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12To configure Stacker payload compression on a point-to-point Frame Relay subinterface, what command(s) is used, and in what configuration modes?
The frame-relay compress stac subinterface subcommand.
13To configure FRF.9 payload compression on a point-to-point Frame Relay subinterface, what command(s) is used, and in what configuration modes?
The frame-relay payload-compress FRF9 stac subinterface subcommand.
14What command lists compression statistics for payload compression on a point-to-point link?
The show compress command.
15What command lists compression statistics for payload compression on a Frame Relay point-to-point subinterface?
The show compress command.
16To configure TCP header compression on a point-to-point Frame Relay subinterface, what command(s) is used, and in what configuration modes?
The frame-relay ip tcp header-compression subinterface subcommand.
17To configure RTP header compression on a point-to-point link, what command(s) is used, and in what configuration modes?
The ip rtp header-compression interface subcommand.
LFI Tools
18List the words represented by the abbreviation LFI.
Link fragmentation and interleaving.
19Describe the main motivation for LFI tools in relation to the support of data, voice, and video traffic.
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.
20To achieve a 20-ms serialization delay on a 128-kbps link, how long can the fragments be?
The formula is max-delay * bandwidth, which is .02 * 128,000 = 2560 bits, or 320 bytes.
21To achieve a 10-ms serialization delay on a 64-kbps link, how long can the fragments be?
The formula is max-delay * bandwidth, which is .01 * 64,000 = 640 bits, or 80 bytes.
812 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections
22Suppose that a 1500-byte packet exits a 56-kbps serial interface, and LFI is not used. How long is the serialization delay?
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.
23Which 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?
All the queuing tools available for use with PPP are also available with MLP LFI. There are no restrictions.
24Which queuing tools can you enable for shaping queues when using FRF.12? Which ones actually interleave the traffic?
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.
25Explain 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.
Unfragmented packets can be interleaved, with fragmented packets not being interleaved.
26Explain the scheduling logic used by MLP LFI to determine which packets can be interleaved in front of fragments of other packets.
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.
27Suppose 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?
MLP fragments the packet into two equal-sized fragments, and sends one over one link, and one over the other.
28What command can you use to determine the fragment size used for MLP LFI? What is the only parameter of the command?
The ppp multilink fragment-delay command sets the maximum serialization delay in milliseconds. IOS calculates the fragment size using the formula max-delay * bandwidth.
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29What command enables the interleaving feature of MLP LFI?
The ppp multilink interleave command.
30What commands list counters for the number of interleaved packets using MLP LFI?
The show queue and show interfaces commands.
31What other QoS feature for Frame Relay must you enable when you also configure FRF.12?
Frame Relay traffic shaping (FRTS).
32What command enables FRF and sets the fragment size?
The frame-relay fragment fragment_size command.
33What command lists counters for the numbers of packets and bytes that were fragmented and unfragmented by FRF.12?
The show frame-relay fragment interface command.
34What command lists counters for the numbers of packets and bytes that would have been sent if FRF.12 fragmentation had not been performed?
The show frame-relay fragment interface command.
35How do FRF.12 and FRF.11-C differ in terms of deciding which packets can be interleaved, and which cannot?
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.
Chapter 8
“Do I Know This Already?” Quiz
1Why is call admission control needed in an environment where LLQ has been properly implemented?
LLQ provides classification, marking, and prioritization of voice packets, but does not differentiate between voice streams. The priority queue can become overburdened with a larger number of voice conversations than expected, leading to degradation in all voice conversations. CAC provides the protection needed to guarantee the quality of service for established voice conversations by limiting the number of simultaneous voice conversations allowed in the priority queue. In short, LLQ protects voice from data, whereas CAC protects voice from voice.
814 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections
2How does a channel-associated signaling circuit, such as E&M or T1 CAS, react to call admission control?
CAS circuits carry signaling in band. This forces the call to be hairpinned back on another channel in the event that a CAC mechanism indicates that the resources are not available to proceed with the call.
3Name four possible measures that a CAC mechanism can take in the event that the resources are not available to proceed with the call.
•Reroute call to alternate LAN/WAN link.
•Reroute call to alternate PSTN link.
•Return call to originating PBX for rerouting.
•Return a reorder tone, or fast-busy, to the caller.
4What is the definition of local-based CAC?
Local CAC mechanisms base the availability of network resources on local nodal information such as the state of the outgoing LAN or WAN link.
5What Cisco IOS command is used to enable CAC on a VoFR network? frame-relay voice-bandwidth bandwidth-in-bps
6What Cisco IOS command is used to enable physical DS0 limitation?
Physical DS0 limitation is not an IOS command; it is a design methodology that limits the number of physical DS0 connections into the gateway.
7What is the definition of measurement-based CAC?
Measurement-based CAC techniques look into the packet network to gauge the current state of the network.
8What is the difference between an SAA packet and a ping packet?
SAA packets can be IP/TCP, or IP/UDP, or (most common) IP/UDP/RTP packets with sizes, frequency, and ToS all set to accurately synthesize the protocol being observed; whereas a ping packet is an ICMP best-effort packet that does not resemble a voice packet in size or protocol.
9What Cisco IOS command is used to allow the destination node to participate in measurement-based CAC?
rtr responder
