands are shown. Which of the following answer best explains why the show command shows that class barney is not matching any packets?
class-map dino
match protocol rtp audio
!
policy-map barney
class dino
set ip dscp ef
!
interface fastethernet0/0
ip address 1.1.1.1 255.255.255.0
service-policy input barney
a. There is no RTP audio traffic currently in the network
b. You cannot enable CB marking as an input function
c. The show command only works when the mls enable counters interface subcommand is used
d. The ip cef global command is required .
e. The show policy-map interface fa0/0 command lists configuration information, but not packet counters
13. Assume that a router is configured correctly so that voice payload packets are marked with DSCP value EF. Which of the following commands could have been used inside the policy-map to cause CB Marking to set that value?
a. set ip dscp ef
b. set ip dscp 46
c. set dscp ef
d. set dscp 46
e. all of the above
f. none of the above
14. Router A is the endpoint of an IPSEC VPN tunnel. Packets entering router A on interface fa 0/0 will be encrypted and then forwarded to a central site out interface S0/0. You want to enable CB marking on egress packets on the serial0/0 interface, but you want to look at the fields in the IP, TCP, and UDP headers of the original packet, before encryption. What must be done to allow your policy map to work?
a. It works automatically, without any commands required
b. The ip cef global command is needed
c. The mls qos global command is needed
d. The qos pre-classify global command is needed
e. The qos pre-classify command is needed on the IPSEC crypto map
15. Which of the following commands could change the length of a hardware queue?
a. hardware queue-length 10
b. tx-queue length 10
c. hardware 10
d. tx-ring-limit 10
1. B is the correct answer.
2. C is the correct answer.
3. C is the correct answer.
4. B and D are the correct answers.
5. C is the correct answer.
6. B and D are the correct answers.
7. F is the correct answer.
8. B, C and D are the correct answers.
9. B and D are the correct answers.
10. C is the correct answer.
11. E is the correct answer.
12. D is the correct answer.
13. E is the correct answer.
14. E is the correct answer.
15. D is the correct answer.
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