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? compression ip header rtp compression header ip rtp
| | 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? The number of bytes that would have been sent without compression, per second The ratio of bytes actually sent, over the number of bytes that would have been sent without compression The ratio of bytes that would have been sent without compression, over the number of bytes actually sent
| | Answer: | C, D
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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: | D
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? ppp multilink fragment-delay 10 ppp multilink fragment-size 160
| | 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? frame-relay traffic-shaping
| | Answer: | B
| | 10. | Which of the following commands list statistics about the number of fragments created with FRF.12? show frame-relay fragment
| | Answer: | D
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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 equallyeither compressing or not compressing.
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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.
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