Cisco QoS FAQ: Link-Efficiency Tools

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by Prasanna
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2 years ago

Cisco QoS FAQ: Link-Efficiency Tools

Q1. 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 headers and data between the two. Specifically, the IP, TCP, UDP, RTP headers as appropriate, and the user data, are compressed.

2Q. 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.

Q3. 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.

Q4. List the three point-to-point payload compression options available in IOS.

Answer: Stacker, Microsoft Point-to-Point Compression (MPPC), and Predictor.

Q5. 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.

Figure : Delay Versus Bandwidth with Compression

cisco-qos-faq-link-efficiency-tools

Answer: The compressed packet experiences longer delay, all other things being equal, because the compression algorithm does take some time to execute. The compressed packet consumes less bandwidth on the link, because the compression algorithm has made it smaller. An overall reduction in queue sizes can occur as well, which can actually then reduce delay and jitter.

Q6. List the three options for Frame Relay payload compression in IOS. Which of the three is not Cisco proprietary?

Answer: Packet by packet, data stream, and FRF.9 are the three options. FRF.9 is not Cisco proprietary.

Q7. Which payload compression tool in IOS supports Link Access Procedure, Balanced (LAPB), High-Level Data Link Control (HDLC) and Point-to-Point Protocol (PPP) encapsulations?

Answer: Stacker.

Q8. 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 26.4 kbps over Frame Relay, but it only needs 11.2 kbps using cRTP.

Q9. 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.

Q10. 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.

Q11. To configure Stacker payload compression on a point-to-point link, what command(s) is used, and in what configuration modes?

Answer: The compress stac interface subcommand.

Q12. To configure Stacker payload compression on a point-to-point Frame Relay subinterface, what command(s) is used, and in what configuration modes?

Answer: The frame-relay compress stac subinterface subcommand.

Q13. To configure FRF.9 payload compression on a point-to-point Frame Relay subinterface, what command(s) is used, and in what configuration modes?

Answer: The frame-relay payload-compress FRF9 stac subinterface subcommand.

Q14. What command lists compression statistics for payload compression on a point-to-point link?

Answer: The show compress command.

Q15. What command lists compression statistics for payload compression on a Frame Relay point-to-point subinterface?

Answer: The show compress command.

Q16 To configure TCP header compression on a point-to-point Frame Relay subinterface, what command(s) is used, and in what configuration modes?

Answer: The frame-relay ip tcp header-compression subinterface subcommand.

17 To configure RTP header compression on a point-to-point link, what command(s) is used, and in what configuration modes?

Answer:
The ip rtp header-compression interface subcommand.
LFI Tools

Q18. List the words represented by the abbreviation LFI.

Answer: Link fragmentation and interleaving.

19. 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.

Q20. 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.

Q21. 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.

Q22. 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 and bits per second, respectively.

Q23. 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), as compared to when you are just using PPP?

Answer: All the queuing tools available for use with PPP are also available with MLP LFI. There are no restrictions.

Q24. Which queuing tools can you enable for shaping queues when using FRF.12? Which ones actually interleave the traffic?

Answer: Weighted Fair Queuing (WFQ), Class-Based WFQ (CBWFQ), Low Latency Queuing (LLQ), and IP RTP Priority can be enabled, with LLQ and IP RTP Priority actually interleaving packets.

Q25. Explain the popularly stated scheduling logic, which is consistent with the Cisco QoS courses, that defines how FRF.12 determines which packets can be interleaved in front of fragments of other packets.

Answer: Unfragmented packets can be interleaved, with fragmented packets not being interleaved.

Q26. 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.

Q27. 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.

Q28. 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.

Q29. What command enables the interleaving feature of MLP LFI?

Answer: The ppp multilink interleave command.

Q30. What commands list counters for the number of interleaved packets using MLP LFI?

Answer: The show queue and show interfaces commands.

Q31. What other QoS feature for Frame Relay must you enable when you also configure FRF.12?

Answer: Frame Relay traffic shaping (FRTS).

Q32. What command enables FRF and sets the fragment size?

Answer: The frame-relay fragment fragment_size command.

Q33. 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 command.

Q34. 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 command.

Q35. How do FRF.12 and FRF.11-C differ in terms of deciding which packets can be interleaved, and which cannot?

Answer: FRF.11-C allows only voice frames to be interleaved. FRF.12 actually interleaves packets that were in a low-latency queue (either with LLQ or IP RTP Priority) in a shaping queue. However, the Cisco courses (upon which the exam is based) states that the FRF.12 classification logic simply interleaves unfragmented frames.

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