Chapter 9

"Do I Know This Already" Quiz

Classification and Marking

1.	What methods does a Catalyst 2950 currently use to classify traffic? CoS CoS and DSCP CoS, DSCP, and access lists CoS, DSCP, access lists, and NBAR
Answer:	C
2.	What QoS map needs to be modified on the Catalyst 2950 to allow the switch to match the Layer 2 and Layer 3 currently marked by Cisco IP Phones? DSCP to CoS CoS to DSCP CoS-to-IP Precedence map IP Precedence-to-CoS map DSCP-to-IP Precedence map IP Precedence-to-DSCP map
Answer:	B
3.	By default, what values does the Catalyst 2950 trust? CoS DSCP CoS and DSCP None of the above
Answer:	D
4.	Which of the following is not a trust option on the Catalyst 2950? Trust CoS Trust IP Precedence Trust DSCP Trust Cisco Phone
Answer:	B
5.	What does the command mls qos trust cos pass-through dscp do? Enables trust of CoS values only if a DSCP value is present Enables trust of CoS values and does not modify the DSCP value Enables trust of CoS only if the DSCP value matches the CoS-to-DSCP map Enables trust of DSCP only if a CoS value is present
Answer:	B This command forces the Catalyst 2950 to pass through the original DSCP value on a received packet rather than use the CoS to DSCP map to determine the value.

Congestion Management

6.	How many egress queues are present on each Ethernet interface of the Catalyst 2950? 1 2 4 8
Answer:	C
7.	Which three scheduling methods are supported on the Catalyst 2950? Strict priority scheduling Custom scheduling Strict priority and WRR scheduling MDRR scheduling WRR scheduling FIFO scheduling
Answer:	A, C, E Answer: Strict priority scheduling, WRR scheduling and strict priority scheduling with WRR scheduling.
8.	Which scheduling method is configured by default on a Catalyst 2950? Strict priority scheduling Custom scheduling Strict priority and WRR scheduling MDRR scheduling WRR scheduling FIFO scheduling
Answer:	A
9.	Which scheduling method is recommended for networks that transport IP telephony traffic? Strict priority scheduling Custom scheduling Strict priority and WRR scheduling MDRR scheduling WRR scheduling FIFO scheduling
Answer:	C
10.	Which command is used to assign a CoS value of 5 to queue 4? wrr-queue cos-map 5 4 wrr-queue cos-assign 4 5 wrr-queue cos-map 4 5 wrr-queue cos-assign 5 4
Answer:	C
11.	Which of the following commands is used to enable strict priority with WRR scheduling on the Catalyst 2950? wrr-queue bandwidth 20 1 80 P wrr-queue bandwidth 0 20 1 80 wrr-queue bandwidth 20 1 80 0 wrr-queue bandwidth P 20 1 80
Answer:	C wrr-queue bandwidth 20 1 80 0 where 20 1 and 80 are the values used by the WRR scheduler and the 0 indicates that queue 4 will use the priority scheduler.

Policing

12.	What is a policer? A policer measures the data rate of arriving packets, identifies conforming and nonconforming traffic flows, and takes action on the traffic flows based upon the traffic contract. A policer measures the available bandwidth of an interface, identifies the traffic peaks, and takes action on peak traffic based upon the traffic contract. A policer measures the data rate of arriving packets, identifies known and unknown traffic flows, and takes action on the unknown traffic flows based upon the traffic contract. A policer measures the available bandwidth of an interface, identifies the known and unknown traffic flows, and takes action on the unknown traffic flows based upon the traffic contract.
Answer:	A A policer measures the available bandwidth of an interface, identifies the traffic peaks and takes action on peak traffic based upon the traffic
13.	What is out-of-profile or nonconforming traffic? Traffic routed out of the wrong interface Received traffic that exceeds the rate specified by the policer Received traffic that does not exceed the rate specified by the policer Received traffic that has not been defined by the policer
Answer:	B
14.	What can a policer do with nonconforming traffic on a Catalyst 2950? Drop it Remark the DSCP value Remark the CoS value A and B B and C
Answer:	D

AutoQoS

15.	Which of the following commands enables AutoQoS on the Catalyst 2950? auto qos trust auto qos voip trust auto qos trust voip auto qos cisco-phone auto qos trust cisco-phone
Answer:	B
16.	What does the command auto qos voip trust do? Enables trust of CoS on applied interface Enables trust of DSCP on applied interface Changes CoS-to-DSCP map Changes DSCP-to-CoS map Enables strict priority scheduling Enables strict priority queuing A, C, and F B, D, and E A, B, C, D, and F
Answer:	G
17.	What does the command auto qos voip cisco-phone do? Enables trust of CoS for a Cisco IP Phone Enables trust of DSCP for a Cisco IP Phone Changes CoS-to-DSCP map Changes DSCP-to-CoS map Enables strict priority scheduling Enables strict priority queuing A, C, and F B, D, and E A, B, C, D, and F
Answer:	G
18.	What command can you use to verify AutoQoS on a Catalyst 2950? show qos auto show auto qos show auto cos show cos auto
Answer:	B

Q&A

1.	Why do you need QoS in the LAN?
Answer:	Buffer Management as well as Classification and Marking as close to the sources as possible are the reasons you need QoS is the LAN.
2.	What is buffer overflow and when does it occur?
Answer:	The term buffer overflow indicates that a buffer on and interface has received more traffic that it can transmit. It occurs when an interface is oversubscribed.
3.	What IOS types are available for the Catalyst 2950 and which one is preferred for QoS?
Answer:	Standard and Enhanced. Enhanced is preferred for QoS
4.	You have a Catalyst 2950 running the standard IOS image and need to migrate to a Catalyst 2950 running the enhanced IOS image. What are your options to migrate to a Catalyst 2950 running the enhanced IOS image?
Answer:	The Catalyst 2950 IOS version is hardware dependent. This means that you cannot upgrade a standard IOS image to an enhanced IOS image. You must order a new switch.
5.	What methods can a Catalyst 2950 currently use to classify traffic?
Answer:	Trust, Port-based, CoS, DSCP and access lists
6.	What map needs to be changed on the Catalyst 2950 to reflect the current markings of Cisco IP Phones?
Answer:	CoS-to-DSCP Map
7.	What command is used to verify the CoS-to-DSCP map?
Answer:	mls qos map cos-dscp
8.	By default, a Catalyst 2950 maps voice-media traffic and voice-signaling traffic to which DSCP values?
Answer:	Voice-Media CS5 (decimal 40) and voice-signaling CS3 (decimal 24)
9.	To keep the DSCP values of the voice-media traffic and the voice-signaling traffic consistent between the IP phones and the Catalyst 2950, which DSCP values need to be configured on the Catalyst 2950?
Answer:	Voice-Media EF (decimal 46) and voice-signaling AF31 (decimal 26)
10.	By default, what values does the Catalyst 2950 trust?
Answer:	None
11.	Name two of the three markings or devices that the Catalyst 2950 can use to extend a trust boundary.
Answer:	Trust of CoS, trust of DSCP or trust of Cisco IP Phone
12.	Where is the trust command configured on a Catalyst 2950?
Answer:	On an Ethernet interface
13.	What does the switchport priority extend cos 0 command do?
Answer:	This command causes any CoS value received from a PC attached to an IP phone to be overwritten with a CoS value of 0.
14.	What does the mls qos trust cos pass-through dscp command do?
Answer:	This command forces the Catalyst 2950 to trust the received CoS value and pass through the original DSCP value on a received packet rather than use the CoS to DSCP map to determine the DSCP value.
15.	What command enables the trust of a Cisco IP Phone on a Catalyst 2950?
Answer:	mls qos trust device cisco-phone
16.	How does the Catalyst 2950 determine whether a Cisco IP Phone is connected to a port configured to trust device cisco-phone?
Answer:	The switch will used CDP version 2 to discover the phone and apply the trust.
17.	What happens if an interface is configured to trust device cisco-phone and a PC is connected to the interface?
Answer:	If CDP version 2 does not detect a Cisco IP Phone, the interface will remain untrusted.
18.	Which command set the default CoS value of an interface to 2 for all untagged frames received?
Answer:	mls qos cos 2
19.	What does the command mls qos cos override do?
Answer:	This command overrides the CoS values of packets received on an interface causing all tagged and untagged frames to use the default CoS value configured on that interface.
20.	What command views the current trust state of interface Fast Ethernet 0/1?
Answer:	show mls qos interface fastethernet 0/1
21.	A router that is not configured for 802.1Q is connected to interface 0/1 of the Catalyst 2950. The router interface forwards a voice-media packet to the switch. What CoS value will the switch receive?
Answer:	None. If there is no 802.1Q trunk there is no CoS value.
22.	A router that is not configured for 802.1Q is connected to interface 0/1 of the Catalyst 2950. The switch is configured to trust CoS on this interface. The router interface forwards a voice-media packet to the switch. What DSPC value will the switch use for this packet by default?
Answer:	DSCP 0. Because the switch is configured to trust CoS and there is no 802.1Q trunk, the switch will use the default CoS for that interface. By default, this value is CoS 0 which will be mapped to DSCP 0. To allow the DSCP value to pass, enable trust of DSCP on interface 0/1.
23.	What can MQC be used for on the Catalyst 2950?
Answer:	Classification, Policing, and Marking.
24.	What is a class map used for?
Answer:	To identify or classify traffic.
25.	What is a policy map used for?
Answer:	To apply actions to the classified traffic.
26.	How is a policy map applied to an interface?
Answer:	Using the service-policy command
27.	In which direction can a service policy be applied on a Catalyst 2950?
Answer:	Inbound only
28.	How many ingress queues are present on each Ethernet interface of the Catalyst 2950?
Answer:	1
29.	How many egress queues are present on each Ethernet interface of the Catalyst 2950?
Answer:	4
30.	What scheduling methods are supported on the Catalyst 2950?
Answer:	Strict priority scheduling, WRR scheduling and strict priority scheduling with WRR scheduling, also known as strict priority queuing.
31.	What mark does the Catalyst 2950 use to determine the proper queue for a packet?
Answer:	CoS value
32.	Which scheduling method is configured by default on a Catalyst 2950?
Answer:	Strict priority scheduling
33.	Which scheduling method is recommended for networks that transport IP telephony traffic?
Answer:	Strict priority queuing
34.	How does strict priority scheduling work?
Answer:	Each queue is services by order of priority with 4 being the highest and 1 being the lowest. A packet in a higher queue will always be transmitted before a packet in a lower queue.
35.	What are the advantages and disadvantages of using strict priority scheduling?
Answer:	Strict priority scheduling offers the ability to guarantee minimum delay to applications in the highest priority queue, but lower priority queues have the potential for queue starvation.
36.	How does Weighted Round Robin scheduling work?
Answer:	Each queue is allocated a minimum amount of bandwidth to be serviced in a weighted round robin fashion.
37.	What are the advantages and disadvantages of WRR scheduling?
Answer:	WRR scheduling offers each queue the capability to transmit packets eliminating queue starvation, but lacks a strict priority queue, which may cause voice quality issues with voice-media traffic due to added delay.
38.	How does strict priority queuing work?
Answer:	Strict priority queuing combines the benefits of strict priority scheduling and WRR scheduling by offering a single strict priority queue and 3 WRR serviced queues. Packets are transmitted from the strict priority queue if present. If no packets are in the priority queue, the WRR scheduler will divide that available bandwidth between the remaining 3 queues.
39.	What queue is configured for the strict priority queue when using strict priority queuing?
Answer:	4
40.	What command assigns a CoS value of 5 to queue 4?
Answer:	wrr-queue cos-map 4 5
41.	What command assigns WRR scheduling with queue 1 servicing 5 packets, queue 2 servicing 10 packets, queue 3 servicing 25 packets, and queue 4 servicing 50 packets?
Answer:	wrr-queue bandwidth 5 10 25 50
42.	What command enables the strict priority queue when using strict priority queuing?
Answer:	wrr-queue bandwidth x x x 0 where x is the value of the WRR scheduler and the 0 indicates that queue 4 will be the priority queue.
43.	What command verifies that queue 4 has been configured for the priority queue?
Answer:	show wrr-queue bandwidth
44.	What is a policer?
Answer:	A policer defines the amount of acceptable traffic in a class and specifies what to do with traffic that exceeded the acceptable rate.
45.	What is out of profile or nonconforming traffic?
Answer:	Received traffic that exceeds the rate specified by the policer.
46.	What can a policer do with nonconforming traffic on a Catalyst 2950?
Answer:	Drop or remark the DSCP of the traffic.
47.	How many policers can be applied to a single 10/100 Ethernet interface on a Catalyst 2950? How about a Gigabit interface on a Catalyst 2950?
Answer:	6. 60.
48.	What is the maximum burst size that a policer can use for a 10/100 interface on a Catalyst 2950?
Answer:	64K.
49.	What is the maximum burst size that a policer can use for a Gigabit interface on a Catalyst 2950?
Answer:	512K.
50.	What is the minimum traffic rate that a policer can use for conforming traffic for a 10/100 interface on a Catalyst 2950?
Answer:	1000000 (1 Meg).
51.	What is the minimum traffic rate that a policer can use for conforming traffic for a Gigabit interface on a Catalyst 2950?
Answer:	8000000 (8 Meg).
52.	What commands enables AutoQoS on the Catalyst 2950?
Answer:	auto qos voip trust and auto qos voip cisco-phone
53.	Where are the auto qos voip trust and auto qos voip cisco-phone commands applied?
Answer:	On an Ethernet interface.
54.	What does the command auto qos voip trust do?
Answer:	It enables trust of CoS for the interface it was applied to, changes the CoS to DSCP map and configures strict priority queuing.
55.	What does the command auto qos voip cisco-phone do?
Answer:	It enables trust of a Cisco IP Phone for the interface it was applied to, changes the CoS to DSCP map and configures strict priority queuing.
56.	When is configuring trust manually preferred over AutoQoS?
Answer:	When trust is based upon DSCP.
57.	When AutoQoS is enabled, what CoS values are mapped to queue 1? What percentage of the WRR scheduler does queue 1 receive?
Answer:	CoS values mapped to queue 1 include 0, 1, 2 and 4. Percentage of the WRR scheduler is 20 percent.
58.	When AutoQoS is enabled, what CoS values are mapped to queue 2? What percentage of the WRR scheduler does queue 2 receive?
Answer:	No CoS values are mapped to queue 2; however, the percentage of the WRR scheduler is 1 percent. Because there will never be traffic in this queue, it will never use the 1 percent.
59.	When AutoQoS is enabled, what CoS values are mapped to queue 3? What percentage of the WRR scheduler does queue 3 receive?
Answer:	CoS values mapped to queue 3 include 3, 6 and 7. Percentage of the WRR scheduler is 80 percent.
60.	When AutoQoS is enabled, what CoS values are mapped to queue 4? What percentage of the WRR scheduler does queue 4 receive?
Answer:	CoS 5 is the only CoS value mapped to queue 4. Queue 4 becomes the priority queue and will not use the WRR scheduler.
61.	What command can you use to verify AutoQoS?
Answer:	show auto qos