- •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
Queuing Concepts 245
Example 4-1 TX Queue Length: Finding the Length, and Changing the Length (Continued)
tx_limited=0(1)
! Lines omitted to save space
The show controllers serial 0/0 command lists the size of the TX Queue or TX Ring. In the shaded output in Example 4-1, the phrase “tx_limited=0(16)” means that the TX Ring holds 16 packets. The zero means that the queue size is not currently limited due to a queuing tool being enabled on the interface. For the first instance of show controllers, no queuing method is enabled on the interface, so a zero signifies that the size of the TX Ring has not been limited automatically. After enabling Priority Queuing with the priority-group interface subcommand, the next show controllers command lists “tx_limited=1(2).” The new length of the TX Ring is 2, and 1 shows that the length is automatically limited as a result of queuing being configured. Next, Priority Queuing is disabled with the no priority-group interface subcommand, but the length of the TX Ring is explicitly defined with the tx-ring-limit 1 interface subcommand. On the final show controllers command, the “tx_limited=0(1)” output implies that the size is not limited, because no queuing is enabled, but that the length of the TX Ring is 1.
The following list summarizes the key points about TX Rings and TX Queues in relation to their effect on queuing:
•The TX Queue/TX Ring always performs FIFO scheduling, and cannot be changed.
•The TX Queue/TX Ring uses a single queue, per interface.
•IOS shortens the interface TX Queue/TX Ring automatically when an output queuing method is configured.
•The TX Ring/TX queue length can be configured to a different value.
Queuing on Interfaces Versus Subinterfaces and Virtual Circuits (VCs)
IOS queuing tools create and manage output queues associated with an interface, and then the packets drain into the TX Ring/Queue associated with the interface. IOS also supports queuing on subinterfaces and individual VCs when traffic shaping is also enabled. Shaping queues, created by the traffic-shaping feature, drain into the interface output queues, which then drain into the TX Ring/Queue. Like the interface output queues, the shaping queues can be managed with IOS queuing tools.
The interaction between shaping queues associated with a subinterface or VC, and queues associated with a physical interface, is not obvious at first glance. So, before moving into the details of the various queuing tools, consider what happens on subinterfaces, VCs, and physical interfaces so that you can make good choices about how to enable queuing in a router.
Figure 4-5 provides a reasonable backdrop from which to explain the interaction between queues. R1 has many permanent virtual circuits (PVCs) exiting its S0/0 physical interface. The
246 Chapter 4: Congestion Management
figure shows queues associated with two of the PVCs, a single queue for the physical interface, and the TX Ring for the interface.
Figure 4-5 Subinterface Shaping Queues, Interface Queues, and TX Ring
Router1
s0/0.1 |
|
|
|
|
|
|
|
|
|
Subinterface |
#1 |
|
|
|
|
|
|
|
|
Shaping Queue |
|
|
|
|
|
|
|
|
|
Interface |
|
|
|
|
|||||
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
Output Queue |
|
|
TX Ring |
|
|
|
|
|
|
|
|
s0/0 |
||
Subinterface |
#2 |
|
|
|
|
|
|
|
|
Shaping Queue |
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
|
s0/0.2 |
|
|
|
|
|
|
|
|
|
In this particular example, each subinterface uses a single FIFO shaping queue; the physical interface uses a FIFO output queue. At first glance, it seems simple enough: A packet arrives, and the forwarding decision dictates that the packet should exit subinterface S0/0.1. It is placed into the subinterface 0/0.1 shaping queue, and then into the physical interface output queue, and then into the TX Ring. Then, it exits the interface.
In some cases, the packet moves from the shaping queues directly to the TX Queue. You may recall that packets are not even placed in the output queue if the TX Ring is not full! If no congestion occurs on the interface, the TX Ring does not fill. If no congestion occurs in the TX Ring, the interface output queue does not fill, and the queuing tool enabled on the interface has no effect on the packets exiting the interface.
In some cases, IOS does not place the packets into a shaping queue as they arrive, but instead the packets are placed into the interface queue or TX Queue. When the shaping features knows that a newly arrived packet does not exceed the shaping rate, there is no need to delay the packet. In that case, a queuing tool used for managing the shaping queue would also have no effect.
Traffic shaping can cause subinterface shaping queues to fill, even when there is no congestion on the physical interface. Traffic shaping, enabled on a subinterface or VC, slows down the flow of traffic leaving the subinterface or VC. In effect, traffic shaping on the subinterface creates congestion between the shaping queues and the physical interface queues. On a physical interface, packets can only leave the interface at the physical clock rate used by the interface; similarly, packets can only leave a shaping queue at the traffic-shaping rate.
Queuing Concepts 247
For example, the VC associated with subinterface S0/0.1 uses a 64 kbps committed information rate (CIR), and S0/0 uses a T/1 circuit. Without traffic shaping, more than 64 kbps of traffic could be sent for that PVC, and the only constraining factor would be the access rate (T/1). The Frame Relay network might discard some of the traffic, because the router may send more (up to 1.5 Mbps) on the VC, exceeding the traffic contract (64-kbps CIR). So, traffic shaping could be enabled on the subinterface or VC, restricting the overall rate for this PVC to 64 kbps, to avoid frame loss inside the Frame Relay network. If the offered load of traffic on the subinterface exceeds 64 kbps for some period, traffic shaping delays sending the excess traffic by placing the packets into the shaping queue associated with the subinterface, and draining the traffic from the shaping queue at the shaped rate.
Figure 4-6 shows an updated version of Figure 4-5; this one’s PVC is currently exceeding the shaping rate, and the other PVC is not exceeding the shaping rate.
Figure 4-6 Shaping Active on One VC, and Not Active on the Other
Shaping
Queue
Bit Rate
Limit
x bps
128 kbps Routed
out Subinterface Incoming 0/0.1
Packets
40 kbps Routed out Subinterface 0/0.2
Slow |
Down |
|
|
|
Bit Rate |
|
Limit |
x bps
Discarded
s0/0.1 Queue Fills, Since
Offered Rate > Shaped Rate
Interface |
|
|
TX Ring |
s0/0 |
Output Queue |
|
|
T1 |
|
|
|
|
||
|
Traffic Rate
Less Than
Shaping Rate
- NO Queuing!
In Figure 4-6, packets arrive and are routed out of each of the two subinterfaces. Traffic for subinterface 0/0.1 exceeds the shaping rate, and packets for subinterface 0/0.2 do not. Therefore, a queue begins to form on the shaping queue for subinterface 0/0.1, because traffic shaping delays packets by queuing the packets. On subinterface 0/0.2, packets will not be enqueued into the shaping queue, because the shaping rate has not been exceeded.
You can configure queuing tools to manipulate the output queue on a physical interface, as well as the shaping queue created by shaping. The concepts in this chapter apply to using queuing on both the main interface, and on any shaping queues. However, this chapter only covers the configuration of queuing to manipulate the interface output queues. Chapter 5, “Traffic Policing and Shaping,” which covers traffic shaping, explains how to configure queuing for use on shaping queues. When reading the next chapter, keep these queuing concepts in mind and watch for the details of how to enable your favorite queuing tools for shaping queues.