Quality of Service Overview
`
`This chapter explains quality of service (QoS) and the service models that embody it. It also suggests
`benefits you can gain from implementing Cisco IOS QoS in your network. Then it focuses on the
`Cisco IOS QoS features and the technologies that implement them.
`
`To identify the hardware platform or software image information associated with a feature, use the
`Feature Navigator on Cisco.com to search for information about the feature or refer to the software
`release notes for a specific release. For more information, see the “Identifying Supported Platforms”
`section in the “Using Cisco IOS Software” chapter in this book.
`
`What Is Quality of Service?
`
`QoS refers to the ability of a network to provide improved service to selected network traffic over various
`underlying technologies including Frame Relay, ATM, Ethernet and 802.1 networks, SONET, and
`IP-routed networks. In particular, QoS features provide improved and more predictable network service
`by providing the following services:
`
`• Supporting dedicated bandwidth
`
`(cid:129)
`
`Improving loss characteristics
`
`(cid:129) Avoiding and managing network congestion
`
`(cid:129) Shaping network traffic
`
`(cid:129) Setting traffic priorities across the network
`
`About QoS Architecture
`
`You configure QoS features throughout a network to provide for end-to-end QoS delivery. The following
`three components are necessary to deliver QoS across a heterogeneous network:
`
`(cid:129) QoS within a single network element, which includes queueing, scheduling, and traffic shaping
`features.
`
`(cid:129) QoS signalling techniques for coordinating QoS for end-to-end delivery between network elements.
`
`(cid:129) QoS policing and management functions to control and administer end-to-end traffic across a
`network.
`
`
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`Cisco IOS Quality of Service Solutions Configuration Guide
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`Exhibit 2021
`IPR2016-00309
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`

`
`Who Could Benefit from Using Cisco IOS QoS?
`
`
`
`Quality of Service Overview
`
`Not all QoS techniques are appropriate for all network routers. Because edge routers and backbone
`routers in a network do not necessarily perform the same operations, the QoS tasks they perform might
`differ as well. To configure an IP network for real-time voice traffic, for example, you would need to
`consider the functions of both edge and backbone routers in the network, then select the appropriate QoS
`feature or features.
`
`In general, edge routers perform the following QoS functions:
`
`(cid:129) Packet classification
`
`(cid:129) Admission control
`
`(cid:129) Configuration management
`
`In general, backbone routers perform the following QoS functions:
`
`(cid:129) Congestion management
`
`(cid:129) Congestion avoidance
`
`Who Could Benefit from Using Cisco IOS QoS?
`
`All networks can take advantage of aspects of QoS for optimum efficiency, whether the network is for a
`small corporation, an enterprise, or an Internet service provider (ISP). Different categories of networking
`users—such as major enterprises, network service providers, and small and medium-sized business
`networking users—have their own QoS requirements; in many areas, however, these requirements
`overlap. The Cisco IOS QoS features described in the section “Cisco QoS Features” later in this chapter
`address these diverse and common needs.
`
`Enterprise networks, for example, must provide end-to-end QoS solutions across the various platforms
`comprising the network; providing solutions for heterogeneous platforms often requires that you take a
`different QoS configuration approach for each technology. As enterprise networks carry more complex,
`mission-critical applications and experience increased traffic from Web multimedia applications, QoS
`serves to prioritize this traffic to ensure that each application gets the service it requires.
`
`ISPs require assured scalability and performance. For example, ISPs that long have offered best-effort
`IP connectivity now also transfer voice, video, and other real-time critical application data. QoS answers
`the scalability and performance needs of these ISPs to distinguish different kinds of traffic, thereby
`enabling them to offer service differentiation to their customers.
`
`In the small and medium-sized business segment, managers are experiencing firsthand the rapid growth
`of business on the Internet. These business networks must also handle increasingly complex business
`applications. QoS lets the network handle the difficult task of utilizing an expensive WAN connection in
`the most efficient way for business applications.
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`Quality of Service Overview
`
`Why Deploy Cisco IOS QoS?
`
`
`
`Why Deploy Cisco IOS QoS?
`
`The Cisco IOS QoS features enable networks to control and predictably service a variety of networked
`applications and traffic types. Implementing Cisco IOS QoS in your network promotes the following
`features:
`
`(cid:129) Control over resources. You have control over which resources (bandwidth, equipment, wide-area
`facilities, and so on) are being used. For example, you can limit bandwidth consumed over a
`backbone link by File Transfer Protocol (FTP) transfers or give priority to an important database
`access.
`
`(cid:129) Tailored services. If you are an ISP, the control and visibility provided by QoS enables you to offer
`carefully tailored grades of service differentiation to your customers.
`
`(cid:129) Coexistence of mission-critical applications. Cisco QoS features make certain of the following
`conditions:
`– That your WAN is used efficiently by mission-critical applications that are most important to
`your business.
`– That bandwidth and minimum delays required by time-sensitive multimedia and voice
`applications are available.
`– That other applications using the link get their fair service without interfering with
`mission-critical traffic.
`
`Moreover, in implementing QoS features in your network, you put in place the foundation for a future
`fully integrated network.
`
`End-to-End QoS Models
`
`A service model, also called a level of service, describes a set of end-to-end QoS capabilities. End-to-end
`QoS is the ability of the network to deliver service required by specific network traffic from one end of
`the network to another. Cisco IOS QoS software supports three types of service models: best effort,
`integrated, and differentiated services.
`
`Note
`
`QoS service models differ from one another in how they enable applications to send data and in the
`ways in which the network attempts to deliver that data. For instance, a different service model
`applies to real-time applications, such as audio and video conferencing and IP telephony, than a
`model that applies to file transfer and e-mail applications.
`
`Consider the following factors when deciding which type of service to deploy in the network:
`
`(cid:129) The application or problem you are trying to solve. Each of the three types of service—best effort,
`integrated, and differentiated—is appropriate for certain applications.
`
`(cid:129) The kind of ability you want to allocate to your resources.
`
`(cid:129) Cost-benefit analysis. For example, the cost of implementing and deploying differentiated service is
`certain to be more expensive than the cost for a best-effort service.
`
`The following sections describe the service models supported by features in Cisco IOS software:
`
`(cid:129) Best-Effort Service
`
`(cid:129)
`
`Integrated Service
`
`(cid:129) Differentiated Service
`
`
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`End-to-End QoS Models
`
`Best-Effort Service
`
`
`
`Quality of Service Overview
`
`Best effort is a single service model in which an application sends data whenever it must, in any quantity,
`and without requesting permission or first informing the network. For best-effort service, the network
`delivers data if it can, without any assurance of reliability, delay bounds, or throughput.
`
`The Cisco IOS QoS feature that implements best-effort service is FIFO queueing. Best-effort service is
`suitable for a wide range of networked applications such as general file transfers or e-mail.
`
`Integrated Service
`
`Integrated service is a multiple service model that can accommodate multiple QoS requirements. In this
`model the application requests a specific kind of service from the network before it sends data. The
`request is made by explicit signalling; the application informs the network of its traffic profile and
`requests a particular kind of service that can encompass its bandwidth and delay requirements. The
`application is expected to send data only after it gets a confirmation from the network. It is also expected
`to send data that lies within its described traffic profile.
`
`The network performs admission control, based on information from the application and available
`network resources. It also commits to meeting the QoS requirements of the application as long as the
`traffic remains within the profile specifications. The network fulfills its commitment by maintaining
`per-flow state and then performing packet classification, policing, and intelligent queueing based on that
`state.
`
`Cisco IOS QoS includes the following features that provide controlled load service, which is a kind of
`integrated service:
`
`(cid:129) The Resource Reservation Protocol (RSVP), which can be used by applications to signal their QoS
`requirements to the router.
`
`(cid:129)
`
`Intelligent queueing mechanisms, which can be used with RSVP to provide the following kinds of
`services:
`– Guaranteed Rate Service, which allows applications to reserve bandwidth to meet their
`requirements. For example, a Voice over IP (VoIP) application can reserve the required amount
`of bandwidth end-to-end using this kind of service. Cisco IOS QoS uses weighted fair queueing
`(WFQ) with RSVP to provide this kind of service.
`– Controlled Load Service, which allows applications to have low delay and high throughput even
`during times of congestion. For example, adaptive real-time applications such as playback of a
`recorded conference can use this kind of service. Cisco IOS QoS uses RSVP with Weighted
`Random Early Detection (WRED) to provide this kind of service.
`
`Differentiated Service
`
`Differentiated service is a multiple service model that can satisfy differing QoS requirements. However,
`unlike in the integrated service model, an application using differentiated service does not explicitly
`signal the router before sending data.
`
`For differentiated service, the network tries to deliver a particular kind of service based on the QoS
`specified by each packet. This specification can occur in different ways, for example, using the IP
`Precedence bit settings in IP packets or source and destination addresses. The network uses the QoS
`specification to classify, mark, shape, and police traffic, and to perform intelligent queueing.
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`Quality of Service Overview
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`
`
`Cisco QoS Features
`
`The differentiated service model is used for several mission-critical applications and for providing
`end-to-end QoS. Typically, this service model is appropriate for aggregate flows because it performs a
`relatively coarse level of traffic classification.
`
`Cisco IOS QoS includes the following features that support the differentiated service model:
`
`(cid:129) Committed access rate (CAR), which performs packet classification through IP Precedence and QoS
`group settings. CAR performs metering and policing of traffic, providing bandwidth management.
`
`(cid:129)
`
`Intelligent queueing schemes such as WRED and WFQ and their equivalent features on the Versatile
`Interface Processor (VIP), which are distributed WRED (DWRED) and distributed WFQ. These
`features can be used with CAR to deliver differentiated services.
`
`For more information on how to implement Differentiated Services using the components of Cisco IOS
`software, see the chapter “Implementing DiffServ for End-to-End Quality of Service Overview” in this
`book.
`
`Cisco QoS Features
`
`The Cisco IOS QoS software provides the major features described in the following sections. Some of
`which have been previously mentioned, and all of them are briefly introduced in this chapter.
`
`(cid:129) Classification
`
`(cid:129) Congestion Management
`
`(cid:129) Congestion Avoidance
`
`(cid:129) Policing and Shaping
`
`(cid:129) Signalling
`
`(cid:129) Link Efficiency Mechanisms
`
`(cid:129) QoS Solutions
`
`(cid:129) Modular QoS Command-Line Interface
`
`(cid:129) Security Device Manager
`
`Classification
`
`The features listed are described more fully in the overview chapters of this book, which is organized
`into parts, one for each of the major features listed. Each book part contains an overview chapter and one
`or more configuration chapters.
`
`Packet classification features provide the capability to partition network traffic into multiple priority
`levels or classes of service. For example, by using the three precedence bits in the Type of service (ToS)
`field of the IP packet header—two of the values are reserved for other purposes—you can categorize
`packets into a limited set of up to six traffic classes. After you classify packets, you can utilize other QoS
`features to assign the appropriate traffic handling policies including congestion management, bandwidth
`allocation, and delay bounds for each traffic class.
`
`Packets can also be classified by external sources, that is, by a customer or by a downstream network
`provider. You can either allow the network to accept the classification or override it and reclassify the
`packet according to a policy that you specify.
`
`
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`Cisco QoS Features
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`Quality of Service Overview
`
`Packets can be classified based on policies specified by the network operator. Policies can be set that
`include classification based on physical port, source or destination IP or MAC address, application port,
`IP protocol type, and other criteria that you can specify by using access lists or extended access lists.
`
`You can use Cisco IOS QoS policy-based routing (PBR) and the classification features of Cisco IOS QoS
`CAR to classify packets. You can use Border Gateway Protocol (BGP) policy propagation to propagate
`destination-based packet classification policy throughout a large network via BGP routing updates. This
`section gives a brief description of these features.
`
`In addition, you can use the QoS for Virtual Private Networks (VPNs) feature to classify packets before
`tunneling and encryption occur. The process of classifying features before tunneling and encryption is
`called preclassification.
`
`The Class-Based Packet Marking feature provides users with a user-friendly command-line interface
`(CLI) for efficient packet marking by which users can differentiate packets based on the designated
`markings.
`
`For more complete conceptual information on packet classification, see the chapter “Classification
`Overview” in this book.
`
`For information on how to configure the various protocols that implement classification, see the
`following chapters:
`
`(cid:129)
`
`(cid:129)
`
`(cid:129)
`
`(cid:129)
`
`(cid:129)
`
`(cid:129)
`
`“Configuring Policy-Based Routing”
`
`“Configuring QoS Policy Propagation via Border Gateway Protocol”
`
`“Configuring Committed Access Rate”
`
`“Configuring Class-Based Packet Marking”
`
`“Configuring QoS for Virtual Private Networks”
`
`“Configuring Network-Based Application Recognition”
`
`For complete command syntax information, refer to the Cisco IOS Quality of Service Solutions
`Command Reference.
`
`IP Precedence
`
`The IP Precedence feature allows you to specify the class of service of a packet using the three
`precedence bits in the ToS field of the IP version 4 (IPv4) header. Other features configured throughout
`the network can then use these bits to determine how to treat the packet in regard to the type of service
`to grant it. For example, although IP Precedence is not a queueing method, other queueing methods such
`as WFQ can use the IP Precedence setting of the packet to prioritize traffic.
`
`Policy-Based Routing
`
`Cisco IOS QoS PBR allows you to perform the following tasks:
`
`(cid:129) Classify traffic based on extended access list criteria.
`
`(cid:129) Set IP Precedence bits.
`
`(cid:129) Route specific traffic to engineered paths, which may be required to allow a specific QoS service
`through the network.
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`Quality of Service Overview
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`
`
`Cisco QoS Features
`
`Classification of traffic through PBR allows you to identify traffic for different classes of service at the
`perimeter of the network and then implement QoS defined for each class of service in the core of the
`network using priority queueing, custom queueing, or WFQ techniques. This process obviates the need
`to classify traffic explicitly at each WAN interface in the core-backbone network.
`
`Some possible applications for policy routing are to provide equal access, protocol-sensitive routing,
`source-sensitive routing, routing based on interactive versus batch traffic, or routing based on dedicated
`links.
`
`BGP Policy Propagation
`
`BGP provides a powerful, scalable means of utilizing attributes, such as community values, to propagate
`destination-based packet classification policy throughout a large network via BGP routing updates.
`Packet classification policy can be scalably propagated via BGP without writing and deploying complex
`access lists at each of a large number of routers. BGP ensures that return traffic to customers is handled
`as premium traffic by the network.
`
`Committed Access Rate (Packet Classification)
`
`CAR is the main feature supporting packet classification. CAR uses the ToS bits in the IP header to
`classify packets. You can use the CAR classification commands to classify and reclassify a packet.
`
`Here are some example packet classification policies:
`
`(cid:129) All packets received on a particular T1 line are classified as high priority (port-based classification).
`
`(cid:129) All HTTP traffic is classified as medium priority (application classification).
`
`(cid:129) Video traffic from a specified IP address is classified as medium priority.
`
`(cid:129) Packets bound for particular destinations are classified as high priority traffic (for example,
`international traffic or traffic bound for a premium customer).
`
`(cid:129) Some packets are classified for subrate IP services. The network operator delivers a physical T1/E1
`or T3/E3 line to the customer, but offers a less expensive subrate service, for example, 1 Mbps on
`an E1 line or 10 Mbps on a T3 line. The customer pays for the subrate bandwidth and may be
`upgraded to additional access bandwidth over time based on demand. CAR limits the traffic rate
`available to the customer and delivered to the network to the agreed-upon rate limit (with the ability
`to temporarily burst over the limit). The network operator may upgrade the service without any
`physical network arrangement.
`
`(cid:129) Traffic is classified for exchange point traffic control. An ISP offers transit services to downstream
`ISPs via exchange point connectivity provided by a Layer 2 switch. The upstream provider utilizes
`MAC-address rate limits provided by CAR to enforce bandwidth usage limitations on the
`downstream ISPs.
`
`Note
`
`CAR also implements rate-limiting services, which are described later in this chapter.
`
`
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`Cisco QoS Features
`
`Class-Based Packet Marking
`
`
`
`Quality of Service Overview
`
`The Class-Based Packet Marking feature provides users with a means for efficient packet marking by
`which users can differentiate packets based on the designated markings. The Class-Based Packet
`Marking feature allows users to perform the following tasks:
`
`(cid:129) Mark packets by setting the IP Precedence bits or the IP differentiated services code point (DSCP)
`in the IP ToS byte.
`
`(cid:129) Mark packets by setting the Layer 2 class of service (CoS) value.
`
`(cid:129) Associate a local QoS group value with a packet.
`(cid:129) Set the cell loss priority (CLP) bit setting in the ATM header of a packet from 0 to 1.
`
`QoS for Virtual Private Networks
`
`When packets are encapsulated by tunnel or encryption headers, QoS features are unable to examine the
`original packet headers and correctly classify the packets. Packets traveling across the same tunnel have
`the same tunnel headers, so the packets are treated identically if the physical interface is congested.
`
`With the growing popularity of VPNs, the need to classify traffic within a traffic tunnel is gaining
`importance. QoS features have historically been unable to classify traffic within a tunnel. With the
`introduction of the QoS for VPNs feature, packets can now be classified before tunneling and encryption
`occur. The process of classifying features before tunneling and encryption is called preclassification.
`
`The QoS for VPNs feature is designed for tunnel interfaces. When the feature is enabled, the QoS
`features on the output interface classify packets before encryption, allowing traffic flows to be adjusted
`in congested environments. The result is more effective packet tunneling.
`
`Network-Based Application Recognition
`
`The Network-Based Application Recognition (NBAR) feature provides intelligent network
`classification to network infrastructures. NBAR is a classification engine that recognizes a wide variety
`of applications, including web-based and other difficult-to-classify protocols that utilize dynamic
`TCP/User Datagram Ports (UDP) port assignments. When an application is recognized and classified by
`NBAR, a network can invoke services for that specific application.
`
`Congestion Management
`
`Congestion management features operate to control congestion once it occurs. One way that network
`elements handle an overflow of arriving traffic is to use a queueing algorithm to sort the traffic, then
`determine some method of prioritizing it onto an output link. Each queueing algorithm was designed to
`solve a specific network traffic problem and has a particular effect on network performance.
`
`The Cisco IOS software congestion management, or queueing, features include the following:
`
`(cid:129) FIFO
`
`(cid:129) Priority queueing (PQ)
`
`(cid:129) Frame Relay permanent virtual circuit (PVC) interface priority queueing (FR PIPQ)
`
`(cid:129) Custom queueing (CQ)
`
`(cid:129) Flow-based, class-based, and distributed WFQ
`
`(cid:129) Distributed class-based WFQ
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`Quality of Service Overview
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`
`
`Cisco QoS Features
`
`(cid:129)
`
`IP RTP Priority and Frame Relay IP RTP Priority
`
`(cid:129) Low latency queueing (LLQ), Distributed LLQ, and LLQ for Frame Relay
`
`For more complete conceptual information on packet classification, see the chapter “Congestion
`Management Overview” in this book.
`
`For information on how to configure the various protocols that implement congestion management, see
`the following chapters:
`
`(cid:129)
`
`(cid:129)
`
`(cid:129)
`
`“Configuring Weighted Fair Queueing”
`
`“Configuring Custom Queueing”
`
`“Configuring Priority Queueing”
`
`For complete command syntax information, refer to the Cisco IOS Quality of Service Solutions
`Command Reference.
`
`What Is Congestion in Networks?
`
`To give you a more definite sense of congestion in networks, this section briefly describes some of its
`characteristics, drawing on the explanation presented by V. Paxson and S. Floyd in a paper titled Wide
`Area Traffic: The Failure of Poisson Modeling.
`
`What does congestion look like? Consideration of the behavior of congested systems is not simple and
`cannot be dealt with in a simplistic manner, because traffic rates do not simply rise to a level, stay there
`a while, then subside. Periods of traffic congestion can be quite long, with losses that are heavily
`concentrated. In contrast to Poisson traffic models, linear increases in buffer size do not result in large
`decreases in packet drop rates; a slight increase in the number of active connections can result in a large
`increase in the packet loss rate. This understanding of the behavior of congested networks suggests that
`because the level of busy period traffic is not predictable, it would be difficult to efficiently size networks
`to reduce congestion adequately. Observers of network congestion report that in reality, traffic “spikes,”
`which causes actual losses that ride on longer-term ripples, which in turn ride on still longer-term swells.
`
`FIFO Queueing
`
`FIFO provides basic store and forward capability. FIFO is the default queueing algorithm in some
`instances, thus requiring no configuration. See “WFQ and Distributed WFQ” later in this section for a
`complete explanation of default configuration.
`
`PQ
`
`
`
`Designed to give strict priority to important traffic, PQ ensures that important traffic gets the fastest
`handling at each point where PQ is used. PQ can flexibly prioritize according to network protocol (such
`as IP, IPX, or AppleTalk), incoming interface, packet size, source/destination address, and so on.
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`Cisco QoS Features
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`Frame Relay PVC PQ
`
`
`
`Quality of Service Overview
`
`The FR PIPQ provides an interface-level PQ scheme in which prioritization is based on destination PVC
`rather than packet contents. For example, FR PIPQ allows you to configure PVC transporting voices
`traffic to have absolute priority over a PVC transporting signalling traffic, and a PVC transporting
`signalling traffic to have absolute priority over a PVC transporting data.
`
`FR PIPQ provides four levels of priority: high, medium, normal, and low. The Frame Relay packet is
`examined at the interface for the data-link connection identifier (DLCI) value. The packet is then sent to
`the correct priority queue based on the priority level configured for that DLCI.
`
`CQ
`
`CQ reserves a percentage of the available bandwidth of an interface for each selected traffic type. If a
`particular type of traffic is not using the bandwidth reserved for it, then other traffic types may use the
`remaining reserved bandwidth.
`
`WFQ and Distributed WFQ
`
`WFQ applies priority (or weights) to identified traffic to classify traffic into conversations and determine
`how much bandwidth each conversation is allowed relative to other conversations. WFQ classifies traffic
`into different flows based on such characteristics as source and destination address, protocol, and port
`and socket of the session.
`
`To provide large-scale support for applications and traffic classes requiring bandwidth allocations and
`delay bounds over the network infrastructure, Cisco IOS QoS includes a version of WFQ that runs only
`in distributed mode on VIPs. This version is called VIP-distributed WFQ (DWFQ). It provides increased
`flexibility in terms of traffic classification, weight assessment, and discard policy, and delivers
`Internet-scale performance on the Cisco 7500 series platforms.
`
`For serial interfaces at E1 (2.048 Mbps) and below, WFQ is used by default. When no other queueing
`strategies are configured, all other interfaces use FIFO by default.
`
`CBWFQ and Distributed CBWFQ
`
`The class-based WFQ (CBWFQ) and distributed class-based WFQ (DCBWFQ) features extend the
`standard WFQ functionality to provide support for user-defined traffic classes. They allow you to specify
`the exact amount of bandwidth to be allocated for a specific class of traffic. Taking into account available
`bandwidth on the interface, you can configure up to 64 classes and control distribution among them.
`
`DCWFQ is intended for use on the VIP-based Cisco 7000 series routers with the Route Switch
`Processors (RSPs), and the Cisco 7500 series routers except those with PA-A3-8T1IMA modules.
`
`IP RTP Priority
`
`The IP RTP Priority feature provides a strict priority queueing scheme that allows delay-sensitive data
`such as voice to be dequeued and sent before packets in other queues are dequeued. This feature can be
`used on serial interfaces and Frame Relay PVCs in conjunction with either WFQ or CBWFQ on the same
`outgoing interface. In either case, traffic matching the range of UDP ports specified for the priority queue
`is guaranteed strict priority over other CBWFQ classes or WFQ flows; packets in the priority queue are
`always serviced first.
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`Quality of Service Overview
`
`Frame Relay IP RTP Priority
`
`
`
`Cisco QoS Features
`
`The Frame Relay IP RTP Priority feature provides a strict priority queueing scheme on a Frame Relay
`PVC for delay-sensitive data such as voice. Voice traffic can be identified by its Real-Time Transport
`Protocol (RTP) port numbers and classified into a priority queue configured by the frame-relay ip rtp
`priority command. The result of using this feature is that voice is serviced as strict priority in preference
`to other nonvoice traffic.
`
`LLQ
`
`Distributed LLQ
`
`LLQ provides strict priority queueing on ATM VCs and serial interfaces. This feature allows you to
`configure the priority status for a class within CBWFQ, and is not limited to UDP port numbers, as is IP
`RTP Priority. LLQ and IP RTP Priority can be configured at the same time, but IP RTP Priority takes
`precedence.
`
`Additionally, the functionality of LLQ has been extended to allow you to specify the Committed Burst
`(Bc) size in LLQ and to change (or vary) the number of packets contained in the hold queue per-VC (on
`ATM adapters that support per-VC queueing). For more information, see the chapter “Congestion
`Management Overview” in this book.
`
`The Distributed LLQ feature provides the ability to specify low latency behavior for a traffic class on a
`VIP-based Cisco 7500 series router except those with PA-A3-8T1IMA modules. LLQ allows
`delay-sensitive data such as voice to be dequeued and sent before packets in other queues are dequeued
`
`The Distributed LLQ feature also introduces the ability to limit the depth of a device transmission ring.
`
`LLQ for Frame Relay
`
`LLQ for Frame Relay is provides strict PQ for voice traffic and WFQs for other classes of traffic. Before
`the release of this feature, LLQ was available at the interface and ATM VC levels. It is now available at
`the Frame Relay VC level when Frame Relay Traffic Shaping is configured.
`
`Strict PQ improves QoS by allowing delay-sensitive traffic such as voice to be pulled from the queue and
`sent before other classes of traffic.
`
`LLQ for Frame Relay allows you to define classes of traffic according to protocol, interface, or access
`lists. You can then assign characteristics to those classes, including priority, bandwidth, queue limit, and
`WRED.
`
`Congestion Avoidance
`
`Congestion avoidance techniques monitor network traffic loads in an effort to anticipate and avoid
`congestion at common network and internetwork bottlenecks before it becomes a problem. These
`techniques are designed to provide preferential treatment for premium (priority) class traffic under
`congestion situations while concurrently maximizing network throughput and capacity utilization and
`minimizing packet loss and delay. WRED and DWRED are the Cisco IOS QoS congestion avoidance
`features.
`
`
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`Cisco IOS Quality of Service Solutions Configuration Guide
`
`QC-11
`
`

`
`Cisco QoS Features
`
`
`
`Quality of Service Overview
`
`Router behavior allows output buffers to fill during periods of congestion, using the tail drop feature to
`resolve the problem when WRED is not configured. During tail drop, a potentially large number of
`packets from numerous connections are discarded because of lack of buffer capacity. This behavior can
`result in waves of congestion followed by periods during which the transmission link is not fully used.
`WRED obviates this situation proactively by providing congestion avoidance. That is, instead of waiting
`for buffers to fill before dropping packets, the router monitors the buffer depth and performs early
`discards on selected packets sent over selected connections.
`
`WRED is the Cisco implementation of the RED class of congestion avoidance algorithms. When RED
`is used and the source detects the dropped packet, the source slows its transmission. RED is primarily
`designed to work with TCP in IP internetwork environments.
`
`WRED can also be configured to use the DSCP value when it calculates the drop probability of a packet,
`enabling WRED to be compliant with the DiffServ standard being developed by the Internet Engineering
`Task Force (IETF).
`
`For more complete conceptual information, see the chapter “Congestion Avoidance Overview” in this