The All-New Switch Book

The Complete Guide to LAN Switching Technology

Rich Seifert, James Edwards

• Publisher: Wiley
• Year: 2011
• ISBN:
  9781118080047
• Edition: 2
• Topic: Computer Science
• Subtopic: Computer Networking
• Index: Computer Science

This much-needed update to the bestselling guide on the extensive changes to the local area networks (LAN) switching technologies explains why LAN switching technologies are critical to network design. This in-depth guide covers the capabilities, application, and design of LAN switches and switched internetworks and examines the significant changes that have taken place since the publication of the first edition seven years ago. You're sure to appreciate the witty writing style and easy-to-follow format on such an often-complicated subject matter.

Part I

Foundations of LAN Switches

In This Part

Chapter 1: Laying the Foundation

Chapter 2: Transparent Bridges

Chapter 3: Bridging Between Technologies

Chapter 4: Principles of LAN Switches

Chapter 5: Loop Resolution

Chapter 6: Source Routing

Chapter 1

Laying the Foundation

Before we delve into the details of Local Area Network (LAN) switch operation, you need to consider the foundation on which LAN switches are built. This chapter examines four important building blocks that will be indispensable to your understanding of LAN switches in later chapters:

• Network architecture
• Device addressing
• LAN technology
• LAN standards

Each is considered specifically in the context of Local Area Networks and its relevance to LAN switching.

In addition, this chapter introduces the terminology that will be used consistently throughout the book. Very often, speakers, writers, equipment vendors, and network operations personnel use different sets of terms to describe the elements and behavior of computer networks: Is it an Ethernet frame or an Ethernet packet that is sent by a station?1 While a name in itself is never inherently wrong—speakers and writers can define their own terminology any way they want—we need to agree on the meaning of a number of key words and phrases so that we can unambiguously describe and understand the behavior of network protocols and devices. We have tried throughout this book to use terminology in a way that both reflects common industry usage and is technically accurate. When there is a conflict between these points of view, we have opted for technical correctness. In any case, we have tried to be consistent and unambiguous.

It is not possible to provide a novice-level tutorial on every facet of networking that may be relevant to LAN switches. This book is not intended to be an introduction to computer networks; it is a comprehensive treatise on the design, operation, and application of switch technology in LANs. Most of the discussions here and in later chapters presume that the reader has some experience with networks and LAN technology. While this first chapter does provide background information, it is not intended as a primer, but as a reminder of the technologies and concepts on which later chapters build.

1.1 Network Architecture

The art of networking comprises a wide range of operations and technologies. Casual end users may think that “the network” is the browser or e-mail screen interface; this is all that they know (and from their perspective, probably all that they need to know) about networking. Programmers writing application code that must communicate among multiple machines may need to know about the programming interfaces and network facilities provided by the local operating system, but are generally unconcerned about the actual mechanisms used to deliver messages. Designers of high-speed optical fiber links used to interconnect network routers and servers should not have to worry about the data structures in the e-mail messages that may traverse a link.

In addition, the applications, functions, and technologies of networking are constantly changing. Every year, new ways of increasing the data rate of the communications channels in which our networks operate are introduced. New applications are constantly being written that use existing network facilities to provide improved or even revolutionary new services for users. You need to make sure that advances in one area of network technology are not constrained by limitations in other areas. For example, you want to be able to install a higher-speed communications link without having to wait for a new application or protocol to be designed that can take advantage of that link. Similarly, you want to ensure that the new communications link does not cause previously working applications to fail because those applications depend on some idiosyncrasy endemic to the older technology.

Padlipsky's Rule

If you know what you're doing, three layers is enough. If you don't, even seventeen won't help.

The key to achieving these goals is to separate the totality of network functions into discrete partitions called layers. Layering allows the appropriate technology to be applied to each function and to be changed without unduly affecting other layers. The number of layers is rather arbitrary; the issue is separation of functions. Architectural layers are defined such that each layer provides a set of distinct, related functions. Ideally, these functions are grouped such that layers can be as independent of each other as possible; only a minimum of information should have to pass between layer entities.

Figure 1.1 depicts the Open Systems Interconnect (OSI) model of network layering developed during the late 1970s and formally standardized in [ISO94]. It comprises seven layers of network system functions.

Figure 1.1 OSI reference model for network communications

In the sections that follow, we will take a look at the functions provided by each of these layers, with particular concern for their relevance to LANs and LAN switches.

1.1.1 Physical Layer

The Physical layer serves requests sent down from the Data Link layer (described in the following section), and comprises those elements involved with the actual transmission and reception of signals from the communications medium. The functions provided typically include line drivers and receivers, signal encoders and decoders, clock synchronization circuits, and so on. The exact nature of the device(s) implementing the Physical layer is a function of the design of the communications channel and the physical medium itself.

Examples of Physical layer interfaces are Token Ring, Ethernet, and FDDI. The Physical layer is also concerned with the actual transmission medium, such as network connectors, cabling types, cabling distance factors, and other mechanical considerations.

While a given networking device (for example, a LAN switch) must obviously include the circuitry needed to connect to the communications channel on which it is to be used, the nature of that channel has little impact on the higher-level operation of the device. For example, a LAN switch performs the same functions regardless of whether it is connected to an optical fiber channel operating at 1,000 Mb/s or a twisted pair copper wire channel operating at 10 Mb/s.

1.1.2 Data Link Layer

The Data Link layer provides services that allow direct communication between devices across the underlying physical channel. The communication can be point-to-point in nature (exactly two communicating stations) or point-to-multipoint (one-to-many), depending on the nature and configuration of the underlying channel.

In general, the Data Link layer must provide mechanisms for:

• Framing: The Data Link typically must provide a way to separate (delimit) discrete message transmissions (frames) in the Physical layer symbol stream.
• Addressing: Particularly when communicating among multiple stations on a common communications channel (as is typical of LANs), there must be a means to identify both the sender and target destination(s).
• Error detection: It is theoretically impossible for the underlying communications channel to be totally error free. While we hope that most transmissions will be received intact, there is always some residual rate of data errors, regardless of the technology employed within the Physical layer.2 It is important that corrupted data not be delivered to higher-layer clients of the Data Link. At a minimum, the Data Link layer must detect virtually all errors. Depending on the design of the Data Link, it may either discard corrupted data (leaving error recovery to higher-layer entities) or take explicit action to correct or recover from the data corruption. These two modes of operation are explored in detail in section 1.1.8.1.

In general, LAN technology exists primarily at the Data Link and Physical layers of the architecture. Likewise, the functions performed by a LAN switch occur mainly at the Data Link layer.3 As a result, this book focuses heavily on Data Link operation and behavior. Throughout the book, we show you how LAN switches significantly enhance the power and capabilities provided by the Data Link layer. As part of the design of these new features and the devices that implement them, you must often consider the impact of such Data Link modifications on the operation of higher-layer protocols.

Because it is so crucial to your understanding of LANs and LAN switching, section 1.1.8 provides an in-depth look at Data Link layer operation.

1.1.3 Network Layer

While the Data Link is concerned with the direct exchange of frames among stations on a single communications channel, the Network layer is responsible for station-to-station data delivery across multiple Data Links. As such, this layer must often accommodate a wide variety of Data Link technologies (both local and wide area) and arbitrary topologies, including partially complete meshes with multiple paths between endpoints. The Network layer is responsible for routing packets across the internetwork, usually through the action of intermediate relay stations known as routers (see section 1.5.3).4

Examples of Network-layer protocols include the Internet Protocol (IP) used in the TCP/IP suite, the Internetwork Packet Exchange protocol (IPX) used in NetWare, and the Datagram Delivery Protocol (DDP) used in AppleTalk.

1.1.4 Transport Layer

In most network architectures, Transport is where the buck stops. While the underlying communications facilities may cause packets to be dropped, delivered out of sequence, or corrupted by errors, the Transport layer shields higher-layer applications from having to deal with these nasty details of network behavior. Transport provides its clients with a perfect pipe: an error-free, sequenced, guaranteed-delivery message service that allows process-to-process communications between stations across an internetwork, as long as a functioning communications path is available.

Seifert's Law of Networking #42

The Layers Keep Lifting Me … Higher and Higher.

To provide this end-to-end reliable delivery service, Transport often needs to include mechanisms for connection establishment, error recovery, traffic pacing (flow control), message sequencing, and segmentation/reassembly of large application data blocks. Examples of Transport protocols include the Transmission Control Protocol (TCP) of the TCP/IP suite, the Sequenced Packet Exchange (SPX) protocol of NetWare, and the AppleTalk Transaction Protocol (ATP).

1.1.5 Session Layer

The Session layer provides for the establishment of communications sessions between applications. It may deal with user authentication and access control (for example, passwords), synchronization and checkpointing of data transfers, and so on. The Session layer serves requests from the Presentation layer and sends requests to the Transport layer.

The Session layer sets up, manages, and ultimately terminates communication between end users and end user applications. It is able to combine different data stream types coming from various sources and synchronize the data so the end users can all be on the same page (so to speak).

Exam...

Table of contents

• Cover
• Titlepage
• Copyright
• Dedication
• Credits
• Preface
• Introduction
• Part I: Foundations of LAN Switches
• Part II: Advanced LAN Switch Concepts
• Appendix
• References
• Glossary
• Index