
- •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

162 Chapter 3: Classification and Marking
Table 3-3 Fields Directly Matchable by Classification and Marking Tools (Continued)
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Comments |
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Host name and URL string |
CB marking |
NBAR can also match URL strings, including the |
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host name, using regular expressions. CB marking |
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can use NBAR to match these strings for |
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classification. |
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Outgoing Interface |
Policy-based |
Checks the routing table and finds all valid routes |
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routing (PBR) |
for the packet; matches based on the outgoing |
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interface. |
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Next-Hop |
PBR |
Similar to the outgoing interface, but it checks the |
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next-hop routers’ IP addresses. |
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Metric |
PBR |
Checks the routing table entry for this packet, and |
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compares the metric value to match the packet. |
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Route type |
PBR |
Checks the routing table, looking at the source of |
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the routing table entry that matches the packet. |
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Dial Peer |
Dial peers |
Based on the dial peer and used to connect a VoIP |
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call. |
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These two tables can be a little intimidating, especially for those of you studying to pass the QoS exams! Rest assured, however, that this book covers approximately 30 QoS tools, and the exams typically have no more than 60 questions on QoS. So, statistically speaking, the exams simply do not have enough questions to ask you about every little item that a tool could use for classification. I just included these tables, and others like them, for reference.
Marking
Marking involves setting some bits inside a data link or network layer header, with the goal of letting other devices’ QoS tools classify based on the marked values. You can mark a wide variety of fields, and each has a particular purpose. Some fields are more widely used, and some are less widely used. Some marking options make sense for all devices on the LAN, whereas others only when using specific hardware platforms. Marking at the WAN is possible, too.
The following sections list the header fields that you can use for marking, along with explanations of when it is most useful to use that particular field. Recommendations follow these sections as to when to use classification and marking.

Classification and Marking Concepts 163
IP Header QoS Fields: Precedence and DSCP
The two most popular marking fields for QoS are the IP Precedence and IP DSCP fields that were introduced in Chapter 2, “QoS Tools and Architectures.” QoS tools frequently use these two fields in part because the IP packet header exists from endpoint to endpoint in a network, as shown in Figure 3-1.
Figure 3-1 Headers Used During Typical Packet Flow
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PC1 |
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another group; send |
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to nearby router. |
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Eth. |
IP packet |
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10.0.0.0 |
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My route |
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R1 |
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to that group is |
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out seriallink. |
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IP packet |
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My route |
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R2 |
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to that group is |
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out Frame |
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Relay. |
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FR |
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IP packet |
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168.11.0.0 |
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R3 |
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Send directly |
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to Barney. |
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TR |
IP packet |
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168.1.0.0 |
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PC2
168.1.1.1
As seen in Figure 3-1, the IP packet en route to host PC2 stays intact throughout the network, whereas the data-link headers only exist while a frame is crossing between a host and a router, or between routers.

164 Chapter 3: Classification and Marking
Figure 3-2 outlines the two marking fields and their positions inside an IP header.
Figure 3-2 IP Precedence and IP DSCP Fields
8 bits
Precedence TOS CU
IP Header, |
Byte 1 |
TOS Byte |
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Bytes 3-20 |
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Before DiffServ |
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IP Header, |
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Byte 1 |
DS Field* |
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DSCP |
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CU |
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8 bits |
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You can mark the Precedence and DSCP fields with any valid binary value of either 3 or 6 bits, respectively. Chapter 2 contains detailed discussion of the recommended values used in these two fields. Briefly, Precedence field values should grow in importance, and in QoS behavior, as the number gets higher. DSCP differs in that several per-hop behavior (PHB) RFCs define suggested DSCP values for which the larger number does not always get a better QoS treatment.
Table 3-4 lists the IP precedence and DSCP values, and their names, for review. Note that not all DSCP values are listed; only the DSCP values suggested by the DiffServ RFCs are listed in the table. QoS tools that are capable of setting DSCP can set any of the actual 64 values.
Table 3-4 IP Precedence and DSCP—Popular Values and Names
Field and |
Binary |
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Value (Decimal) |
Value |
Name |
Defined by This RFC |
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Precedence 0 |
000 |
routine |
791 |
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Precedence 1 |
001 |
priority |
791 |
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Precedence 2 |
010 |
immediate |
791 |
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Precedence 3 |
011 |
flash |
791 |
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Precedence 4 |
100 |
flash override |
791 |
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Precedence 5 |
101 |
critic |
791 |
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Precedence 6 |
110 |
internetwork control |
791 |
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Precedence 7 |
111 |
network control |
791 |
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DSCP 0 |
000000 |
best effort or default |
2475 |
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Classification and Marking Concepts 165 |
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Table 3-4 IP Precedence and DSCP—Popular Values and Names (Continued) |
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Field and |
Binary |
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Value (Decimal) |
Value |
Name |
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Defined by This RFC |
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DSCP 8 |
001000 |
CS1 |
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2475 |
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DSCP 16 |
010000 |
CS2 |
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2475 |
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DSCP 24 |
011000 |
CS3 |
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2475 |
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DSCP 32 |
100000 |
CS4 |
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2475 |
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DSCP 40 |
101000 |
CS5 |
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2475 |
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DSCP 48 |
110000 |
CS6 |
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2475 |
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DSCP 56 |
111000 |
CS7 |
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DSCP 10 |
001010 |
AF11 |
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2597 |
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DSCP 12 |
001100 |
AF12 |
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2597 |
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DSCP 14 |
001110 |
AF13 |
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2597 |
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DSCP 18 |
010010 |
AF21 |
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2597 |
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DSCP 20 |
010100 |
AF22 |
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2597 |
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DSCP 22 |
010110 |
AF23 |
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2597 |
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DSCP 26 |
011010 |
AF31 |
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2597 |
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DSCP 28 |
011100 |
AF32 |
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2597 |
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DSCP 30 |
011110 |
AF33 |
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2597 |
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DSCP 34 |
100010 |
AF41 |
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2597 |
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DSCP 36 |
100100 |
AF42 |
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2597 |
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DSCP 38 |
100110 |
AF43 |
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2597 |
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DSCP 46 |
101110 |
EF |
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2598 |
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CS = Class Selector
AF = Assured Forwarding
EF = Expedited Forwarding
The two IP header QoS marking fields do not provide all the QoS marking fields needed today. One day, all other Layer 3 protocols besides IP may no longer be used. One day, all LAN switches will be capable of looking at IP headers, including IP DSCP and Precedence, and perform QoS based on those fields. Likewise, one day, all WAN services, including Frame Relay and ATM switches, will be able to perform QoS based on these same fields. However, today’s reality is that even as more and more devices become capable of marking and reacting to IP precedence and DSCP, it will be a long time before all networking devices are both capable and configured to use these fields for QoS purposes. So, other QoS marking fields are needed.