- •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
248 Chapter 4: Congestion Management
Summary of Queuing Concepts
For the remainder of this chapter, queuing tools are compared based on the six general points listed in this section. Figure 4-7 outlines these points in the same sequence that each point is listed in the upcoming sections on each queuing tool.
Figure 4-7 Six Comparison Points for IOS Queuing Tools
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Scheduling gets the most attention when network engineers choose which queuing tool to use for a particular application. However, the other components of queuing are important as well. If the classification part of a queuing tool cannot classify the traffic as defined in the QoS policy for the network, for instance, either the policy must be changed or another tool must be used. For instance, PQ and CQ cannot take direct advantage of network-based application recognition (NBAR), but CBWFQ and LLQ can. In addition, some queuing tools allow a drop policy for each queue, which becomes particularly important when voice and video compete with data traffic in a converged network.
Queuing Tools
Cisco IOS provides a large variety of different queuing tools. Some of the queuing tools have been available for quite some time, whereas others have only become available in recent releases. Some of the tools may not be popular to implement because one queuing tool may provide a better solution than another queuing tool. However, all the queuing tools mentioned in this chapter are covered on one of the current Cisco QoS exams; therefore, we cover them in this book.
The following sections of this book include coverage of First-In, First-Out Queuing (FIFO); Priority Queuing (PQ); Custom Queuing (CQ); Weighted Fair Queuing (WFQ); Class-Based WFQ (CBWFQ); Low Latency Queuing (LLQ); and IP RTP Priority.
Queuing Tools 249
FIFO Queuing
NOTE This section covers FIFO Queuing, which like TX Queues, is not currently covered on the DQOS 9E0-601 exam. This section does cover some useful information for all people interested in QoS. Therefore, for you DQOS exam takers, you might choose to just focus on the concepts about FIFO, and not worry about memorization or configuration. And as always, check the websites listed in the Introduction for any news about changes to the exams.
The first reason that a router or switch needs output queues is to hold a packet while waiting for the interface to become available for sending the packet. Whereas the other queuing tools in this chapter also perform other functions, like reordering packets, FIFO Queuing just provides a means to hold packets while they are waiting to exit an interface.
FIFO Queuing does not need the two most interesting features of the other queuing tools, namely classification and scheduling. FIFO Queuing uses a single queue for the interface. Because there is only one queue, there is no need for classification to decide the queue into which the packet should be placed. Also there is no need for scheduling logic to pick which queue from which to take the next packet. The only really interesting part of FIFO Queuing is the queue length, which is configurable, and how the queue length affects delay and loss.
FIFO Queuing uses tail drop to decide when to drop or enqueue packets. If you configure a longer FIFO queue, more packets can be in the queue, which means that the queue will be less likely to fill. If the queue is less likely to fill, fewer packets will be dropped. However, with a longer queue, packets may experience more delay and jitter. With a shorter queue, less delay occurs, but the single FIFO queue fills more quickly, which in turn causes more tail drops of new packets. These facts are true for any queuing method, including FIFO.
Figure 4-8 outlines simple FIFO Queuing. R1 uses FIFO Queuing on the interface connected to R2. The only decision required when configuring FIFO Queuing is whether to change the length of the queue.
Figure 4-8 Simple FIFO Queuing
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250 Chapter 4: Congestion Management
Remember to consider two steps when configuring FIFO Queuing. First, configuring FIFO Queuing actually requires you to turn off all other types of queuing, as opposed to just configuring FIFO Queuing. Cisco IOS uses WFQ as the default queuing method on serial interfaces running at E1 speeds and slower. However, IOS does not supply a command to enable FIFO Queuing; to enable FIFO Queuing, you must first disable WFQ by using the no fair-queue interface subcommand. If other queuing tools have been explicitly configured, you should also disable these. Just by removing all other queuing configuration from an interface, you have enabled FIFO!
The second FIFO configuration step that you might consider is to override the default queue length. To do so, use the hold-queue x out interface subcommand to reset the length of the queue.
Example 4-2 shows a sample FIFO Queuing configuration.
Example 4-2 FIFO Queuing Configuration
R3#conf t
Enter configuration commands, one per line. End with CNTL/Z. R3(config)#int s 0/0
R3(config-if)#no fair-queue R3(config-if)#^Z
R3#sh int s 0/0
Serial0/0 is up, line protocol is up Hardware is PowerQUICC Serial
Description: connected to FRS port S0. Single PVC to R1. MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255 Encapsulation FRAME-RELAY, loopback not set Keepalive set (10 sec)
LMI enq sent 80, LMI stat recvd 73, LMI upd recvd 0, DTE LMI up LMI enq recvd 0, LMI stat sent 0, LMI upd sent 0
LMI DLCI 1023 LMI type is CISCO frame relay DTE
Broadcast queue 0/64, broadcasts sent/dropped 171/2, interface broadcasts 155 Last input 00:00:02, output 00:00:03, output hang never
Last clearing of "show interface" counters 00:13:48
Input queue: 0/75/0/0 (size/max/drops/flushes); Total output drops: 0 Queueing strategy: fifo
Output queue :0/40 (size/max)
30 second input rate 0 bits/sec, 0 packets/sec
30 second output rate 0 bits/sec, 0 packets/sec
235 packets input, 14654 bytes, 0 no buffer
Received 0 broadcasts, 0 runts, 0 giants, 0 throttles
2 input errors, 0 CRC, 2 frame, 0 overrun, 0 ignored, 0 abort 264 packets output, 15881 bytes, 0 underruns
0 output errors, 0 collisions, 6 interface resets
0 output buffer failures, 0 output buffers swapped out
10 carrier transitions
DCD=up DSR=up DTR=up RTS=up CTS=up
continues
Queuing Tools 251
Example 4-2 FIFO Queuing Configuration (Continued)
R3#conf t
Enter configuration commands, one per line. End with CNTL/Z. R3(config)#int s 0/0
R3(config-if)#hold-queue 50 out
R3(config-if)#^Z
!
R3#sh int s 0/0
Serial0/0 is up, line protocol is up Hardware is PowerQUICC Serial
!Lines omitted for brevity Queueing strategy: fifo Output queue :0/50 (size/max)
!Line omitted for brevity
Example 4-2 shows FIFO Queuing being configured by removing the default WFQ configuration with the no fair-queue command. The show interface command lists the fact that FIFO Queuing is used, and the output queue has 40 entries maximum. After configuring the output queue to hold 50 packets with the hold-queue 50 out command, the show interface output still lists FIFO Queuing, but now with a maximum queue size of 50.
FIFO Queuing is pretty basic, but it does provide a useful function: It provides the basic queuing function of holding packets until the interface is no longer busy.
Priority Queuing
Priority Queuing’s most distinctive feature is its scheduler. PQ schedules traffic such that the higher-priority queues always get serviced, with the side effect of starving the lower-priority queues. With a maximum of four queues, called High, Medium, Normal, and Low, the complete logic of the scheduler can be easily represented, as is shown in Figure 4-9.
As seen in Figure 4-9, if the High queue always has a packet waiting, the scheduler will always take the packets in the High queue. If the High queue does not have a packet waiting, but the Medium queue does, one packet is taken from the Medium queue—and then the process always starts over at the High queue. The Low queue only gets serviced if the High, Medium, and Normal queues do not have any packets waiting.
The PQ scheduler has some obvious benefits and drawbacks. Packets in the High queue can claim 100 percent of the link bandwidth, with minimal delay, and minimal jitter. The lower queues suffer, however. In fact, when congested, packets in the lower queues take significantly longer to be serviced than under lighter loads. In fact, when the link is congested, user applications may stop working if their packets are placed into lower-priority queues.
252 Chapter 4: Congestion Management
Figure 4-9 PQ Scheduling Logic
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Most of the rest of the details about PQ can be easily understood. PQ classifies packets based on the content of the packet headers. It uses a maximum of four queues, as mentioned earlier. The only drop policy is tail drop—in other words, after classifying the packet, if the appropriate queue is full, the packet is dropped. The length of each queue, which of course affects packet loss and delay, can be changed—in fact, PQ can set the queue length to a value of zero, which means the queue length is infinite. (Infinite really means that when the router runs out of memory, the packet cannot be queued; however, you have worse problems than queuing a packet if the router is out of memory!) Figure 4-10 summarizes these key features of PQ.
The figure represents the internals of a router, after the routing decision has identified the output interface for the packet. The following list describes each component of the queuing process, with the numbers in the list matching the numbers in the figure.
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1PQ can classify packets using access-control lists (ACLs) for most Layer 3 protocols, matching anything allowed by any of the types of ACLs. PQ can also directly match, without using an ACL, the incoming interface, packet length, and TCP and UDP port numbers.
2Tail drop is the only available drop policy.
3Four queues maximum
4Maximum queue length can be set to zero, which means the queue has theoretically infinite length. Defaults are 20, 40, 60, and 80 packets for High, Medium, Normal, and Low queues, respectively.
5Inside a queue, PQ uses FIFO logic.
6When scheduling among the queues, PQ always services the highest-priority queue.
Independent of the process leading up to placing the packet into the queue, the scheduler continually reacts when the TX Ring empties a packet out the interface, implying that there is now more room in the TX Ring/TX Queue for another packet. When the TX Ring frees space, the PQ scheduler then performs the logic described in Figure 4-9, taking a packet from the highestpriority queue that has a packet waiting. The PQ scheduler moves the packet to the TX Ring, for later transmission on the interface.