Chapter 7

“Do I Know This Already” Quiz

Congestion-Avoidance Concepts and RED Questions

1.

TCP Slow Start controls the rate a TCP sender sends data by controlling:

 

Answer: B

2.

For which of the following WRED categories will WRED discard all packets?

 

Answer: B

Answer: While “Tail Drop” may be considered to be the same thing as “Full Drop” by many people, the important distinction is that WRED acts based on the average queue depth, whereas Tail Drop acts on the actual queue depth.

3.

For which of the following WRED categories will WRED discard a subset of the packets?

 

Answer: C

WRED Questions

4.

On which of the following types of queues can you enable WRED on routers that are not part of the 7500-series router line?

The next three questions refer to the following configuration snippet:

     ip cef
     !
     ! The following classes are used in the LLQ configuration applied to S0/0
     !
     class-map match-all class1
       match protocol http url "*important*"
     class-map match-all class2
       match protocol http url "*not-so*"
     class-map match-all class3
       match protocol http
     !
     policy-map wred-q
       class class1
        bandwidth percent 25
        random-detect dscp-based
        random-detect dscp af22 25 35 50
       class class2
        bandwidth percent 20
        random-detect
        random-detect precedence 2 25 35 50
       class class3
        bandwidth percent 15
        random-detect dscp-based
        random-detect dscp af22 50 25 35
       class class-default
        random-detect dscp-based
        random-detect dscp af22 2 25 50
     !
     interface s0/0
      ip address 1.1.1.1 255.255.255.0
      random-detect dscp-based
      random-detect dscp af22 50 25 35
     !
     interface s0/1
     ip address 2.2.2.2 255.255.255.0
     service-policy output wred-q

 

Answer: A, B

WRED can be enabled on a physical interface, but it creates a single FIFO queue, and can’t be used with another queuing tool (like WFQ) at the same time. It can also be enabled in a CBWFQ, but it cannot be enabled in an LLQ.

5.

For which of the following will WRED discard 2 percent of packets of some precedence or DSCP value, when the average queue depth approaches the maximum threshold?

 

Answer: B, C, E

Please refer to the answer to the next question for some background information.

6.

Imagine a packet marked as AF22. Out which interface or class must the packet be forwarded in order to have a 35 percent chance of being discarded, assuming that WRED’s average queue depth calculation was approaching the maximum threshold?

 

Answer: F

The last of the parameters on the random-detect dscp and random-detect precedence commands set the mark probability denominator (MPD). MPD is not set as a percentage, but the percentage is 1/MPD. None of the settings give a percentage of 35. Note that on the previous question, with a setting of 50 in some cases for MPD, that 2 percent of the packets would be dropped (1/50 = 2 percent).

7.

Assuming the commands in the configuration snippet were typed into configuration mode in a router, one of the random-detect commands would be rejected. Under which configuration mode can that erroneous command be found?

 

Answer: D

The random-detect dscp AF22 50 25 35 command attempts to set the minimum threshold to 50, and the maximum to 25, which is not allowed.

ECN Questions

8.

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?

 

Answer: B

9.

Referring to the configuration snippet before question 5, what command would be required to enable ECN for class2 in policy-map wred-q?

 

Answer: C

Q&A

Congestion-Avoidance Concepts and Random Early Detection (RED)

1.

Describe the function of the congestion window in TCP, and how it is changed as a result of packet loss.

 

Answer: The TCP congestion window, or CWND, is one of two windowing mechanisms that limit TCP senders. CWND can be split in half as a result of packet loss, slowing the sending rate. CWND can also be slammed shut to the size of a single segment in some cases.

2.

Identify the two TCP windowing mechanisms, and describe when each is used.

 

Answer: The TCP congestion window, or CWND, and the TCP receiver window, are the two windowing mechanisms. The lower of the two values is used at all times.

3.

Describe the process of TCP slow start, and when it occurs.

Answer: TCP slow start governs the growth of the TCP congestion window after the window has been lowered in reaction to a packet drop. Slow start increases the window by one segment size for each positively acknowledged packet received.

4.

Describe the meaning of the term “global synchronization,” and discuss what causes it.

Answer: Global synchronization describes a condition in which many TCP connections have their congestion windows lowered due to unacknowledged or lost segments at around the same instant in time. The connections all grow CWND at about the same rate, re-creating the same congestion levels again, causing more drops, which in turn reduces again the TCP congestion windows. Global synchronization is caused by a large number of packet drops in a very short period, typically the result of tail drops.

5.

Define the meaning of the term “tail drop.”

Answer: When a queue fills, and a new packet must be placed into the queue, the packet is dropped. Because the packet would be placed into the end, or tail, of the queue, it is called tail drop.

6.

Define the meaning of the term “TCP starvation.”

Answer: When packets are dropped, TCP connections slow down, but UDP flows do not slow down. UDP packets can consume a disproportionate amount of queue space as a result, which could get to the point that the TCP connections simply get little or no queue space; this is called TCP starvation.

7.

Does RED compare the actual queue depth or the average queue depth to queue thresholds when deciding whether it should discard a packet? Why this one, and not the other?

Answer: RED uses average queue depth. By using the average, rather than the actual queue depth, RED behaves more consistently, rather than more erratically, which helps prevent synchronization of TCP flows.

8.

Describe how RED uses actual queue depth to calculate average queue depth. Do not list the formula, but just describe the general idea.

Answer: RED calculates the average by adjusting the previously calculated average a small amount based on the current actual queue depth. By default, the current queue depth is weighted at about .2 percent in the formula.

9.

Assume the RED minimum threshold is 20, the maximum threshold is 40, and the mark probability denominator is 10. What must be true for RED to discard all new packets?

Answer: The average queue depth must be above 40.

10.

Assume the RED minimum threshold is 20, the maximum threshold is 40, and the mark probability denominator is 10. What must be true for RED to discard 5 percent of all new packets?

Answer: The average queue depth must be at 30. Because the discard percentage grows linearly from 0 percent to 10 percent (in this case), between average queue depth of 20 through 40, average queue depth of 30 would mean that the discard percentage had grown to 5 percent.

11.

Define how RED uses the mark probability denominator. Give one example.

Answer: RED calculates the discard percentage based on the formula 1/MPD. For instance, with an MPD of 20, the discard percentage is 1/20, or 5 percent.

12.

Define the term “exponential weighting constant.” If the value is lowered compared to the default setting of 9, how does RED behave differently?

Answer: The exponential weighting constant defines how quickly the average queue depth changes, by determining how much the actual queue depth affects the rolling average queue depth. If EWC is lowered, the average changes more quickly, because the formula weights the current actual queue depth more than before. Therefore, a larger constant provides more handling of bursty traffic, but too large and congestion avoidance will be ineffective.

13.

Define the term “WRED Profile.”

Answer: A WRED profile is a collection of WRED parameters applied to a single IP Precedence or DSCP value. The parameters include the minimum threshold, the maximum threshold, and the Mark Probability Denominator (MPD).

14.

Explain how you can tune how fast or slow that WRED changes the calculated average queue depth over time.

Answer: WRED calculates a new average based on the old average and the current queue depth. You can tell WRED to count the current queue depth as a larger or smaller part of the calculation by tuning the exponential weighting constant. The formula is:

New average = (Old_average * (1 – 2–n)) + (Current_Q_depth * 2–n)

Where “n” is the exponential weighting constant.

Weighted RED (WRED)

15.

Spell out the words represented by the initials RED, WRED, and FRED.

Answer: Random Early Detection (RED), Weighted Random Early Detection (WRED), Flow-Based Weighted Random Early Detection (FRED).

16.

List the three WRED terms that name the separate states in which WRED discards no packets, a percentage of packets, and all packets.

Answer: No Discard, Random Discard, and Full Discard, respectively.

17.

List the queuing tools that can be concurrently supported on an interface when WRED has been enabled directly on a serial interface, assuming no retrictions on the particular model of router.

Answer: FIFO Queuing only.

18.

Identify the most important difference between RED operation and WRED operation.

Answer: WRED weights its discard decisions based on precedence or DSCP, whereas RED ignores precedence and DSCP.

19.

Describe how WRED “weights” packets.

Answer: WRED weights packets based on precedence or DSCP by assigning different minimum threshold, maximum threshold, and mark probability denominator values for each precedence or DSCP.

20.

List the queuing tools that can enable WRED for use with some or all of their queues, effectively enabling WRED concurrently with the queuing tool, assuming no retrictions on the particular model of router.

Answer: CBWFQ and LLQ.

21.

What command enables you to look at WRED drop statistics when WRED is configured inside an MQC class?

Answer:

   show policy-map interface

22.

Taking as many defaults as possible, list the configuration commands needed to configure precedence-based WRED on interface S1/1.

Answer:

   interface serial 1/1
         random-detect

23.

Taking as many defaults as possible, list the configuration commands needed to configure DSCP-based WRED on interface S1/1.

Answer:

   interface serial 1/1
         random-detect dscp-based

24.

Taking as many defaults as possible, list the configuration commands needed to configure DSCP-based WRED inside class class1, inside policy map my-policy. (You can assume that the CBWFQ configuration has already been completed, and you just entered global configuration mode. Assume that you need just to enable WRED in class class1.)

Answer:

   policy-map my-policy
         class class1
           random-detect dscp-based

25.

List the command needed to set the minimum threshold to 25, the maximum threshold to 50, and the mark probability denominator to 4, for precedence 2.

Answer:

   random-detect precedence 2 25 50 4

26.

What show command lists detailed statistics about random drops on interface S1/1?

Answer:

   show queueing interface s1/1

Explicit Congestion Notification

27.

For a single WRED profile, WRED can be either dropping no packets, randomly choosing packets to discard, or dropping all packets. For which of these three states does ECN impact WRED’s discard actions? How does it change what WRED does to the packets?

Answer: For Random Discard only. WRED forwards the packets instead of discarding them, but only after setting the ECN bits to “11”.

28.

Identify the bits in the IP header used with ECN, by name and location.

Answer: The low-order 2 bits of the DSCP byte are called the ECN field. The first bit is called the ECN Capable Transport (ECT) bit, and the second one is the Congestion Experienced (CE) bit.

29.

Imagine a router on which WRED and ECN are enabled, and WRED decides to randomly discard a packet. What must be true in order for WRED to discard the packet, instead of using ECN logic to mark and forward the packet? Explain the role of any other devices besides the router.

Answer: With ECN enabled, it would set the ECN bits to “11”, unless the ECN field was set to 00. An ECN field of 00 means that the sender did not support ECN for that TCP connection.

30.

Imagine a router on which WRED and ECN are enabled, and WRED decides to randomly discard a packet. What must be true in order for WRED to use ECN logic to mark and forward the packet, instead of discarding the packet? Explain the role of any other devices besides the router.

Answer: The ECN field must be set to something besides 00. The sender of the packet would choose to set ECN to one of those two values if it did support ECN for that TCP connection.

31.

Imagine a policy map with WRED already configured for class class-web. What additional command is required to also enable ECN for the packets in that class?

Answer: The random-detect ecn command.

Chapter 8

“Do I Know This Already” Quiz

Compression Questions

1.

With CB RTP header compression, which of the following are compressed?

 

Answer: A, C, D

2.

With CB TCP header compression, which of the following are compressed?

 

Answer: B, C

3.

With Layer 2 payload compression, which of the following could be compressed?

 

Answer: A, B, C, D, E

Payload compression compresses everything inside the layer 3 packet.

4.

Which of the following Modular QoS Command-Line Interface (MQC) class subcommands enables CB RTP header compression?

 

Answer: B, E

the compress header ip command enables both RTP and TCP header compression.

5.

In the show policy-map interface command output, with TCP or RTP header compression enabled, what does “efficiency improvement factor” mean?

 

Answer: C, D

Link Fragmentation and Interleave Questions

6.

What fragment size, in bytes, should be used on a 256-Kbps link in order to ensure each fragment has less than or equal to 10 ms of serialization delay?

 

Answer: E

The answer is derived by multiplying the speed times the delay, and converting to bytes: 256,000 bits/second * .01 seconds / 8 bits/byte = 320 bytes.

7.

What serialization delay, in milliseconds, would be experienced by a 160 byte fragment on a 64-kbps link?

 

Answer: D

The answer is derived by dividing the number of bits in the fragment by the link speed, and converting to milliseconds: 160 bytes * 8 bits/byte / 64,000 bits/second = .02 seconds = 20ms.

8.

A router has MLP LFI configured on interface s0/0. The bandwidth 128 command is already configured on the multilink interface. Which of the following commands is used under a multilink interface in order to set the fragment size to 160 bytes?

 

Answer: A

The command ppp multilink interleave is required for LFI to work, but the ppp multilink fragment-delay 10 command sets the fragment delay. The fragment size is then calculated using the formula size = delay * bandwidth. In this case, it would be .01 seconds * 128,000 bits/second = 1280 bits = 160 bytes.

9.

A router has FRF.12 configured for all VC’s on interface s0/0. The bandwidth 128 command is already configured on the interface. Which of the following commands is used under the map-class frame-relay command in order to set the fragment size to 160 bytes?

 

Answer: B

10.

Which of the following commands list statistics about the number of fragments created with FRF.12?

 

Answer: D

Q&A

Compression Tools

1.

Describe what is compressed, and what is not compressed, when using payload compression. Be as specific as possible regarding headers and data.

 

Answer: Payload compression does not compress the data-link header and trailer, but it does compress all the higher-layer headers and data between the two. Specifically, the IP, TCP, UDP, RTP headers as appropriate, and the user data, are compressed.

2.

Describe what is compressed, and what is not compressed, when using TCP header compression. Be as specific as possible regarding headers and data.

 

Answer: IP packets that also have TCP headers are compressed. The compression algorithm does not compress the data link header or trailer. It does compress both the IP and TCP headers. It does not compress any user data that follows the TCP header.

3.

Describe what is compressed, and what is not compressed, when using RTP header compression. Be as specific as possible regarding headers and data.

 

Answer: IP packets that also have RTP headers are compressed. The compression algorithm does not compress the data-link header or trailer. It does compress the IP, UDP, and RTP headers. It does not compress any user data that follows the RTP header.

4.

Suppose a packet is sent across a network with no compression. Later, a packet of the exact same size and contents crosses the network, but payload compression is used on the one serial link in the network. Describe the difference in bandwidth and delay in the network between these two packets.

 

Answer: The packet experiences longer processing delay as a result of the compression algorithm. However, the packet requires less time to be serialized onto the link, resulting in less serialization delay. Overall queuing delay should be decreased, because the shorter compressed packets take less time to serialize, thereby causing packets to exit the queues more quickly. The overall reduction in queue sizes can reduce delay and jitter.

5.

How much bandwidth should a G.729 call require over Frame Relay, and how much should be required with cRTP?

 

Answer: A single G.729 call requires 28 kbps over Frame Relay, but it only needs 12.8 kbps using cRTP.

6.

When TCP header compression is used, what is the range of sizes of the part of the frame that can be compressed, and what is the range of sizes for this field of the frame after compression?

 

Answer: TCP header compression compresses the 20-byte IP header and 20-byte TCP header, with the combined field size of 40 bytes. The compressed field will be between 3 and 5 bytes.

7.

When RTP header compression is used, what is the range of sizes of the part of the frame that can be compressed, and what is the range of sizes for this field of the frame after compression?

 

Answer: RTP header compression compresses the 20-byte IP header, 8-byte UDP header, and 12-byte RTP header, with the combined field size of 40 bytes. The compressed field will be between 2 and 4 bytes.

The next several questions refer to the following configuration snippet.

            !
            hostname R1
            !
            ip cef
            !
             class-map match-all class1

              match protocol telnet
             class-map match-all class2
              match protocol rtp audio
            !
            !
             policy-map test-compress
              class class2
               compression header ip
               class class1
                compression header ip tcp
                bandwidth 20
            !
            !
            interface Serial0/1
             ip address 10.1.1.1 255.255.255.0
             service-policy output test-compress
             load-interval 30
             clockrate 64000
             !
             !
          
hostname R2
             !
             ip cef
             !
              policy-map catch-all
               class class-default
                compression header ip
             !
             !
             interface Serial0/1
              ip address 10.1.1.1 255.255.255.0
              service-policy output catch-all
              load-interval 30

8.

The configuration snippet shows the configuration on two routers, R1 and R2, on the serial interfaces attached to a common serial link. Will VoIP RTP sessions be compressed, or even work at all? Explain why or why not in relation to what has been configured on the two routers.

 

Answer: CB RTP Compression will occur. On R1, all RTP audio sessions match class class2, inside which both RTP and TCP header compression has been enabled. CB RTP must be enabled on R2 as well, and it is, because all packets fall into the class-default class, and RTP header compression is enabled in that class.

9.

The configuration snippet shows the configuration on two routers, R1 and R2, on the serial interfaces attached to a common serial link. Will telnet connections be compressed, or even work at all? Explain why or why not in relation to what has been configured on the two routers.

 

Answer: CB TCP Compression will occur. On R1, all Telnet connections match class classs, inside which TCP header compression has been enabled. CB TCP Compression must be enabled on R2 as well, and it is, because all packets fall into the class-default class, and TCP header compression is enabled in that class.

10.

The configuration snippet shows the configuration on two routers, R1 and R2, on the serial interfaces attached to a common serial link. Will web TCP connections be compressed, or even work at all? Explain why or why not in relation to what has been configured on the two routers.

 

Answer: The web TCP connections will fail. On R1, all web TCP connections match class-default, inside which TCP header compression has NOT been enabled. CB TCP Compression is enabled on R2, because all packets fall into the class-default class, and TCP header compression is enabled in that class. Because compression has been enabled on only one side, the TCP connection will fail.

11.

Without creating all the exact configuration commands, generally describe the types of changes to the configuration on R2 that would be required in order to ensure that none of the flows would fail due to one router wanting to compress, and the other one not wanting to compress.

 

Answer: R2’s policy map would need classes that match on exactly the same criteria as those on R1. In each class, the same compression command would be used on both routers, either enabling RTP, TCP, or both. The class map names and policy map names would not have to match, but by matching the exact same types of packets, both routers would treat the packets in a single flow equally—either compressing or not compressing.

LFI Tools

12.

List the words represented by the abbreviation LFI.

 

Answer: Link fragmentation and interleaving.

13.

Describe the main motivation for LFI tools in relation to the support of data, voice, and video traffic.

 

Answer: LFI tools interleave some packets between the fragments of other packets. Voice and two-way video traffic are particularly sensitive to delay. LFI reduces the delay for voice and video packets by interleaving voice and video packets between fragments of the data packets.

14.

To achieve a 20-ms serialization delay on a 128-kbps link, how long can the fragments be?

 

Answer: The formula is max-delay * bandwidth, which is .02 * 128,000 = 2560 bits, or 320 bytes.

15.

To achieve a 10-ms serialization delay on a 64-kbps link, how long can the fragments be?

 

Answer: The formula is max-delay * bandwidth, which is .01 * 64,000 = 640 bits, or 80 bytes.

16.

To achieve a 10-ms serialization delay on a 56-kbps link, how long can the fragments be?

 

Answer: The formula is max-delay * bandwidth, which is .01 * 56,000 = 560 bits, or 70 bytes.

17.

To achieve a 30-ms serialization delay on a 128-kbps link, how long can the fragments be?

 

Answer: The formula is max-delay * bandwidth, which is .03 * 128,000 = 3840 bits, or 480 bytes.

18.

Suppose that a 1500-byte packet exits a 56-kbps serial interface, and LFI is not used. How long is the serialization delay?

 

Answer: The formula is packet length/link speed, which is 1500 * 8/56,000, or .214 seconds. The units used in the formula are bits, bits per second, and seconds, respectively.

19.

Which queuing tools can you enable directly on a serial interface when using multilink Point-to-Point Protocol with link fragmentation and interleaving (MLP LFI), in order to interleave packets?

 

Answer: PQ, LLQ and IP RTP Priority. CBWFQ can be configured, but because it does not have a PQ-like function, it does not interleave packets.

20.

Which queuing tools can you enable with FRTS in order to actually interleave the traffic?

 

Answer: LLQ and IP RTP Priority actually interleave packets.

21.

Explain the scheduling logic used by MLP LFI to determine which packets can be interleaved in front of fragments of other packets.

 

Answer: MLP LFI does not define scheduling logic. Instead, it relies on the scheduler of the queuing tool enabled on the interface to decide which packets to send next. If LLQ were used, for instance, packets from the low-latency queue would be interleaved in front of packets from other queues.

22.

Suppose a 1500-byte packet arrives and needs to be sent over an MLP bundle that has two active links. LFI has not been configured. Which link does the packet flow across to achieve MLP load balancing?

 

Answer: MLP fragments the packet into two equal-sized fragments, and sends one over one link, and one over the other.

23.

What command can you use to determine the fragment size used for MLP LFI? What is the only parameter of the command?

 

Answer: The ppp multilink fragment-delay command sets the maximum serialization delay in milliseconds. IOS calculates the fragment size using the formula max-delay * bandwidth.

24.

What command enables the interleaving feature of MLP LFI?

 

Answer: The ppp multilink interleave command.

25.

What commands list counters for the number of interleaved packets using MLP LFI?

 

Answer: The show queue and show interfaces commands.

26.

What other QoS feature for Frame Relay must you enable when you also configure FRF.12?

 

Answer: Frame Relay Traffic Shaping (FRTS).

27.

What command enables FRF and sets the fragment size?

 

Answer: The frame-relay fragment fragment_size command.

28.

What command lists counters for the numbers of packets and bytes that were fragmented and unfragmented by FRF.12?

 

Answer: The show frame-relay fragment interface subcommand.

29.

What command lists counters for the numbers of packets and bytes that would have been sent if FRF.12 fragmentation had not been performed?

 

Answer: The show frame-relay fragment interface subcommand.

30.

What command lists counters for the number of packets that end up in the High and Normal Dual-FIFO siftware queues, when using FRF.12?

 

Answer: The show queueing interface x/y command.

Chapter 9

“Do I Know This Already” Quiz

Classification and Marking

1.

What methods does a Catalyst 2950 curren use to classify traffic?

 

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?

 

Answer: B

3.

By default, what values will the Catalyst 2950 trust?

 

Answer: D

4.

Which of the following is NOT a trust option on the Catalyst 2950?

 

Answer: B

5.

What does the command mls qos trust cos pass-through dscp do?

 

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?

 

Answer: C

7.

Which three scheduling methods are supported on the Catalyst 2950?

 

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?

 

Answer: A

9.

Which scheduling method is recommended for networks that transport IP Telephony traffic?

 

Answer: C

10.

Which command is used to assign CoS value of 5 to queue 4?

 

Answer: C

11.

Which of the following commands is used to enable strict priority with WRR scheduling on the Catalyst 2950?

 

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?

 

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?

 

Answer: B

14.

What can a policer do with nonconforming traffic on a Catalyst 2950?

 

Answer: D

AutoQoS

15.

Which of the following commands enables Auto QoS on the Catalyst 2950?

 

Answer: B

16.

What does the command auto qos voip trust do?

 

Answer: G

17.

What does the command auto qos voip cisco-phone do?

 

Answer: G

18.

What command can you use to verify Auto QoS on a Catalyst 2950?

 

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 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 current 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 will map 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 will the Catalyst 2950 trust?

 

Answer: None

11.

Name two of the three markings and / 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 is used to enable 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 if 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 is used to 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 is used to view 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 is used to assign CoS value of 5 to queue 4?

 

Answer: wrr-queue cos-map 4 5

41.

What command is used to assign 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 is used to enable 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 is used to verify 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 interface on a Catalyst 2950?

 

Answer: 512K.

50.

What is the minimum traffic rate that a policer used 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 used for conforming traffic for a Gigabit interface on a Catalyst 2950?

 

Answer: 8000000 (8 Meg).

52.

What commands can be used to enable 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 doe 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 Auto QoS?

 

Answer: When trust is based upon DSCP.

57.

When Auto QoS 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 Auto QoS 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 Auto QoS 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 Auto QoS 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 Auto QoS?

 

Answer: show auto qos

Chapter 10

“Do I Know This Already” Quiz

1.

Which of the following traffic types provides a consistent packet exchange?

 

Answer: B

2.

Which of the following factures influence the bandwidth requirements of a VoIP call?

 

Answer: B, C, D

3.

The International Telecommunications Union (ITU) G.114 specification states that the one-way delay of a voice packet should not exceed how many milliseconds (ms)?

 

Answer: B

4.

How much bandwidth should be provisioned for a 384-kbps interactive video stream?

 

Answer: B

5.

How sensitive is mission-critical data to jitter?

 

Answer: B

6.

A converged network should be designed for less than what percentage of packet loss in the class that supports real-time applications?

 

Answer: B

7.

A converged network should be designed for less than how many milliseconds of jitter in the class that supports real-time applications?

 

Answer: C

8.

Where should traffic be classified?

 

Answer: B

9.

What is the recommended differentiated services code point (DSCP) value for scavenger traffic?

 

Answer: C

10.

What is the recommended DSCP class for bulk data applications?

 

Answer: A

11.

What are the preferred methods of queuing on Cisco routers?

 

Answer: C

12.

When using LFI, what is the recommended serialization delay of a voice packet?

 

Answer: B

13.

Where does WRED perform the most effectively?

 

Answer: C

14.

What QoS mechanism should never be used in the core of the campus LAN?

 

Answer: A

15.

If the service provider manages the CE router, what inbound policy is required on the PE router?

 

Answer: E

Q&A

1.

What is end-to-end QoS?

 

Answer: End-to-end QoS is a means of offering the same treatment to a packet on each node it traverses.

2.

What do service providers use to contractually guarantee specific delay, jitter, and packet loss to their customers?

 

Answer: A service-level agreement (SLA)

3.

When would an SLA be offered on a per PVC basis?

 

Answer: When the physical interface services multiple customers.

4.

In a Cisco IP Telephony environment, which codec is best suited for the LAN?

 

Answer: G.711

5.

In a Cisco IP Telephony environment, which codec is best suited for the WAN?

 

Answer: G.729

6.

By default, how many milliseconds of speech are placed into every packet in a voice media stream?

 

Answer: 20 ms

7.

How much bandwidth is required to transport a G.729 VoIP call @ 50 packets per second over a Frame Relay circuit? Assume that cRTP and VAD are not enabled.

 

Answer: 28 kbps

8.

Enabling cRTP can reduce the overhead of the IP/UDP/RTP header from 40 bytes to how many bytes?

 

Answer: 2 or 4

9.

Cisco DSPs have the capability to compensate for how many milliseconds of lost speech?

 

Answer: 30 ms

10.

A Cisco IP Phone can compensate for how much jitter?

 

Answer: 20–50 ms

11.

Data applications should be separated into no more than how many distinct classes?

 

Answer: Four or five

12.

In a converged network supporting voice, interactive video, streaming video, mission-critical data, transactional data, and best-effort traffic, which class or classes should use the low latency queue?

 

Answer: Voice and interactive video

13.

What tool is recommended to discover applications on your network?

 

Answer: NBAR

14.

What DSCP value do Cisco IP Phones currently mark voice media traffic with?

 

Answer: EF

15.

What CoS value do Cisco IP Phones currently mark voice media traffic with?

 

Answer: 5

16.

What DSCP value do Cisco IP Phones currently mark voice signaling traffic with?

 

Answer: AF31

17.

What CoS value do Cisco IP Phones currently mark voice media signaling with?

 

Answer: 3

18.

What is the recommended CoS value for IP routing protocols?

 

Answer: 6

19.

What is the recommended DSCP value for IP routing protocols?

 

Answer: CS6

20.

What is the recommended DSCP class for transactional applications?

 

Answer: The AF2x class

21.

Which type of queue should voice signaling traffic use in an enterprise network?

 

Answer: CBWFQ

22.

Which command is used to allocate 100 percent of the link to MQC?

 

Answer: max-reserved-bandwidth 100

23.

What value should the Tc be configured for on a Frame Relay circuit that will transport voice media traffic?

 

Answer: 10 ms

24.

What is needed in a campus LAN before QoS is implemented?

 

Answer: A hierarchical and redundant campus design

25.

Where in the campus LAN is policing typically configured?

 

Answer: The access and distribution layers

26.

What is the recommended CoS value for IP routing protocols?

 

Answer: CoS value of 6

27.

What QoS mechanism should never be implemented in the core of the campus LAN?

 

Answer: Classification and marking

28.

If Cisco IP Phones have the capability to mark voice media and voice signaling traffic, why is it necessary to use access lists to classify these types of traffic?

 

Answer: Cisco Softphone and certain CTI applications might not have the capability to mark this traffic, yet they have the same QoS requirements.

29.

When is a CoS-to-DSCP map used?

 

Answer: When a switch receives a trusted frame with a CoS value, it will assign a DSCP value to the packet based upon this map.

30.

When should the CoS-to-DSCP map be modified?

 

Answer: When the CoS value does not match the desired DSCP value by default. For example, a received packet with a CoS value of 5 will be marked with a DSCP value of decimal 40, which is not the same as DSCP EF (decimal 46). The CoS-to-DSCP map must be modified to map a CoS value of 5 to a DSCP value of EF (decimal 46).

31.

Which two queuing methods should be used in the service provider’s backbone network?

 

Answer: LLQ/CBWFQ and MDRR
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