
- •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

694 Chapter 10: LAN QoS
Example 10-1 DSCP-to-CoS Mapping (Continued)
!
interface FastEthernet0/0.1 encapsulation dot1Q 102
service-policy input map-cos-to-dscp service-policy output map-dscp-to-cos
!
interface FastEthernet0/0.2 encapsulation dot1Q 2 native
After classification has been established, QoS tools can be used to differentiate the traffic and direct the desired classification to the proper queue.
Layer 2 Queues
Although queues on LAN switches behave similarly as compared with router queues, a slightly different perspective helps with understanding the problems solved by queuing on a LAN switch. Think of a Layer 2 queue as a bucket that buffers packets until they are transported. The larger the bucket, the more packets it holds. In a switch that has one queue, refered to as 1q, for example, all traffic is placed in this queue regardless of traffic classification and traffic is serviced on a first-in, first-out (FIFO) basis. If a packet arrives at the queue during a period of congestion, the packet may be dropped if the queue cannot hold additional packets.
Using multiple queues on a switch interface allocates some of the finite number of switch buffers in each queue. You can protect the voice queue against the possibility of running out of buffers by putting delay-sensitive voice into one queue, and all else into another. A switch that has two queues, refered to as 2q, for example, has the capability to direct traffic matching a specific classification, such as voice traffic with a CoS value of 5, into one queue while directing other traffic that does not meet this critera into the other queue. Because voice packets are smaller and more predictable than data packets, classifying and scheduling high-priority traffic into this second queue decreases the likelihood that the second queue will expreience buffer overrun and discard the high-priority traffic.
Keep in mind that each port has a finite amount of buffer space to support the buckets. One queue will take all of the buffer space, for instance, two queues will divide the buffer space into two parts, three queues divide the buffer space into three parts, and so on. If the buffer space is too small, it will not be effective in momentarily holding the traffic before transport. Because nonpriority queues are serviced in either a round-robin or a Weighted Round-Robin manner, there is no guarantee that the traffic in the buffer is transported next. This limitation can lead to instaneous buffer overrun for the classified traffic of the last example due to the smaller buffer size available.
One remmedy for this situation is the introduction of a single strict-priority queue, refered to as 1p—meaning “one-priority queue.” In a 1p queue, all traffic within the priority queue is prioritized over the traffic in a standard queue by being transported as it is received. For example, a

The Need for QoS on the LAN 695
switch configured with one priority queue and one standard queue, called 1p1q, can be configured to direct traffic matching a specific classification, such as voice traffic with a CoS value of 5, into the priority queue, ensuring immediate transport, while directing other traffic that does not meet this critera into the standard queue.
Table 10-2 defines a few of the Layer 2 queues available in Cisco Catalyst switches.
Table 10-2 Layer 2 Queues
Layer 2 Queue |
Description |
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1q |
A single Layer 2 queue. All traffic crossing the interface flows through this queue. |
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2q |
2 Layer 2 queues. Traffic can be directed to the desired queue based on |
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classification. |
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1p1q |
1 priority Layer 2 queue and 1 standard Layer 2 queue. Traffic can be directed to |
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the priority queue based on classification. Other traffic can be directed to the |
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standard queue. |
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1p2q |
1 priority Layer 2 queue and 2 standard Layer 2 queues. Traffic can be directed to |
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the priority queue based on classification. Traffic can be directed to the desired |
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standard queues based on additional classification. |
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It is highly recommended that you place real-time applications into the priority queue.
Drop Thresholds
Drop thresholds define the amount of the total Layer 2 buffer use that must be reached before a specified class of traffic is dropped. In other words, this is how much the bucket needs to fill before a decision is made to begin dropping traffic of a specific class. Some switches have one priority queue and one standard queue, for instance, with four drop thesholds on the nonpriority queue, refered to as 1p1q4t. For the traffic placed into the nonpriority queue, the four drop thresholds indicate which class of traffic, based on CoS, should be dropped most agresssively in the event that a percentage of the bucket fills.
Table 10-3 illustrates a possible configuration of the theshold values from the previous example.
Table 10-3 Drop Thesholds
Queue Use Threshold |
Traffic to Drop |
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50% |
CoS 0–1 |
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60% |
CoS 2–3 |
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80% |
CoS 4–5 |
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100% |
CoS 6–7 |
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696 Chapter 10: LAN QoS
After the queue has reached 50 percent of capacity, any traffic classified with CoS of 0 or 1 becomes drop candidates to avoid congestion. If the queue continues to fill in spite of the drops, at 60 percent of capacity any traffic classified with a CoS of 0, 1, 2, or 3 becomes drop candidates to avoid congestion. If the queue still continues to fill in spite of the drops, at 80 percent of capacity any traffic classified with a CoS of 0, 1, 2, 3, 4, or 5 becomes drop candidates to avoid congestion. At 100 percent of capacity, all traffic, regardless of classification, becomes drop candidates.
Figure 10-4 illustrates the drop thresholds.
Figure 10-4 Drop Thresholds
Drop Threshold 4: 100%
Reserved for
CoS 6 and 7
Reserved for CoS 4 and Higher
Reserved for CoS 2 and Higher
Available for Traffic with Any CoS Value
Drop Threshold 3: 80%
Drop Threshold 2: 60%
Drop Threshold 1: 50%
100% Available for CoS 6 and 7
80% Available for CoS 4 and 5 |
Receive Queue |
60% Available for CoS 2 and 3
50% Available for CoS 0 and 1
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(Default Values Shown)
Thesholds provide an alternative to dividing the buffer space to add more standard queues, eliminating the risk of starving one queue while flooding another. Setting a drop theshold allows the entire buffer space to be used, decreasing the potential for instaneous buffer overrun for higher-priority traffic in the standard queue.
Trust Boundries
As discussed in Chapter 3, trust boundaries represent the point in the network that you begin to trust the packet markings. Establishing a trust boundary becomes increasingly important as PC

The Need for QoS on the LAN 697
network interface cards (NICs) gain the capability to mark traffic and alter the desired QoS design of your network. Figure 10-5 shows a network that uses a trust boundary at the IP Phone. A trust boundary can also be configured on an access switch in the event that an IP Phone is not present.
Figure 10-5 Trust Boundaries
Mark
X |
Y |
Z |
Voice Bearer Traffic Mark CoS 5, IP Precedence 5 and IP DSCP EF
Voice Signaling Traffic Mark CoS 3, IP Precedence 3 and IP DSCP AF31
Re-Mark PC Traffic CoS 0, IP Precedence 0 and IPDSCP Default
Mark
X |
Y |
Z |
If SW1 Is Layer 3 Capable:
Classification and Marking Based on IP Precedence or IP DSCP for Ingress Traffic
If ISL/802.1q to SW1:
Map Incoming CoS Value to IP Precedence or IP DSCP Values
Hannah
IP |
SW1 |
R1 |
Jessie |
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Mark |
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Trust |
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Boundary |
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Mark |
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Layer 3 Switch: |
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Classification and Marking |
Based on IP |
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Layer 2 Switch: |
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IP Precedence and IP DSCP, |
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and CoS if Trunking |
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