
- •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

706 Chapter 10: LAN QoS
Example 10-6 The show qos info config Command (Continued)
Default CoS = 0 |
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Queue and Threshold Mapping: |
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Queue Threshold |
CoS |
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0 1 |
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2 3 |
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4 5 |
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drop |
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drop |
thresholds are disabled for untrusted ports. |
Queue # |
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150% 60% 80% 100% Tx drop thresholds:
Queue # Thresholds - percentage (abs values)
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140% 100%
2 40% 100% Tx WRED thresholds:
WRED feature is not supported for this port_type. Queue Sizes:
Queue # Sizes - percentage (abs values)
------- -------------------------------------
180%
220%
WRR Configuration of ports with speed 1000MBPS: Queue # Ratios (abs values)
------- -------------------------------------
1100
2255
QoS Flow on the Catalyst 6500
Now that you have an understanding of the various hardware options available on the Catalyst 6500 as they relate to QoS, we can begin to discuss the QoS flow as the frames are received and forwarded through the switch. Figure 10-6 shows the QoS flow overview.
Figure 10-6 QoS Flow Overview
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CoS = 0 for All Traffic |
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Cisco Catalyst Switch QoS Features 707
The frame arrives at the switch Ethernet ingress port. The existing frame marking, Ethernet ingress queuing structure, and the current trust state of the ingress Ethernet port determine the level of service the frame receives. After the ingress queue and threshold have been determined, the ingress port forwards the frame to the Layer 2 switching engine, or the PFC Layer 3 switching engine if one is installed.
When the supervisor does not have a PFC, the Layer 2 switching engine forwards the packet to the egress Ethernet port. The egress switch port uses the CoS marking on the frame to determine the queue that is used for the frame as it exits the switch.
When the supervisor does have a PFC, the PFC consults its cache to determine whether it has a known path to the destination address. If the path exists in the cache, the PFC forwards the packet to the correct egress port. Because the PFC can operate at Layer 3, it has the capability to switch the packet between VLANs without losing the Layer 2 CoS markings, so the frame retains its CoS marking, and the packet retains its DSCP or IP precedence marking.
In the event that the PFC does not have the path in cache, the packet is forwarded to the MSFC to determine which Ethernet egress port to use. When the MFSC makes a routing decision, and determines the correct Ethernet egress port needed to reach the destination address, the PFC populates its cache with this information for future use. This means the first packet in a flow loses the CoS value, because the first packet is routed at Layer 3 across the MSFC. However, subsequent packets are switched by the PFC and retain their original marked values. After the egress Ethernet port has been identified, the packet is assigned to the proper queue and threshold indicated by the Layer 2 or Layer 3 markings on the packet.
Ingress Queue Scheduling
The first opportunity for applying any kind of QoS tool to an incoming frame occurs when the frame enters the switch. Even before the supervisor or PFC forwards the frame to the egress line card, ingress QoS features occur. For instance, ingress queue scheduling defines the treatment that a frame or packet receives as it enters the switch. If QoS has been enabled, and the ingress port is trusted, the switch uses receive queue drop thresholds to schedule the arriving frames. Figure 10-7 shows the treatment received by the frame or packet in the ingress queue.
The first variable encountered as the frame enters the switch is trust. This asks the questions “Where is our trust boundary? Do we want to trust the markings on a frame as it enters the switch on this port?” If the port is untrusted, there is no need to continue to classify the frame. The ingress port just applies the configured port QoS value, typically 0, and forwards the frame to the switching engine.
If the port is trusted, the next questions asked are “Is this an 802.1Q or ISL trunk port? Does the incoming frame have an 802.1Q or ISL header?” If this ingress Ethernet port is not a trunk port, the configured port CoS value is assigned to the frame. If this ingress Ethernet port is configured as a trunk port, the CoS value marked on the frame is trusted.

708 Chapter 10: LAN QoS
Figure 10-7 Ethernet Ingress Port Treatment
Frame
Enters
Switch Ethernet Ingress Port Classification, Marking,
Scheduling, and Congestion Avoidance
Port Set to |
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ISL or |
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Port Set to Yes |
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Strict Priority Queue |
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(Default Values |
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Standard Queue |
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Shown) |
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80% for CoS 4 and 5 |
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60% for CoS 2 and 3 |
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50% for CoS 0 and 1 |
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1q1q01 port |
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(tail-drop thresholds) |
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Strict Priority Queue |
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100% for CoS 5 |
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Standard Queue |
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100% for CoS 0, 1, 2, 3, 4, 6, 7 |
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To
Switching Engine

Cisco Catalyst Switch QoS Features 709
The next question depends on the hardware installed in the switch. If a PFC has been installed, the switch has the capability to classify and mark traffic based on Layer 3 markings, and so the question is asked “Do we want to trust IP precedence or DSCP from packets entering this Ethernet ingress port?” If the answer is yes, the packet is forward to the PFC. If we do not want to trust the Layer 3 markings, the switch relies on the CoS value to determine the correct queue and threshold.
After the proper queue and threshold has been identified, the question is asked “Is there room in the ingress queue for this frame, or has the drop threshold for this CoS value been met?” If the threshold has been met, the frame is dropped, otherwise the frame is forwarded on to the switching engine. If a PFC is not installed in the Catalyst switch, the frame flows to the Layer 2 switching engine, otherwise the frame is passed to the Layer 3 switching engine (PFC).
Layer 2 Switching Engine QoS Frame Flow
The Layer 2 switching engine has few options for classification and marking because it operates on the Layer 2 network. Figure 10-8 shows the options available.
Figure 10-8 Layer 2 Switching Engine Classification and Marking
From
Ingress Port
Match
Destination No
MAC
Address /
VLAN?
Yes
Mark CoS
To Egress
Port
The Layer 2 switching engine receives the frame from the ingress Ethernet port. The only variable that the Layer 2 switching engine has is the destination MAC address / VLAN match. If the arriving/received frame matches this criteria, the CoS value can be overwritten. If the received frame does not match this criteria, the frame is forwarded to the egress Ethernet port, retaining the received CoS value.