Call Admission Control and QoS Signaling

Suppose that you and a passenger are inside a vehicle traveling on a northbound highway. The highway consists of four regular lanes and one high-occupancy-vehicle (HOV) lane. To gain entry into the HOV lane, your vehicle must contain two or more people. In this instance, the HOV lane represents a priority queue, whereas the remaining four lanes represent lower-priority queues. As traffic begins to build in the four regular lanes, you decide to merge into the HOV lane. Over the next few miles...

Call Admission Control and RSVP

Call admission control (CAC) protects network bandwidth by preventing more concurrent voice and video flows than the network can support. By doing so, it not only protects data traffic, but it also protects the quality of voice and video calls that are already set up. If a network engineer designs a network to support 3 concurrent G.729 calls, for instance, which take roughly 85 kbps, depending on the data links used, but if 10 concurrent calls occur, taking roughly 285 kbps, many bad things...

WRED Summary

WRED provides a valuable tool for managing congestion in queues. Cisco IOS uses defaults that conform to the DiffServ Assured Forwarding conventions, which reduce the likelihood that you will need to configure thresholds for WRED. WRED can be particularly effective when used with MQC-based queuing tools, but when enabled directly on an interface, WRED has the unfortunate side effect of disallowing other queuing tools to be used. Table 6-8 lists some of WRED's key points. Discards packets to...

Qa

As mentioned in the Introduction, you have two choices for review questions. The questions that follow next give you a more difficult challenge than the exam itself by using an open-ended question format. By reviewing now with this more difficult question format, you can exercise your memory better, and prove your conceptual and factual knowledge of this chapter. You can find the answers to these questions in Appendix A. The second option for practice questions is to use the CD-ROM included...

Local Voice Busyout

Several CAC mechanisms generate a busy signal to the originating PBX to indicate that an alternate route must be found to successfully place a call. The preceding section discussed trunk conditioning, which operates on connection trunk networks only. Similar functionality is needed for switched networks. Local Voice Busy-Out (LVBO) is the first of two features that achieve this. LVBO enables you to take a PBX trunk connection to the attached gateway completely out of service when WAN conditions...

Classification and Marking

Almost every QoS tool uses classification to some degree. To put one packet into a different queue than another packet, the IOS must somehow differentiate between the two packets. To perform header compression on RTP packets, but not on other packets, the IOS must determine which packets have Real Time Protocol (RTP) headers. To shape data traffic going into a Frame Relay network, so that the voice traffic gets enough bandwidth, the IOS must differentiate between Voice over IP (VoIP) and data...

Subnet Bandwidth Management

Subnet bandwidth management (SBM) specifies a signaling method and protocol for LAN-based CAC for RSVP flows. SBM allows RSVP-enabled Layer 2 and Layer 3 devices to support reservation of LAN resources for RSVP-enabled data flows. SBM uses the concept of a designated entity elected to control the reservations for devices on the managed LAN. The elected candidate is called the designated subnetwork bandwidth manager (DSBM). It is the DSBM's responsibility to exercise admission control over...

Foundation Summary

The Foundation Summary is a collection of tables and figures that provide a convenient review of many key concepts in this chapter. For those of you already comfortable with the topics in this chapter, this summary could help you recall a few details. For those of you who just read this chapter, this review should help solidify some key facts. For any of you doing your final prep before the exam, these tables and figures are a convenient way to review the day before the exam. ISPs make the...

Video Basics

IP packet video can be categorized into two main categories Interactive video Includes H.323-compliant video conferencing systems, such as Cisco's IP VC 3500 series of products, and Microsoft's NetMeeting desktop videoconferencing product. H.323-compliant video-conferencing tools use the familiar RTP protocol for transmission of the voice and audio payload, typically sending the audio in a separate RTP stream than the video. Noninteractive video Includes typical e-learning video services and...

RSVP Configuration

The following three tasks are performed on a gateway to originate or terminate voice traffic using RSVP 1 Turn on the synchronization feature between RSVP and H.323. This is a global command and is turned on by default when Cisco IOS Release 12.1(5)T or later is loaded. 2 Configure RSVP on both the originating and terminating sides of the VoIP dial peers. Configure both the requested QoS (req-qos) and the acceptable QoS (acc-qos) commands. The guaranteed-delay option must be chosen for RSVP to...

Distributed Traffic Shaping DTS Configuration

DTS shapes traffic identically to CB shaping, but with processing distributed to VIPs in a 7500 series router. In fact, DTS and CB shaping configurations are very similar, but with a few extra requirements. Distributed CEF must be configured for the interfaces on which DTS should operate. DTS also has a few idiosyncrasies that are not mentioned in either of the QoS courses, so they are unlikely to be on the exam. Ifyou want to read further about DTS, look for the Cisco AVVID Enterprise Quality...

Frame Relay Fragmentation Configuration

The configuration of FRF.12 requires very little effort in itself. However, FRF. 12 requires FRTS, and for the FRF.12 interleaving function to actually work, you need to enable IP RTP Priority or LLQ for the shaping queues on one or more VCs. Therefore, although the FRF.12 new configuration details are brief, the related tools make the configuration a little longer. The show commands related to FRF.12 give a fairly detailed view into what is actually happening with fragmentation and are covered...

Traffic Characteristics of Voice Video and Data

So why do you need QoS QoS can affect a network's bandwidth, delay, jitter, and packet loss properties. Applications have different requirements for bandwidth, delay, jitter, and packet loss. With QoS, a network can better provide the right amounts of QoS resources for each application. Table 1-10 lists some major application categories, along with their typical needs for network resources and behavior. Table 1-10 Applications and Their QoS Needs Fragile (For Instance, SNA*) * SNA Systems...

PSTN Fallback Summary

PSTN fallback is a widely deployable, topology-independent CAC mechanism that can be used over any backbone. Consider the following attributes of PSTN fallback when designing a network Because it is based on IP probes, PSTN fallback applies to VoIP networks only. PSTN fallback does not reroute calls in progress when network conditions change. A slight increase in postdial delay will apply to the first call to a destination not yet in the cache. No interaction occurs between the SAA probe timer...

GOCS Class Based QoS

Most QoS tools do not need to differentiate between each flow. In the Figure 2-10, for instance, flows to web Serverl were identified. Most network engineers would want to treat those collective web flows the exact same way with their QoS tools. Therefore, most QoS tools tend to operate on the idea of a category, or class, of flows and packets. Consider Figure 2-11, for example, which has thousands of flows, all of which are classified into four types of traffic. Figure 2-11 GOCS Approach to...

Gatekeeper Zone Bandwidth Summary

Gatekeeper CAC works well in network designs where the desire is to limit the number of calls between sites. This may be required due to either bandwidth limitations or business policy. If bandwidth limitations are on the WAN links, manual calculations can be performed to translate the maximum number of calls allowed between sites into a bandwidth figure that will cause the gatekeeper to deny calls when the calculated number is exceeded. Gatekeepers do not share database information. If the...

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 Figure 3-1 Headers Used During Typical Packet Flow As seen in Figure 3-1, the IP packet en route to host PC2 stays intact throughout the network,...

LAN Class of Service CoS

Many LAN switches today can mark and react to a Layer 2 3-bit field called the Class of Service (CoS) located inside an Ethernet header. The CoS field only exists inside Ethernet frames when 802.1Q or Inter-Switch Link (ISL) trunking is used. You can use the field to set 8 different binary values, which can be used by the classification features of other QoS tools, just like IP precedence and DSCP. Figure 3-3 shows the general location of the CoS field inside ISL and 802.1P headers. The term...

Cisco Press

Cavanaugh Copyright 2004 Cisco Systems, Inc. Cisco Press logo is a trademark of Cisco Systems, Inc. All rights reserved. No part of this book may be reproduced or transmitted in any form or by any means, electronic or mechanical, including photocopying, recording, or by any information storage and retrieval system, without written permission from the publisher, except for the inclusion of brief quotations in a review. Printed in the United States of America 1 2 3 4 5...

WRED Configuration

WRED requires very little configuration if you want to take the IOS defaults for the various tunable settings, such as per-precedence and per-DSCP thresholds. If you want to change the defaults, the configuration details can become quite large. This section begins with a table of configuration commands (Table 6-6) and show commands (Table 6-7), followed by three separate examples. Table 6-6 Command Reference for WRED random-detect dscp-based prec-based Interface or class configuration mode...

Diff Serv Classifiers and Traffic Conditioners

DiffServ focuses on scalable service differentiation in the Internet according to RFC 2475. In times past, the Internet provided a best-effort service networks generated and sent traffic, and the Internet forwarded it and everyone hoped that market forces helped the Internet grow to accommodate the loads. With the advent of premium offerings from ISPs, new QoS tools and models were needed, with DiffServ being just one component. Because DiffServ focuses on Internet services, the RFCs focus on...

VoIP Dial Peer

IOS voice gateways provide many services to connect the packetized, VoIP network to non-packetized, traditional voice services, including analog and digital trunks. IOS gateways perform many tasks, but one of the most important tasks is to convert from packetized voice to nonpacketized voice, and vice versa. In other words, voice traffic entering a router on an analog or digital trunk is not carried inside an IP packet, but the IOS gateway converts the incoming voice to a digital signal (analog...

TCP and RTP Header Compression Configuration

Unlike payload compression, Cisco IOS Software does not have different variations on the compression algorithms for TCP and RTP header compression. To enable TCP or RTP compression, you just enable it on both sides of a point-to-point link, or on both sides of a Frame Relay VC. Note that when enabling compression, it is best practice to enable the remote side of the WAN link before enabling the local side of the WAN link. This enables the administrator to retain control of WAN connectivity. If...

The Integrated Services QoS Model

Integrated services (IntServ) defines a different model for QoS than does DiffServ. IntServ defines a signaling process by which an individual flow can request that the network reserve the bandwidth and delay needed for the flow. The original work grew out of the experiences of the IETF in multicasting the audio and video for IETF meetings in the early to mid-1990s. To provide guarantees per flow, IntServ RFC 1633 describes two components resource reservation and admission control. Resource...

WFQ Configuration

Although WFQ requires a little deeper examination to understand all the underlying concepts, configuration is simple. IOS uses WFQ by default on all serial interfaces with bandwidths set at T 1 and E 1 speeds and below. None of WFQ's parameters can be set for an individual queue, so at most, the WFQ configuration will be one or two lines long. An example configuration for WFQ follows Tables 4-6 and 4-7. Tables 4-6 and 4-7 list the configuration and exec commands related to WFQ respectively....

How WRED Weights Packets

WRED bases its decisions about when to discard packets, and what percentage to discard, on the following four factors First, just like RED, WRED calculates the average queue depth. WRED then compares the average queue depth to the minimum and maximum thresholds to decide whether it should discard packets. If the average queue depth is between the two thresholds, WRED discards a percentage of the packets, with the percentage based on the MPD if the average queue depth exceeds the maximum...

Traffic Shaping Adaption

The rate at which the shaping function shapes traffic can vary over time. The adaption or adaptation process causes the shaper to recognize congestion and reduce the shaping rate temporarily, to help reduce congestion. Similarly, adaption notices when the congestion abates and returns the shaping rate to the original rate. Two features define how adaption works. First, the shaper must somehow notice when congestion occurs, and when it does not occur. Second, the shaper must adjust its rate...

Class Based Marking CB Marking

Cisco added CB marking to IOS after all the other classification and marking tools discussed in this book. As of IOS 12.1(5)T and 12.2 mainline, CB marking represents the only classification and marking tool specifically intended for the classification and marking function. NBAR, CAR, PBR, and dial peers have other purposes, whereas CB marking focuses entirely on classification and marking. You must use a new IOS syntax called the Modular QoS command-line interface (MQC) to configure CB...

Ip Rtp Priority Configuration

To configure IP RTP Priority, you just add one additional configuration command to an interface that already uses WFQ. There are no additional show commands to list information about IP RTP Priority beyond the commands that apply to the underlying queuing tools (WFQ and CBWFQ). Table 4-15 lists the two configuration commands, and Example 4-9 lists several example configurations. Table 4-15 Command Reference for IP RTP Priority ip rtp priority starting-rtp-port-number port-number-range bandwidth...

Shaping and Policing

Because shaping and policing provide two different functions, you may wonder why shaping and policing are covered here at the same time. The simple answer is this Networks that use policing typically need shaping as well. Also both shaping and policing measure the rates at which traffic is sent and received in a network, so some of the underlying features are similar. Both can be described using similar metaphors of token buckets. Finally, from a business perspective, shaping and policing are...

Traffic Shaping When and Where

Networks use traffic shaping for two main reasons To shape the traffic at the same rate as policing (if the service provider polices traffic) To avoid the effects of egress blocking For instance, consider Branches 1 and 24 in Figure 5-3. Branch 1 does not shape, whereas Branch 24 does shape to 96 kbps. In both cases, the Frame Relay switches they are connected to police packets at a 96-kbps rate. (The CIR in each case is 64 kbps. Therefore, the service provider is not policing aggressively. The...

Voice Jitter Considerations

The previous section about delay explained most of the technical details of voice flows relating to jitter. Ifjitter were to cause no degradation in voice call performance, it would not be a problem. However, jitter causes hesitation in the received speech pattern and lost sounds, both when the jitter increases quickly and when it decreases quickly. For instance, consider Figure 1-25, where packets 3 and 4 experience jitter. Figure 1-25 De-Jitter Buffer Underrun Due to Jitter T X - Instant That...

Traffic Policing and Shaping

Traffic policing allows devices in one network to enforce a traffic contract. Traffic contracts define how much data one network can send into another, typically expressed as a committed information rate (CIR) and a committed burst (Bc). Policing measures the flow of data, and discards packets that exceed the traffic contract. Similarly, traffic shaping allows devices to conform to a traffic contract. In cases where packets that exceed the traffic contract might be discarded, the sending device...

Bandwidth Engineering

To successfully implement CAC mechanisms in your packet network, you must begin with a clear understanding of the bandwidth required by each possible call that can be placed. In Chapter 7, Link-Efficiency Tools, you learned about bandwidth requirements for two of the most popular codecs deployed in converged networks, G.711 and G.729. The G.711 codec specification carries an uncompressed 64-kbps payload stream, known in the traditional telephony world as pulse code modulation (PCM). G.711...

QoS Exam Objectives

Describe the Integrated Services model List the key benefits and drawbacks of the IntServ model Describe the Differentiated Services model List the key benefits of the DiffServ model compared to the IntServ model Describe the building blocks of IP QoS mechanisms (classification, marking, metering, policing, shaping, dropping, forwarding, queuing) List the IP QoS mechanisms available in the Cisco IOS Describe what QoS features are supported by different IP QoS mechanisms Describe the...

Frame Relay Traffic Shaping FRTS Configuration

Frame Relay traffic shaping (FRTS) differs from the other three shaping tools in several significant ways. The most obvious difference is the most important FRTS applies to Frame Relay only. But the basic shaping function is the same, with the same parameters a shaped rate, which is often set to CIR a Tc interval, which defaults to 125 ms and the Bc value is either set, or calculated based on the Tc Bc CIR formula. The first big difference between FRTS and the other shapers has to do with...

Delay

All packets in a network experience some delay between when the packet is first sent and when it arrives at its destination. Most of the concepts behind QoS mechanisms relate in some way to delay. Therefore, a deeper look into delay is useful. Take a look at Figure 1-5 this sample network is used often in this book. Figure 1-5 Sample Network for Discussion ofDelay Figure 1-5 Sample Network for Discussion ofDelay At what points will delay occur in this network Well, at all points, in actuality....

Queuing Concepts

Most people already understand many of the concepts behind queuing. First, this section explains the basics and defines a few terms. Afterward, some of the IOS-specific details are covered. IOS stores packets in memory while processing the packet. When a router has completed all the required work except actually sending the packet, if the outgoing interface is currently busy, the routerjust keeps the packet in memory waiting on the interface to become available. To manage the set of packets...

Voice over Frame Relay Voice Bandwidth

In a Voice over Frame Relay (VoFR) network, the frame-relay voice-bandwidth command is used in a Frame Relay map class to set aside the bandwidth required to successfully transport the desired number of calls. This method of bandwidth provisioning operates in much the same way as IP RTP Priority and Low Latency Queuing features that reserve bandwidth for traffic flows. Unlike LLQ or RTP Priority, the frame-relay voice-bandwidth command has the capability to provide CAC. Because VoFR operates at...

Class Based Shaping Configuration

The underlying processes of class-based (CB) shaping are the same as GTS. Just like GTS, CB shaping can be enabled on a large variety of interfaces. It can also adapt the rate based on BECN signals, and reflect BECNs on a VC after receiving an FECN. It can also perform shaping on a subset of the traffic on an interface. Finally, the configuration is simple, because like all other QoS features starting with the words class based, CB shaping uses the Modular QoS command-line interface (MQC) for...

Network Based Application Recognition NBAR

CB marking, and other MQC-based tools, can use NBAR to help classify traffic. By using the match protocol class-map subcommand, MQC can match protocols recognized by NBAR. This section describes NBAR, and includes examples of CB marking with NBAR. NBAR classifies packets that are normally difficult to classify. For instance, some applications use dynamic port numbers, so a statically configured match command, looking for a particular UDP or TCP port number, just could not classify the traffic....

WFQ Scheduling The Process

WFQ gives each flow a weighted percentage of link bandwidth. However, WFQ does not predefine queues like class-based queuing tools do, because WFQ dynamically classifies queues based on the flow details. And although WFQ ends up causing each flow to get some percentage of link bandwidth, the percentage changes, and changes rapidly, because flows come and go frequently. Because each flow may have different precedence values, the percentage of link bandwidth for each flow will change, and it will...

Voice Traffic Characteristics

Voice traffic can degrade quickly in networks without QoS tools. This section explains enough about voice traffic flows to enable the typical reader to understand how each of the QoS tools applies to voice. NOTE This book does not cover voice in depth because the details are not directly related to QoS. For additional information, refer to the following sources Deploying Cisco Voice over IP Solutions, Cisco Press, Davidson and Fox IP Telephony, Hewlett-Packard Professional Books, Douskalis...

Shaping with Frts Gts Dts and CB Shaping

9 Along with the class-map, policy-map, and service-policy commands, CB shaping requires one specific command that actually sets values used for the shaping function. List the command, with the correct syntax, that sets a shaped rate of 128 kbps, a Bc of 8000, and a Be of 8000, when using CB shaping. Do not assume any defaults explicitly set the values in the command. 10 Compare and contrast the use of the class-map and map-class commands in terms of how each is used by FRTS and CB shaping. 11...

LLQ Configuration

LLQ configuration requires one more command in addition to the commands used for CBWFQ configuration. Instead of using the bandwidth command on a class, use the priority command. This single additional command is listed in Table 4-24. The syntax for this command is as follows priority bandwidth-kbps percent percentage burst This class subcommand enables LLQ in this class, reserves bandwidth, and enables the policing function. You can also configure the burst for the policer with this command,...

Random Early Detection RED

Random Early Detection (RED) reduces the congestion in queues by dropping packets so that some of the TCP connections temporarily send fewer packets into the network. Instead of waiting until a queue fills, causing a large number of tail drops, RED purposefully drops a percentage of packets before a queue fills. This action attempts to make the computers sending the traffic reduce the offered load that is sent into the network. The name Random Early Detection itself describes the overall...

Frame Relay LFI Using FRF12

Cisco IOS Software supports two flavors of Frame Relay LFI. The more popular option, FRF.12, is based on Frame Relay Forum Implementation Agreement 12, with the other option, FRF.11-C, being based on Frame Relay Forum Implementation Agreement 11, Annex C. FRF.12 applies to data VCs, and FRF.11-C applies to voice VCs. Because most Frame Relay VCs are data VCs, and because most service providers do not offer FRF.11 (VoFR) VCs, this section focuses on FRF.12. The final part of the FRF...

Summary of Classification and Marking QoS Features

Classification and marking tools can be easily compared based on three general categories. First, some classification and marking tools are specialized, and some are more general. CB marking, CAR, and PBR all perform the same basic function of matching packets based on fields inside the packet header, and marking based on those fields, so these three tools are the more generalized classification and marking tools. Dial peers perform specialized classification and marking functions. This tool is...

Max Connections

Similar to physical DS0 limitation, Max-Connections uses the concept of limiting the number of simultaneous calls to help protect the quality of voice conversations. Unlike physical DS0 limitation, the max-conn command is a physical gateway configuration applied on a per-dial peer basis. The first advantage that Max-Connections offers over DS0 limitation is the capability to provision for the oversubscription of TDM interfaces on an originating gateway without compromising the quality of the...

CAC Mechanism Evaluation Criteria

As each CAC method in this chapter is described, it is evaluated against various factors and criteria that will help determine which CAC mechanism is the most appropriate for the network design under consideration. As seen in the wording of the DQOS exam topics, an important part of the DQOS exam includes identifying these CAC tools and their basic features. Table 8-4 describes the criteria that is used to evaluate the different CAC tools. Table 8-4 CAC Feature Evaluation Criteria The voice...

How Shaping Works

Shaping only makes sense when the physical clock rate of a transmission medium exceeds a traffic contract. The most typical case for shaping involves a router connected to a Frame Relay or ATM network. More often today, however, connections to ISPs use a point-to-point serial link or an Ethernet link between an enterprise and the ISP, with a traffic contract defining lower traffic volumes than the physical link. Starting with a physical installation using a higher-speed link allows for simple...

RSVP Synchronization Configuration

To configure RSVP for use with H.323, you need to enable RSVP as normal and add a few commands to the H.323 dial peers. By default, RSVP is disabled on each interface to remain backward compatible with systems that do not implement RSVP. To enable RSVP for IP on an interface, use the ip rsvp bandwidth command. This command starts RSVP and sets the maximum bandwidth for all RSVP flows combined, and a single-flow bandwidth limit. The default maximum bandwidth is up to 75 percent of the bandwidth...

Physical DS0 Limitation

Physical DS0 limitation is a design methodology that limits the number of physical DS0 connections into the gateway. This limitation, in conjunction with other queuing methods, ensures that the gateway can successfully provide IP bandwidth across the WAN for each voice conversation originating from the individual DS0 trunks. If you have determined that there is 158.4 kbps of WAN bandwidth available to handle 6 simultaneous G.729 calls, for example, DS0 limitations can be implemented by allowing...

Multilink PPP Interleaving Configuration

Before you configure MLP LFI, think about why you would use MLP at all. If you have a point-to-point link, and need to perform LFI, you must migrate from your current Layer 2 protocol to MLP, to use MLP LFI. However, MLP itself has many benefits, and even a few brief thoughts about what MLP does will help you through some of the configuration tasks. MLP enables you to have multiple parallel point-to-point links between a pair of devices, such as routers. The main motivation for MLP was to allow...

Shaping and Policing Tools

QoS shaping and policing tools provide you with a variety of methods. As usual, you may consider many factors when comparing these tools. (Table 2-4 lists a few of these factors.) First, not all shaping and policing tools support every data-link protocol. Second, some tools can be enabled on a subinterface, but not on a per data-link connection identifier (DLCI) therefore, in cases where a network uses multipoint subinterfaces, one tool may give more granularity for shaping policing. With...

TCP and UDP Reactions to Packet Loss

UDP and TCP behave very differently when packets are lost. UDP, by itself, does not react to packet loss, because UDP does not include any mechanism with which to know whether a packet was lost. TCP senders, however, slow down the rate at which they send after recognizing that a packet was lost. Unlike UDP, TCP includes a field in the TCP header to number each TCP segment (sequence number), and another field used by the receiver to confirm receipt of the packets (acknowledgment number). When a...

Voice Delay Considerations

Voice call quality suffers when too much delay occurs. The symptoms include choppy voice, and even dropped calls. Interactivity also becomes difficult ever had a call on a wireless phone, when you felt like you were talking on a radio Hey Fred, let's go bowling OVER Okay, Barney, let's go while Betty and Wilma are out shopping OVER. With large delays, it sometimes becomes difficult to know when it is your turn to talk. Voice traffic experiences delays just like any other packet, and that delay...

Output Queues TX Rings and TX Queues

NOTE This section covers the concepts behind TX Rings and TX Queues. Interestingly, the DQOS exam 9E0-601 does not mention these topics at all, but the QoS exam does. For a true understanding of queuing inside IOS, however, you must understand these concepts. Therefore, for you DQOS exam takers, you might choose to just focus on the concepts about TX Queues and TX Rings, and not worry about memorization. As always, check the websites listed in the Introduction for any news about changes to the...

WFQ Summary

WFQ works well for networks where the most delay-sensitive traffic requires less bandwidth than the average flow. Also flows with higher precedence work well, with low-volume, high-precedence flows receiving exceptional treatment. Best of all, WFQ requires no classification configuration. As a result, WFQ provides a great default queuing choice, particularly when traffic characteristics are unpredictable and difficult to qualify. WFQ works poorly for voice and interactive video traffic, because...

The Good Old Common Sense QoS Model

Most people already have some idea of what would be useful, and not so useful, regarding implementing QoS in a network. If you have been reading this book from the beginning of Chapter 1, you already know about the how QoS tools affect bandwidth, delay, jitter, and loss. You also know some of the traffic characteristics of voice, video, and data flows. You have seen the basics of how each general category of QoS tool works (Chapter 1), and you have at least seen a list of QoS tools in IOS...

Committed Access Rate CAR Configuration

CAR has more similarities than differences when compared to CB policing. Both perform policing on all traffic on either an interface or subinterface. Both can classify traffic to police a subset of traffic as well. Both use the same units when configuring policing parameters bits per second for the policing rate, bytes for the normal and Be values, with the configured Be value actually representing Bc + Be. CAR differs from CB policing regarding four main features. The most obvious is that CAR...

Congestion Avoidance Through Drop Policies

Quality of service (QoS) congestion-avoidance tools help prevent congestion before it occurs. These tools monitor queue depth, and before the queue fills, they drop some packets. The computers sending the packets might reduce the frequency of sending packets in reaction to the packet loss, particularly if the application sending the data uses TCP. In the moments after the congestion-avoidance tool discards packets, congestion is reduced, because less traffic is sent into the network. Cisco...

CBWFQ Summary

CBWFQ combines some of the best features of the various queuing tools into a single tool. Like CQ, CBWFQ can reserve a guaranteed amount of bandwidth per queue, but without the negative side effects of the CQ scheduler. CBWFQ can use WFQ as the default behavior for unclassified traffic. Packet loss behavior can take advantage of WRED, which reduces the possibilities of global synchronization. In addition, of all the queuing tools, CBWFQ has the largest variety of directly matchable fields for...

Congestion Avoidance Concepts and Random Early Detection RED

1 Describe the function of the congestion window in TCP, and how it is changed as a result of packet loss. 2 Identify the two TCP windowing mechanisms, and describe when each is used. 3 Describe the process of TCP slow start, and when it occurs. 4 Describe the process of TCP congestion avoidance, and when it occurs. 5 Describe the meaning of the term global synchronization, and discuss what causes it. 6 Define the meaning of the term tail drop. 7 Define the meaning of the term TCP starvation. 8...

Policing When and Where

Whenever the physical clock rate exceeds the traffic contract, policing may be needed. Suppose, for instance, that ISP1 has 1000 customers, just like PB Tents, each with a 100-Mbps connection, and a contract for support of 2 Mbps. What happens over time Well, without something to prevent it, each customer will send and receive more and more traffic. For a while, all the customers are happy, because their packets make it through the overbuilt ISP1 core. Even if ISP1 has enough capacity to...

Custom Queuing

Custom Queuing (CQ) followed PQ as the next IOS queuing tool added to IOS. CQ addresses the biggest drawback of PQ by providing a queuing tool that does service all queues, even during times of congestion. It has 16 queues available, implying 16 classification categories, which is plenty for most applications. The negative part of CQ, as compared to PQ, is that CQ's scheduler does not have an option to always service one queue first like PQ's High queue so CQ does not provide great service for...

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...

Car with Be

The previous discussion revolved around the simpler case, in which there was no Be. With a Be configured, CAR uses a slightly more complex algorithm to decide whether a packet conforms to or exceeds the traffic contract. This more complex algorithm has a simple goal to soften the blow when the policer must discard packets. CAR's algorithm recognizes when the extra bytes defined by Be are being consumed, and classifies some packets as exceed, and some packets as conform, before all of Be has...

Tail Drop Global Synchronization and TCP Starvation

Tail drop occurs when a packet needs to be added to a queue, but the queue is full. Yes, tail drop is indeed that simple. However, tail drop results in some interesting behavior in real networks, particularly when most traffic is TCP based, but with some UDP traffic. Of course, the Internet today delivers mostly TCP traffic, because web traffic uses HTTP, and HTTP uses TCP. The preceding section described the behavior of a single TCP connection after a single packet loss. Now imagine an...

Calculated Planning Impairment Factor

The ITU standardizes network transmission impairments in ITU G.113. This standard defines the term calculated planning impairment factor (ICPIF), which is a calculation based on network delay and packet loss figures. ICPIF yields a single value that can be used as a gauge of network impairment. ITU G.113 provides the following interpretations of specific ICPIF values 45 Exceptional limiting case 55 Customers likely to react strongly SAA probe delay and loss information is used in calculating an...

Trunk Conditioning

Cisco IOS supports a function called a permanent trunk connection, sometimes called a connection trunk. A connection trunk creates a permanent trunk connection across the VoIP part of the network. To accomplish this, the connection trunk command is configured on a voice port to emulate a permanent connection across a packet network. The bandwidth required by the connection trunk is allocated at the creation of the trunk and remains reserved until the trunk is torn down. Figure 8-11 illustrates...

Multizone Topology

To solve the single-zone problem of reducing the call volume of the network to the capabilities of the lowest-capacity WAN link, you can design multiple gatekeeper zones into the network. A good practice in designing multiple zones is to create one zone per site, as shown in Figure 8-24. Figure 8-24 Simple Enterprise Multizone Topology HQ-Gatekeeper Site 1-Gatekeeper Zone 1 to Any Zone Limited to Internal Zone HQ Traffic Is Unlimited Zone HQ to Zone 1 Limited to 128 kbps Zone HQ to Zone 2...

Do I Know This Already Quiz

Table 1-1 outlines the major topics discussed in this chapter and the Do I Know This Already quiz questions that correspond to those topics. Table 1-1 Do I Know This Already Foundation Topics Section-to-Question Mapping Table 1-1 outlines the major topics discussed in this chapter and the Do I Know This Already quiz questions that correspond to those topics. Table 1-1 Do I Know This Already Foundation Topics Section-to-Question Mapping Foundation Topics Section Covering These Questions QoS...

The Expedited Forwarding PHB and DSCP Values

RFC 2598 defines the expedited forwarding per-hop behaviors. This RFC defines a very simple PHB (low latency, with a cap on bandwidth), and a single DSCP (EF) to represent it. Expedited forwarding simply states that a packet with the EF DSCP should minimize delay, jitter, and loss, up to a guaranteed bandwidth level for the class. Like AF, the EF RFC suggests two QoS actions be performed to achieve the PHB. First, queuing must be used to minimize the time that EF packets spend in a queue. A...

FRF11C and FRF12 Comparison

Most of the coverage of LFI over Frame Relay has focused on FRF.12. However, IOS offers another Frame Relay LFI service called FRF.11-C. You can use each of these two LFI options only when you use particular types of Frame Relay VCs. To appreciate how the two options differ, you first need to understand these two types of VCs. The Frame Relay Forum (FRF) created data VCs originally to carry multiprotocol data traffic, as defined in the FRF. 3 Implementation Agreements. Service providers around...

Channel Partner IP Telephony Technology Specializations

The most popular reason for taking the DQOS exam relates to the Cisco Channel Partner program. Cisco calls their resellers and services partners Channel Partners. The way the program works is that Cisco moves more than 90 percent of their product sales, in dollar volumes, through their Channel Partners. Thus, Cisco is motivated to help themselves by working well with their Channel Partner community. Cisco also focuses heavily on customer satisfaction. So Cisco uses both a carrot and a stick to...

Generic Traffic Shaping Configuration

GTS performs traffic shaping using the same logic and features discussed in the introductory section of this chapter. It can be enabled on a large variety of interfaces. It can also adapt the rate based on BECN signals, and reflect BECNs on a VC after receiving a FECN. It supports a single queuing tool for the shaping queues Weighted Fair Queuing (WFQ). It can also classify traffic, performing shaping on a subset of the traffic on an interface by classifying packets based on access-control...

DeJitter Buffer Delay

De-jitter buffer delay is the third voice delay component. Jitter happens in data networks. You can control it, and minimize it for jitter-sensitive traffic, but you cannot eliminate it. Buy why talk about jitter in the section on delay Because a key tool in defeating the effects of jitter, the de-jitter buffer (sometimes called the jitter buffer) actually increases delay. The de-jitter buffer collects voice packets and delays playing out the voice to the listener, to have several ms of voice...

Policy Based Routing PBR

PBR enables you to route a packet based on other information, in addition to the destination IP address. In most cases, engineers are happy with the choices of routes made by the routing protocol, with routing occurring based on the destination IP address in each packet. For some specialized cases, however, an engineer may want some packets to take a different path. One path through the network may be more secure, for instance, so some packets could be directed through a longer, but more...

FRED Configuration

FRED configuration requires only slightly more effort than does WRED configuration, as long as default configuration values are acceptable. This section shows two FRED configuration examples that involve the same basic scenarios as the first two WRED examples in the previous section. FRED configuration and show commands are listed in Tables 6-11 and 6-12. Table 6-11 Command Reference for FRED Interface configuration mode enables FRED on the interface. random-detect flow average-depth-factor...

The Assured Forwarding PHB and DSCP Values

RFC 2597 defines something called the assured forwarding per-hop behaviors. This RFC suggests that one good DiffServ design choice would be to allow for four different classes for queuing purposes. Within each queue, three levels of drop probability could be implied. The RFC title rightfully suggests that the focus is on the QoS behavior the PHB at each node. Most engineers also think of the RFC as defining the 12 DSCPs that are used in conjunction with the AF PHBs. An individual PHB describes...

WRED and Queuing

WRED relies on the average queue depth concept, which calculates a rolling average of the queue depth of some queue. But which queue Well, first consider a serial interface on a router, on which Weighted Fair Queuing (WFQ) is enabled by default. In this case, however, WFQ has been disabled, leaving a single first-in, first-out (FIFO) output queue on the interface. Figure 6-9 shows the basic idea. Figure 6-9 FIFO Output Queue and WRED Interaction Figure 6-9 FIFO Output Queue and WRED Interaction...

Voice Basics

Voice over data includes Voice over IP (VoIP), Voice over Frame Relay (VoFR), and Voice over ATM (VoATM). Each of these three voice over data technologies transports voice, and each is slightly different. Most of the questions you should see on an exam will be related to VoIP, and not VoFR or VoATM, because of the three options, VoIP is the most pervasive. Also calls between Cisco IP Phones use VoIP, not VoFR or VoATM. Imagine a call between the two analog phones in Figure 1-17, extensions 201...

Payload Compression Configuration

Payload compression requires little configuration. You must enable compression on both ends of a point-to-point serial link, or on both ends of a Frame Relay VC for Frame Relay. The compress command enables compression on point-to-point links, with the frame-relay payload-compression command enabling compression over Frame Relay. Table 7-5 lists the various configuration and show commands used with payload compression, followed by example configurations. Table 7-5 Configuration Command...

Classification for Voice Packets into LLQ

As you learned in previous chapters, LLQ is one of the most important Cisco QoS mechanisms to ensure quality for voice conversations, because it prioritizes voice packets over data packets at the router egress interface. For this to work, voice packets must be classified such that they are placed in the priority queue (PQ) portion of LLQ. Cisco IOS provides the service levels that RSVP accepts by working in conjunction with IOS queuing tools. As a general Cisco IOS feature, RSVP has its own set...

PSTN Fallback Configuration

PSTN fallback is configured on the originating gateway and applies only to calls initiated by the originating gateway. Inbound call attempts are not considered. PSTN fallback is configured at the global level and therefore applies to all outbound calls attempted by the gateway. You cannot selectively apply PSTN fallback to calls initiated by certain PSTN PBX interfaces. The SAA responder feature is configured on the destination node, also referred to as the terminating gateway. To apply PSTN...

Queuing and Traffic Shaping

Shaping tools support a variety of queuing tools that can be applied to the packets waiting in the shaping queue(s). At the same time, IOS supports queuing tools for the interface output queue(s) associated with the physical interface. Deciding when to use queuing tools on shaping queues, when to use them on the interface, and how the configurations differ in each case, can be a little confusing. This section clears up some of that confusion. To begin, Table 5-5 lists the traffic-shaping tools,...

CBWFQ Configuration

CBWFQ configuration uses the same MQC commands as does CB marking, with a few additional commands added. If you remember how to configure CB marking from Chapter 3, CBWFQ configuration will be easy for you. The commands used for CP marking and CBWFQ configuration are repeated here in Tables 4-11 and 4-12. Table 4-11 Command Reference for CBWFQ Global config names a class map, where classification options are configured. Class map subcommand defines specific classification parameters. match...

Link Fragmentation and Interleaving

Both types of QoS tools covered in this chapter address bandwidth constraints to some degree. Compression tools directly attack bandwidth constraints by lowering the bandwidth required to forward packets. Link fragmentation and interleaving (LFI) tools directly lower delay by defeating a side effect of a small transmit clock speed, namely serialization delay. A quick review of serialization delay should help you make more sense out of LFI tools. Serialization is the time required to send a...

Table of Contents

Do I Know This Already Quiz 3 QoS Tuning Bandwidth, Delay, Jitter, and Loss Questions 4 Traffic Characteristics of Voice, Video, and Data Questions 5 QoS Tuning Bandwidth, Delay, Jitter, and Loss 7 Bandwidth 9 The clock rate Command Versus the bandwidth Command 11 QoS Tools That Affect Bandwidth 11 Delay 13 Serialization Delay 14 Propagation Delay 16 Queuing Delay 18 Forwarding Delay 20 Shaping Delay 20 Network Delay 22 Delay Summary 23 QoS Tools That Affect Delay 24 Jitter 27 QoS Tools That...

Advanced Voice Busyout

Whereas LVBO provides for busyout based on local conditions of the originating gateway, AVBO adds the capability to trigger an SAA probe to one or more configured IP destinations. The information returned by the probe, which can be either the explicit loss and delay values, or the ICPIF congestion threshold, is used to trigger a busyout of the TDM trunk connection to the PBX. AVBO therefore introduces the capability to busy out a PBX trunk, or individual voice...

Low Latency Queuing LLQ

Cbwfq Scheduler Logic

Low Latency Queuing sounds like the best queuing tool possible, just based on the name. What packet wouldn't want to experience low latency As it turns out, for delay (latency) sensitive traffic, LLQ is indeed the queuing tool of choice. LLQ is simple to understand and simple to configure, assuming you already understand CBWFQ. LLQ is not really a separate queuing tool, but rather a simple option of CBWFQ applied to one or more classes. CBWFQ treats these classes as strict-priority queues. In...

Classification and Marking Tools

A variety of classification and marking tools exist. Classification and marking tools first classify by looking at something inside each packet you can compare these tools by listing the fields the tool can examine. Classification and marking tools mark the frame or packet based on the earlier comparisons you can compare these tools by listing the fields that can be marked. Some classification and marking tools also perform other functions, as noted in Table 2-2. Chapter 3 explains the details...

LLQ with More Than One Priority Queue

Some Cisco documentation claims that you can only have one low-latency queue inside a single policy map. In other words, only one class can use the priority command, making it a low-latency queue. Other Cisco documentation claims that you can have more than one low-latency queue in a single policy map. As it turns out, you can have multiple low-latency queues in a single policy map. Why would you need more than one low-latency queue in a policy map, and how would it work Well, it's actually...

Voice Bandwidth Considerations

Voice calls create a flow with a fixed data rate, with equally spaced packets. Voice flows can be described as isochronous, which, according to Dictionary.com, means characterized by or occurring at equal intervals of time. Consider Figure 1-19, where a call has been placed between analog phones at extensions 201 and 301. Figure 1-19 Isochronous Packet Flow for Voice Call Figure 1-19 Isochronous Packet Flow for Voice Call R1 creates the IP UDP RTP voice payload packets and sends them, by...

Class Based Policing Configuration

CB policing performs policing using three separate actions for packets that conform, exceed, or violate the traffic contract. (The exact meanings of each of these three categories were covered in the How Policing Works section earlier in this chapter.) Generally speaking, CB policing considers packets that happen to arrive when enough Bc tokens are available as conforming packets. Packets that arrive when Bc is consumed, but Be is not, are considered exceeding and packets that arrive after Bc...

Classification and Marking at the Edge

Class-based tools have advantages and disadvantages as well. The engineer can exercise great control over the packets placed into the classes or categories used in a network. Because a small number of categories are used, in most cases fewer than 10, the configuration scales. For instance, you build a network and choose four categories for packets. As the network grows, you can keep using the same four categories, and even with growing numbers of packets and routers, the number of classes can...

The Class Selector PHB and DSCP Values

Classs Selector And Dscp Values

RFC 2475, which defines DiffServ, became an RFC in December 1998. Even today, some QoS features in IOS do not support DiffServ Some QoS features will never support DiffServ, because newer, better tools that can do the same thing may have been introduced. All tools that support Cisco's strategic direction for QoS configuration, using the Modular QoS command-line interface (MQC), support DSCP. However, depending on the tools you need to use, and the IOS revisions you use in your network, you may...

Bandwidth

The term bandwidth refers to the number of bits per second that can reasonably be expected to be successfully delivered across some medium. In some cases, bandwidth equals the physical link speed, or the clock rate, of the interface. In other cases, bandwidth is smaller than the actual speed of the link. Consider, for example, Figure 1-2, which shows two typical networks, one with a point-to-point serial link, and the other using Frame Relay. Figure 1-2 Two Similar Networks, One with...

WFQ Classification

Flow-Based WFQ, or just WFQ, classifies traffic into flows. Flows are identified by at least five items in an IP packet Transport layer protocol (TCP or UDP) as defined by the IP Protocol header field TCP or UDP destination port Depending on what document you read, WFQ also classifies based on the ToS byte. In particular, the CCIP QoS exam expects WFQ to classify based on the ToS byte as well as the five items listed previously. The DQOS course, and presumably the DQOS exam, claims that it...