31 Taking as many defaults as possible, list the configuration commands needed to configure precedence-based FRED on interface S1/1.
interface serial 1/1
random-detect flow-based
By using the random-detect flow-based command, Cisco IOS Software adds the random-detect command automatically.
32 Taking as many defaults as possible, list the configuration commands needed to configure DSCP-based FRED on interface S1/1.
interface serial 1/1
random-detect dscp-based random-detect flow-based
If you had just used the random-detect flow-based command, and not the random- detect dscp-based command, Cisco IOS Software would have added the random- detect command automatically, which enables precedence-based WRED.
Chapter 7
“Do I Know This Already?” Quiz
Compression
1 Describe what is compressed, and what is not compressed, when using payload compres- sion. Be as specific as possible regarding headers and data.
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.
2 Describe what is compressed, and what is not compressed, when using TCP header compression. Be as specific as possible regarding headers and data.
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.
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 List the three point-to-point payload compression options available in IOS.
Stacker, Microsoft Point-to-Point Compression (MPPC), and Predictor.
5 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?
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.
6 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?
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.
Link Fragmentation and Interleave
7 List the words represented by the abbreviation LFI.
Link fragmentation and interleaving.
8 To 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.
9 What 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.
10 What 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.
11 What command enables FRF and sets the fragment size?
The frame-relay fragment fragment_size command.
12 What other QoS feature for Frame Relay must you enable when you configure FRF.12 as well?
Frame Relay traffic shaping (FRTS).
Q&A
Compression Tools
1 Describe what is compressed, and what is not compressed, when using payload compres- sion. Be as specific as possible regarding headers and data.
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.
2 Describe what is compressed, and what is not compressed, when using TCP header compression. Be as specific as possible regarding headers and data.
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.
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 List the three point-to-point payload compression options available in IOS.
Stacker, Microsoft Point-to-Point Compression (MPPC), and Predictor.
5 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.
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.
6 List the three options for Frame Relay payload compression in IOS. Which of the three is not Cisco proprietary?
Packet by packet, data stream, and FRF.9 are the three options. FRF.9 is not Cisco proprietary.
7 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?
Stacker.
8 How much bandwidth should a G.729 call require over Frame Relay, and how much should be required with cRTP?
A single G.729 call requires 26.4 kbps over Frame Relay, but it only needs 11.2 kbps using cRTP.
9 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?
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.
10 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?
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.
11 To configure Stacker payload compression on a point-to-point link, what command(s) is used, and in what configuration modes?
The compress stac interface subcommand.
12 To 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.
13 To 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.
14 What command lists compression statistics for payload compression on a point-to-point link?
The show compress command.
15 What command lists compression statistics for payload compression on a Frame Relay point-to-point subinterface?
The show compress command.
16 To 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.
17 To 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
18 List the words represented by the abbreviation LFI.
Link fragmentation and interleaving.
19 Describe 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.
20 To 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.
21 To 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.
22 Suppose 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.
23 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?
All the queuing tools available for use with PPP are also available with MLP LFI.
There are no restrictions.
24 Which 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.
25 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.
Unfragmented packets can be interleaved, with fragmented packets not being interleaved.
26 Explain 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.
27 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?
MLP fragments the packet into two equal-sized fragments, and sends one over one link, and one over the other.
28 What 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.
29 What command enables the interleaving feature of MLP LFI?
The ppp multilink interleave command.
30 What commands list counters for the number of interleaved packets using MLP LFI?
The show queue and show interfaces commands.
31 What other QoS feature for Frame Relay must you enable when you also configure FRF.12?
Frame Relay traffic shaping (FRTS).
32 What command enables FRF and sets the fragment size?
The frame-relay fragment fragment_size command.
33 What 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.
34 What 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.
35 How 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
1 Why 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 overbur- dened with a larger number of voice conversations than expected, leading to degra- dation 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 pro- tects voice from data, whereas CAC protects voice from voice.
2 How 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.
3 Name 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.
4 What 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.
5 What Cisco IOS command is used to enable CAC on a VoFR network?
frame-relay voice-bandwidth bandwidth-in-bps
6 What 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.
7 What is the definition of measurement-based CAC?
Measurement-based CAC techniques look into the packet network to gauge the current state of the network.
8 What 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.
9 What Cisco IOS command is used to allow the destination node to participate in measurement-based CAC?
rtr responder
10 What is the definition of resource-based CAC?
Resource-based CAC calculates the resources required to facilitate and protect the call on each leg the call traverses toward the destination and then attempts to reserve these resources for use by the voice conversation.
11 For a gatekeeper to provide CAC, what must be configured for each link that requires protection?
Each link must be associated with a zone for a gatekeeper to provide CAC.
12 What is IntServ and how does it work?
The integrated services (IntServ) model includes provisions for best-effort traffic, real-time traffic, and controlled-link sharing. In the IntServ model, an application requests a specific level of service from the network before sending a data flow. If the requested level of service can be achieved from the network, a reservation is placed in each intermediary router to guarantee the requested level of service for the life of the flow.
13 Name the messages used by RSVP to provide resource reservation and CAC.
Path and resv message
14 What level of service must be in place to provide CAC?
Guaranteed-delay
Q&A
Call Admission Control Concepts
1 Why is CAC 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.
2 Name 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.
3 How does a common channel signaling circuit, such as PRI, react to CAC?
CCS circuits have the capability to send out-of-band signaling to tear down the circuit and reroute the call in the event that a CAC mechanism indicates that the resources are not available to proceed with the call.
4 How does a channel-associated signaling circuit, such as E&M or T1 CAS, react to CAC?
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.
5 What is the difference between payload bandwidth and the bandwidth per call required for a voice conversation?
Payload bandwidth is the amount of bandwidth required to digitize the analog voice stream, whereas the bandwidth per call for a voice conversation takes the Layer 2 overhead, IP, UDP, and RTP headers, and the payload into consideration.
Local-Based CAC
6 What 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.
7 Name three of the five available local-based CAC mechanisms.
• Physical DS0 limitation
• Max-Connections dial-peer configuration
• VoFR, Voice-Bandwidth configuration
• Trunking condition
• Local Voice Busyout
8 What 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.
9 What Cisco IOS command is used to enable CAC on a VoFR network?
frame-relay voice-bandwidth bandwidth-in-bps
10 What type of circuit, CAS or CSS, will Local Voice Busyout (LVB) be most beneficial for?
A CAS circuit is better suited to use LVBO because a CAS circuit can reroute the call in the event that the resources are not available to proceed with the call.
Measurement-Based CAC
11 What is the definition of measurement-based CAC?
Measurement-based CAC techniques look into the packet network to gauge the current state of the network
12 What is the difference between LVBO and Advanced LVBO?
Whereas LVBO provides for busyout based on local conditions of the originating gateway, AVBO adds the capability to trigger an SAA probe to one or more configured IP destinations.
13 What 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.
14 What measurements are gathered by SSA probes for PSTN fallback?
Delay and loss measurements
15 What Cisco IOS command is used to allow the destination node to participate in measurement based CAC
rtr responder