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- •QoS Overview
- •“Do I Know This Already?” Quiz
- •QoS: Tuning Bandwidth, Delay, Jitter, and Loss Questions
- •Foundation Topics
- •QoS: Tuning Bandwidth, Delay, Jitter, and Loss
- •Bandwidth
- •The clock rate Command Versus the bandwidth Command
- •QoS Tools That Affect Bandwidth
- •Delay
- •Serialization Delay
- •Propagation Delay
- •Queuing Delay
- •Forwarding Delay
- •Shaping Delay
- •Network Delay
- •Delay Summary
- •QoS Tools That Affect Delay
- •Jitter
- •QoS Tools That Affect Jitter
- •Loss
- •QoS Tools That Affect Loss
- •Summary: QoS Characteristics: Bandwidth, Delay, Jitter, and Loss
- •Voice Basics
- •Voice Bandwidth Considerations
- •Voice Delay Considerations
- •Voice Jitter Considerations
- •Voice Loss Considerations
- •Video Basics
- •Video Bandwidth Considerations
- •Video Delay Considerations
- •Video Jitter Considerations
- •Video Loss Considerations
- •Comparing Voice and Video: Summary
- •IP Data Basics
- •Data Bandwidth Considerations
- •Data Delay Considerations
- •Data Jitter Considerations
- •Data Loss Considerations
- •Comparing Voice, Video, and Data: Summary
- •Foundation Summary
- •QoS Tools and Architectures
- •“Do I Know This Already?” Quiz
- •QoS Tools Questions
- •Differentiated Services Questions
- •Integrated Services Questions
- •Foundation Topics
- •Introduction to IOS QoS Tools
- •Queuing
- •Queuing Tools
- •Shaping and Policing
- •Shaping and Policing Tools
- •Congestion Avoidance
- •Congestion-Avoidance Tools
- •Call Admission Control and RSVP
- •CAC Tools
- •Management Tools
- •Summary
- •The Good-Old Common Sense QoS Model
- •GOCS Flow-Based QoS
- •GOCS Class-Based QoS
- •The Differentiated Services QoS Model
- •DiffServ Per-Hop Behaviors
- •The Class Selector PHB and DSCP Values
- •The Assured Forwarding PHB and DSCP Values
- •The Expedited Forwarding PHB and DSCP Values
- •The Integrated Services QoS Model
- •Foundation Summary
- •“Do I Know This Already?” Quiz Questions
- •CAR, PBR, and CB Marking Questions
- •Foundation Topics
- •Marking
- •IP Header QoS Fields: Precedence and DSCP
- •LAN Class of Service (CoS)
- •Other Marking Fields
- •Summary of Marking Fields
- •Class-Based Marking (CB Marking)
- •Network-Based Application Recognition (NBAR)
- •CB Marking show Commands
- •CB Marking Summary
- •Committed Access Rate (CAR)
- •CAR Marking Summary
- •Policy-Based Routing (PBR)
- •PBR Marking Summary
- •VoIP Dial Peer
- •VoIP Dial-Peer Summary
- •Foundation Summary
- •Congestion Management
- •“Do I Know This Already?” Quiz
- •Queuing Concepts Questions
- •WFQ and IP RTP Priority Questions
- •CBWFQ and LLQ Questions
- •Comparing Queuing Options Questions
- •Foundation Topics
- •Queuing Concepts
- •Output Queues, TX Rings, and TX Queues
- •Queuing on Interfaces Versus Subinterfaces and Virtual Circuits (VCs)
- •Summary of Queuing Concepts
- •Queuing Tools
- •FIFO Queuing
- •Priority Queuing
- •Custom Queuing
- •Weighted Fair Queuing (WFQ)
- •WFQ Scheduler: The Net Effect
- •WFQ Scheduling: The Process
- •WFQ Drop Policy, Number of Queues, and Queue Lengths
- •WFQ Summary
- •Class-Based WFQ (CBWFQ)
- •CBWFQ Summary
- •Low Latency Queuing (LLQ)
- •LLQ with More Than One Priority Queue
- •IP RTP Priority
- •Summary of Queuing Tool Features
- •Foundation Summary
- •Conceptual Questions
- •Priority Queuing and Custom Queuing
- •CBWFQ, LLQ, IP RTP Priority
- •Comparing Queuing Tool Options
- •“Do I Know This Already?” Quiz
- •Shaping and Policing Concepts Questions
- •Policing with CAR and CB Policer Questions
- •Shaping with FRTS, GTS, DTS, and CB Shaping
- •Foundation Topics
- •When and Where to Use Shaping and Policing
- •How Shaping Works
- •Where to Shape: Interfaces, Subinterfaces, and VCs
- •How Policing Works
- •CAR Internals
- •CB Policing Internals
- •Policing, but Not Discarding
- •Foundation Summary
- •Shaping and Policing Concepts
- •“Do I Know This Already?” Quiz
- •Congestion-Avoidance Concepts and RED Questions
- •WRED Questions
- •FRED Questions
- •Foundation Topics
- •TCP and UDP Reactions to Packet Loss
- •Tail Drop, Global Synchronization, and TCP Starvation
- •Random Early Detection (RED)
- •Weighted RED (WRED)
- •How WRED Weights Packets
- •WRED and Queuing
- •WRED Summary
- •Flow-Based WRED (FRED)
- •Foundation Summary
- •Congestion-Avoidance Concepts and Random Early Detection (RED)
- •Weighted RED (WRED)
- •Flow-Based WRED (FRED)
- •“Do I Know This Already?” Quiz
- •Compression Questions
- •Link Fragmentation and Interleave Questions
- •Foundation Topics
- •Payload and Header Compression
- •Payload Compression
- •Header Compression
- •Link Fragmentation and Interleaving
- •Multilink PPP LFI
- •Maximum Serialization Delay and Optimum Fragment Sizes
- •Frame Relay LFI Using FRF.12
- •Choosing Fragment Sizes for Frame Relay
- •Fragmentation with More Than One VC on a Single Access Link
- •FRF.11-C and FRF.12 Comparison
- •Foundation Summary
- •Compression Tools
- •LFI Tools
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Call Admission Control Overview
- •Call Rerouting Alternatives
- •Bandwidth Engineering
- •CAC Mechanisms
- •CAC Mechanism Evaluation Criteria
- •Local Voice CAC
- •Physical DS0 Limitation
- •Max-Connections
- •Voice over Frame Relay—Voice Bandwidth
- •Trunk Conditioning
- •Local Voice Busyout
- •Measurement-Based Voice CAC
- •Service Assurance Agents
- •SAA Probes Versus Pings
- •SAA Service
- •Calculated Planning Impairment Factor
- •Advanced Voice Busyout
- •PSTN Fallback
- •SAA Probes Used for PSTN Fallback
- •IP Destination Caching
- •SAA Probe Format
- •PSTN Fallback Scalability
- •PSTN Fallback Summary
- •Resource-Based CAC
- •Resource Availability Indication
- •Gateway Calculation of Resources
- •RAI in Service Provider Networks
- •RAI in Enterprise Networks
- •RAI Operation
- •RAI Platform Support
- •Cisco CallManager Resource-Based CAC
- •Location-Based CAC Operation
- •Locations and Regions
- •Calculation of Resources
- •Automatic Alternate Routing
- •Location-Based CAC Summary
- •Gatekeeper Zone Bandwidth
- •Gatekeeper Zone Bandwidth Operation
- •Single-Zone Topology
- •Multizone Topology
- •Zone-per-Gateway Design
- •Gatekeeper in CallManager Networks
- •Zone Bandwidth Calculation
- •Gatekeeper Zone Bandwidth Summary
- •Integrated Services / Resource Reservation Protocol
- •RSVP Levels of Service
- •RSVP Operation
- •RSVP/H.323 Synchronization
- •Bandwidth per Codec
- •Subnet Bandwidth Management
- •Monitoring and Troubleshooting RSVP
- •RSVP CAC Summary
- •Foundation Summary
- •Call Admission Control Concepts
- •Local-Based CAC
- •Measurement-Based CAC
- •Resources-Based CAC
- •“Do I Know This Already?” Quiz
- •QoS Management Tools Questions
- •QoS Design Questions
- •Foundation Topics
- •QoS Management Tools
- •QoS Device Manager
- •QoS Policy Manager
- •Service Assurance Agent
- •Internetwork Performance Monitor
- •Service Management Solution
- •QoS Management Tool Summary
- •QoS Design for the Cisco QoS Exams
- •Four-Step QoS Design Process
- •Step 1: Determine Customer Priorities/QoS Policy
- •Step 2: Characterize the Network
- •Step 3: Implement the Policy
- •Step 4: Monitor the Network
- •QoS Design Guidelines for Voice and Video
- •Voice and Video: Bandwidth, Delay, Jitter, and Loss Requirements
- •Voice and Video QoS Design Recommendations
- •Foundation Summary
- •QoS Management
- •QoS Design
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •The Need for QoS on the LAN
- •Layer 2 Queues
- •Drop Thresholds
- •Trust Boundries
- •Cisco Catalyst Switch QoS Features
- •Catalyst 6500 QoS Features
- •Supervisor and Switching Engine
- •Policy Feature Card
- •Ethernet Interfaces
- •QoS Flow on the Catalyst 6500
- •Ingress Queue Scheduling
- •Layer 2 Switching Engine QoS Frame Flow
- •Layer 3 Switching Engine QoS Packet Flow
- •Egress Queue Scheduling
- •Catalyst 6500 QoS Summary
- •Cisco Catalyst 4500/4000 QoS Features
- •Supervisor Engine I and II
- •Supervisor Engine III and IV
- •Cisco Catalyst 3550 QoS Features
- •Cisco Catalyst 3524 QoS Features
- •CoS-to-Egress Queue Mapping for the Catalyst OS Switch
- •Layer-2-to-Layer 3 Mapping
- •Connecting a Catalyst OS Switch to WAN Segments
- •Displaying QoS Settings for the Catalyst OS Switch
- •Enabling QoS for the Catalyst IOS Switch
- •Enabling Priority Queuing for the Catalyst IOS Switch
- •CoS-to-Egress Queue Mapping for the Catalyst IOS Switch
- •Layer 2-to-Layer 3 Mapping
- •Connecting a Catalyst IOS Switch to Distribution Switches or WAN Segments
- •Displaying QoS Settings for the Catalyst IOS Switch
- •Foundation Summary
- •LAN QoS Concepts
- •Catalyst 6500 Series of Switches
- •Catalyst 4500/4000 Series of Switches
- •Catalyst 3550/3524 Series of Switches
- •QoS: Tuning Bandwidth, Delay, Jitter, and Loss
- •QoS Tools
- •Differentiated Services
- •Integrated Services
- •CAR, PBR, and CB Marking
- •Queuing Concepts
- •WFQ and IP RTP Priority
- •CBWFQ and LLQ
- •Comparing Queuing Options
- •Conceptual Questions
- •Priority Queuing and Custom Queuing
- •CBWFQ, LLQ, IP RTP Priority
- •Comparing Queuing Tool Options
- •Shaping and Policing Concepts
- •Policing with CAR and CB Policer
- •Shaping with FRTS, GTS, DTS, and CB Shaping
- •Shaping and Policing Concepts
- •Congestion-Avoidance Concepts and RED
- •WRED
- •FRED
- •Congestion-Avoidance Concepts and Random Early Detection (RED)
- •Weighted RED (WRED)
- •Flow-Based WRED (FRED)
- •Compression
- •Link Fragmentation and Interleave
- •Compression Tools
- •LFI Tools
- •Call Admission Control Concepts
- •Local-Based CAC
- •Measurement-Based CAC
- •Resources-Based CAC
- •QoS Management Tools
- •QoS Design
- •QoS Management
- •QoS Design
- •LAN QoS Concepts
- •Catalyst 6500 Series of Switches
- •Catalyst 4500/4000 Series of Switches
- •Catalyst 3550/3524 Series of Switches
- •Foundation Topics
- •QPPB Route Marking: Step 1
- •QPPB Per-Packet Marking: Step 2
- •QPPB: The Hidden Details
- •QPPB Summary
- •Flow-Based dWFQ
- •ToS-Based dWFQ
- •Distributed QoS Group–Based WFQ
- •Summary: dWFQ Options
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770 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections
26Discuss the delay requirements of video traffic.
Interactive video (video conferencing, for instance) requires low delay because it is interactive. Delay budgets up to 200 ms are the norm. However, streaming video— one-way video—can tolerate long delays. When playing an e-learning video, for instance, the playout may start after 30 seconds of video has been received into a de-jitter buffer—but each packet may have experienced several seconds of delay.
27List the basic differences between TCP and UDP traffic.
TCP performs error recovery, whereas UDP does not. TCP also uses dynamic windowing to perform flow control, whereas UDP does not. Both use port numbers to multiplex among various applications running on a single computer.
28Contrast the QoS characteristics needed by interactive data applications, as compared to the QoS needs of voice payload flows.
Answering such a question requires one to understand that QoS requirements for data applications are more subjective than those for voice. Generally, interactive data wants consistent delay (low jitter), but relative to voice, more jitter is tolerable. Bandwidth demands vary greatly for data applications, whereas a single voice call uses a static amount of bandwidth. Delay for interactive data can be relatively longer than for voice, but the key measurement for data is application response time, which includes round-trip packet delays. Finally, data applications are much more tolerant of packet loss, because either the application will resend the data, or rely on TCP to resend the data, or just not care whether some data is lost.
Chapter 2
“Do I Know This Already?” Quiz
QoS Tools
1List four queuing tools, including the full names and popular acronyms.
Priority Queuing (PQ), Custom Queuing (CQ), Weighted Fair Queuing (WFQ), IP RTP Priority, Class-Based WFQ (CBWFQ), Low Latency Queuing (LLQ), Modified Deficit Round-Robin (MDRR).
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2List four link-efficiency tools, including the full names and popular acronyms.
Payload compression, RTP header compression (cRTP), TCP header compression, Multilink PPP fragmentation and interleaving (MLPPP LFI), Frame Relay fragmentation (FRF), link fragmentation and interleaving for Frame Relay and ATM virtual circuits (VCs).
3Which of the following tools can be used for classification and marking? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
CB marking, NBAR, CAR, and QPPB.
4Which of the following tools can be used for policing? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
CAR.
Differentiated Services
5Define the DiffServ term “DSCP,” including what the acronym stands for.
According to RFC 2475, DSCP refers to “a specific value of the DSCP portion of the DS field, used to select a PHB.” The acronym stands for differentiated services code point. It is the 6-bit field in the redefined ToS byte in the IP header used for marking packets for DiffServ.
6Define the DiffServ term “PHB,” including what the acronym stands for.
According to RFC 2475, PHB refers to “the externally observable forwarding behavior applied at a DS-compliant node to a DS behavior aggregate.” The acronym stands for per-hop behavior. It is the collection of QoS actions that occur at one router (hop) in a network for a particular behavior aggregate (BA).
7Compare and contrast the terms “shaper,” “meter,” and “dropper,” according to DiffServ specifications. Suggest typical points in the network where each is used.
The meter function measures the packet/bit rate of ingress or egress traffic, as well as the sizes of bursts of traffic, with the goal of determining whether the traffic meets the agreed-upon traffic contract. A shaper delays packets on egress so that the bit rate and burst size do not exceed a configured level. Droppers, called policers by Cisco IOS, drop packets that exceed a configured bit rate and burst size. Shapers are typically used on egress from one differentiated services domain; policers are typically used on ingress boundary nodes.
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772 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections
8Compare and contrast the contents of the IP ToS byte before and after the advent of DiffServ.
Before DiffServ, the ToS byte contained a 3-bit Precedence field, 4 bits in a ToS field, and 1 reserved bit. DiffServ redefined the ToS byte to contain a 6-bit DSCP field, which contains the DSCP values, and 2 reserved bits.
Integrated Services
9Imagine an enterprise network, connected to an Internet service provider (ISP), that is connected to a second ISP, which is then connected to another enterprise network. The second ISP does not support IntServ directly. Discuss the two options that allow the other three networks to support IntServ for flows that pass through the nonsupporting ISP.
Two options exist for support of RSVP and IntServ. Both require ISP2 to pass the RSVP messages through it just like any other packet. Additionally, with one option, no guarantees are made in ISP2; the packets in the reserved flows are treated as besteffort traffic. The other option maps the RSVP flows to a DSCP, so ISP2 can provide DiffServ QoS treatment.
10Compare and contrast DiffServ and IntServ in terms of using classes, flows, and scalability.
IntServ applies to individual flows, whereas DiffServ differentiates traffic into classes. With large networks and the Internet, the number of IntServ-managed flows does not scale, because information retained about each flow, and the RSVP signaling messages for each flow, continue throughout the life of each flow. DiffServ uses classes, and the number of classes does not increase when packet volumes increase, which allows better scalability.
11Describe the two options available to a router to perform IntServ admission control.
Routers can perform local admission control by just examining the amount of currently reserved bandwidth, versus the maximum allowed, on each of its interfaces. Routers can also use a Common Open Policy Server (COPS) policy decision point (PDP), asking it to decide whether enough bandwidth is available in the network.
12What is the QoS framework, and what does it define?
The QoS framework is a drawing and description that lists the major components of QoS functions in a network.
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Q&A
1List four classification and marking tools, including the full names and popular acronyms.
Committed access rate (CAR), policy-based routing (PBR), class-based marking (CB marking), dial peers, network-based application recognition (NBAR), QoS policy propagation with BGP (QPPB).
2List four queuing tools, including the full names and popular acronyms.
Priority Queuing (PQ), Custom Queuing (CQ), Weighted Fair Queuing (WFQ), IP RTP Priority, Class-Based WFQ (CBWFQ), Low Latency Queuing (LLQ), Modified Deficit Round-Robin (MDRR).
3List four policing and shaping tools, including the full names and popular acronyms.
Committed access rate (CAR), class-based policing (CB policing), Frame Relay traffic shaping (FRTS), generic traffic shaping (GTS), distributed traffic shaping (DTS), class-based shaping (CB shaping).
4List three congestion-avoidance tools, including the full names and popular acronyms.
Random Early Detection (RED), Weighted RED (WRED), Flow-Based RED (FRED).
5List four link-efficiency tools, including the full names and popular acronyms.
Payload compression, RTP header compression (cRTP), TCP header compression, Multilink PPP fragmentation and interleaving (MLPPP LFI), Frame Relay fragmentation (FRF), link fragmentation and interleaving for Frame Relay and
ATM VCs.
6List seven VoIP CAC tools, including the full names and popular acronyms.
Physical DS0 limitation, Max-connections, voice-bandwidth for Frame Relay, trunk conditioning, Local Voice Busy-Out (LVBO), Advanced Voice Busy-Out (AVBO), PSTN Fallback, Resource Availability Indicator (RAI), Gatekeeper Zone Bandwidth (GK Zone Bandwidth), Resource Reservation Protocol (RSVP).
7List four QoS management tools, including the full names and popular acronyms.
QoS Device Manager (QDM), QoS Policy Manager (QPM), Service Assurance Agent (SAA), Internetwork Performance Monitor (IPM), Service Management Solution (SMS).
8List the QoS tools that perform some classification function.
This is a bit of a trick question. Almost all IOS QoS tools perform classification—for instance, to place two different types of packets into two different queues, the queue tool performs classification.
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774 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections
9Which of the following tools can be used for classification and marking? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
CB marking, NBAR, CAR, and QPPB.
10Which of the following tools can be used for queuing? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
WFQ, LLQ, PQ, CQ.
11Which of the following tools can be used for policing? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
CAR
12Which of the following tools can be used for shaping? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
CB shaping, FRTS, GTS
13Which of the following tools can be used for link efficiency? CAR, CB marking, PQ, CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
FRF, cRTP, MLPPP LFI
14Which of the following tools can be used for network management? CAR, CB marking, PQ CB shaping, QDM, WFQ, WRED, FRTS, LLQ, GTS, RAI, FRF, RSVP, SAA, MLPPP LFI, AVBO, CQ, NBAR, QPM, CAR, FRED, QPPB, cRTP
QPM, QDM, SAA
15Define the DiffServ term “behavior aggregate.”
According to RFC 2475, a behavior aggregate is “a collection of packets with the same DS code point crossing a link in a particular direction.” The key points are that the DSCP has been set; the packets all move the same direction; and the packets collectively make up a class.
16Define the DiffServ term “DSCP,” including what the acronym stands for.
According to RFC 2475, DSCP refers to “a specific value of the DSCP portion of the DS field, used to select a PHB.” The acronym stands for differentiated services code point. It is the 6-bit filed in the redefined ToS byte in the IP header used for marking packets for DiffServ.
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17Define the DiffServ term “PHB,” including what the acronym stands for.
According to RFC 2475, PHB refers to “the externally observable forwarding behavior applied at a DS-compliant node to a DS behavior aggregate.” The acronym stands for per-hop behavior. It is the collection of QoS actions that occur at one router (hop) in a network for a particular BA.
18Define the DiffServ term “MF classifier,” including what the acronym stands for.
According to RFC 2475, an MF classifier is “a multi-field (MF) classifier which selects packets based on the content of some arbitrary number of header fields; typically some combination of source address, destination address, DS field, protocol ID, source port and destination port.” It is the classification function used to classify packets before the DSCP has been set.
19Define the DiffServ term “DS ingress node,” including what the acronym stands for.
According to RFC 2475, a DS ingress node is “a DS boundary node in its role in handling traffic as it enters a DS domain.” DS stands for differentiated services. The term defines a node at which packets enter the DiffServ domain.
20Compare and contrast the terms “BA classifier” and “MF classifier,” according to DiffServ specifications. Suggest typical points in the network where each is used.
A classifier is a DiffServ function that classifies or categories packets based on the contents of fields in the packet headers. A BA classifier performs this function only based on the DSCP field. An MF classifier can look at many fields in the packet header. MF classifiers typically classify ingress traffic near the edge of a network, and work with markers to set the DSCP field. BA classifiers are used at points in the network after an MF classifier and marker have set the DSCP field values.
21Compare and contrast the terms “shaper,” “meter,” and “dropper,” according to DiffServ specifications. Suggest typical points in the network where each is used.
The meter function measures the packet/bit rate of ingress or egress traffic, as well as the sizes of bursts of traffic, with the goal of determining whether the traffic meets the agreed-upon traffic contract. A shaper delays packets on egress so that the bit rate and burst size do not exceed a configured level. Droppers, called policers by Cisco IOS, drop packets that exceed a configured bit rate and burst size. Shapers are typically used on egress from one DS domain; policers are typically used on ingress boundary nodes.
22Compare and contrast the contents of the IP ToS byte before and after the advent of DiffServ.
Before DiffServ, the ToS byte contained a 3-bit Precedence field, 4 bits in a ToS field, and 1 reserved bit. DiffServ redefined the ToS byte to contain a 6-bit DSCP field, which contains the DSCP values, and 2 reserved bits.
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776 Appendix A: Answers to the “Do I Know This Already?” Quizzes and Q&A Sections
23Describe the QoS behavior at a single DS node when using the AF PHB. Also explain what the acronym “AF PHB” represents and identify the RFC that defines it.
The assured forwarding per-hop behavior, as defined in RFC 2597, defines a PHB with two components. The first part defines four BAs or classes, each which should be placed in a separate queue and given a configured guaranteed minimum amount of bandwidth. The second component provides three different drop probabilities for a congestion-avoidance tool such as RED.
24Explain (by comparing and contrasting) whether AF and CS PHB DSCPs conform to the concept that “bigger DSCP values are better than smaller values.”
CS uses values that have three binary 0s at the end, and the eight IP precedence values for the first three bits. In other words, CS includes the eight binary values for a 6-bit number for which the last three digits are 0s. CS conforms to the idea that a bigger value is better, to be backward compatible with IP precedence. AF uses 12 different values. Of the three AF DSCPs in each class, the highest of the three values actually receives the worst drop preference.
25Describe the QoS behavior at a single DS node when using the EF PHB. Also explain what the acronym “EF PHB” represents and identify the RFC that defines it.
The expedited forwarding per-hop behavior, as defined in RFC 2598, defines a PHB with two components. The first part defines queuing, with features that reserve bandwidth for a single BA, with the added feature on minimizing latency, delay, and loss. The other action of the PHB provides a policing/dropper function, disallowing traffic beyond a configured maximum bandwidth for the class.
26Describe the process used by RSVP to reserve bandwidth in a network.
A host signals to the network using an RSVP reservation request using an RSVP path message. The request passes along the route to the destination host; at each intermediate router, if that router can guarantee the right bandwidth, the request is forwarded. When received by the destination host, it replies with an RSVP resv message. The process is reversed, with each router passing the reserve message if it can guarantee the bandwidth in the opposite direction. If the original host receives the reservation message, the bandwidth has been reserved.
27Imagine an enterprise network, connected to an Internet service provider (ISP), that is connected to a second ISP, which is then connected to another enterprise network. The second ISP does not support IntServ directly. Discuss the two options that allow the other three networks to support IntServ for flows that pass through the nonsupporting ISP.
Two options exist for support of RSVP and IntServ. Both require ISP2 to pass the RSVP messages through it just like any other packet. Additionally, with one option, no guarantees are made in ISP2; the packets in the reserved flows are treated as besteffort traffic. The other option maps the RSVP flows to a DSCP, so ISP2 can provide DiffServ QoS treatment.
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28List and describe the two main features of IntServ.
IntServ provides resource reservation and admission control. Resource reservation uses the RSVP protocol to reserve bandwidth and set a maximum delay value. To prevent too many reserved flows from occurring, however, routers can reject new reservations, which is called admission control.
29Compare and contrast DiffServ and IntServ in terms of using classes, flows, and scalability.
IntServ applies to individual flows, whereas DiffServ differentiates traffic into classes. With large networks and the Internet, the number of IntServ-managed flows does not scale, because information retained about each flow, and the RSVP signaling messages for each flow, continues throughout the life of each flow. DiffServ uses classes, and the number of classes does not increase when packet volumes increase, which allows better scalability.
30Describe the two options available to a router to perform IntServ admission control.
Routers can perform local admission control by just examining the amount of currently reserved bandwidth, versus the maximum allowed, on each of its interfaces. Routers can also use a Common Open Policy Server (COPS) policy decision point (PDP), asking it to decide whether enough bandwidth is available in the network.
31What is the QoS framework, and what does it define?
The QoS framework is a drawing and description that lists the major components of QoS functions in a network.
32Which five categories of QoS tools are shown in the bottom half of the Cisco QoS framework?
Classification and marking, congestion management, link efficiency, traffic conditioners, congestion avoidance.
Chapter 3
“Do I Know This Already?” Quiz
Classification and Marking Concepts
1Describe the difference between classification and marking.
Classification processes packet headers, or possibly other information, to differentiate between multiple packets. Marking changes a field inside the frame or packet header.