Introduction

Today’s Service Provider (SP) marketplace is a highly competitive environment in which it is important to make the most of the assets you have. The halcyon days of the late 1990s, when providers had vast amounts of money to spend on equipment and transmission services and spent it at a prodigious rate, are long gone. It is essential that network architects today can design IP and MPLS networks, which are flexible enough to expand and, possibly more importantly, can cope with new and profitable services being added to the network.

WHAT IS THIS BOOK ALL ABOUT?

As a network engineer working and learning my job in the mid-1990s I often wished I had a book that would help me understand the decisions being made by our design and development team. I wanted to join that team and, if I was to design scalable networks for my employer, I needed to have that understanding. Unfortunately, no such book existed. There were a number of books describing the operation and behaviour of various IP routing protocols (useful resources in themselves) but none which explained the principles of how to build and operate large networks. I was extremely fortunate to be able to learn from other more senior members of the various engineering groups and this book is the amalgamation of what I was taught and what I have learned through self-study and painful experience.

In this book I will examine the architectural and design principles that can be applied to designing and building scalable IP and MPLS networks. Each chapter in this book presents a particular aspect of the overall process of designing a scalable IP network and provides examples of configurations using both Juniper Networks JUNOS software and Cisco Systems IOS software. I have chosen these two vendors as theirs are by far the most widely used devices in the Internet today.

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This book will not provide in-depth descriptions of the protocols associated with IP routing and switching. For those seeking an in-depth treatment of those protocols, there are a number of other books that provide an excellent reference (e.g. Routing TCP/IP, Volume 1 by Jeff Doyle, Routing TCP/IP, Volume 2 by Jeff Doyle and Jennifer Carroll, and Complete Reference: Juniper Networks Routers by Jeff Doyle and Matt Kolon).1

There will be detailed discussion, where necessary, to explain the way particular features and protocol behaviours impact the scalability of a network.

WHO IS THIS BOOK FOR?

This book is intended to provide a guide to those designing IP networks. It will provide both a guide and a reference to network architects and engineers. In addition to those designing networks, it will also be useful to network operations staff who want to understand some of the principles being applied in the architecture and design of their network.

It is also hoped that this book will provide a valuable resource for people studying for the CCIE and JNCIE exams from Cisco Systems and Juniper Networks, respectively. While design does not actually form part of those exams, a solid understanding of the design process can only be beneficial to candidates. Also, while not referring directly to any other vendors’ software or hardware, the principles described herein should be applicable to designing or building a network using any vendor’s equipment.

While this book focuses on processes and mechanisms with respect to the design of service provider networks, much of the material in this book can be applied to the design of any IP network, irrespective of whether it is a service provider’s network or an enterprise’s network.

It is assumed that the reader is familiar with the basic principles of IP networking and the protocols associated with IP routing and switching.

SCALING: GETTING BIGGER AND DOING MORE

Scaling means different things to different people. Most will agree that it means getting bigger. Getting bigger is certainly an extremely important issue but it is important that in order to grow, it is not necessary to entirely redesign the architecture of your network. The initial design of your network will dictate how easily you can make subsequent changes.

1 Jeff Doyle Routing TCP/IP, Volume 1, Cisco Press, ISBN 1578500418. Jeff Doyle and Jennifer DeHaven Carroll, Routing TCP/IP, Volume 2, Cisco Press, ISBN 1578700892. Jeff Doyle and Matt Kolon (eds) Complete Reference: Juniper Networks Routers, Osborne McGraw-Hill, ISBN 0072194812.

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In addition, it is important to recognize that scaling does not just mean building bigger networks but also means being able to offer a wider range of services to your customers using your existing assets. In today’s marketplace, it is no longer sufficient to provide a network that provides simple IP access. Customers are demanding new and different services (e.g. VPNs, CoS/QoS-backed SLAs, etc.). The ability to add new services to your network will be pivotal to your potential to grow your business and maintain (or move into) profitability.

DESIGNING A NETWORK FROM SCRATCH

Few of us are lucky enough to have the luxury of designing a network from scratch. For those of us who are, there are opportunities that are not available when trying to enlarge or extend the functionality of an existing network. In a new network it is possible, within financial constraints, ‘to do the right thing’. For example, it is possible to create efficient IP addressing and efficient aggregation schemes. This can significantly reduce the size of the routing tables in all your routers. In hierarchical routing models in which the core routers see all routes, this is particularly important. Reducing the size of your routing tables has a positive impact not only on the memory required to maintain them but also on the processing power required to calculate routing updates.

SCALING AN EXISTING NETWORK

In the previous section, it was pointed out how much easier it is to start from scratch. However, that does not mean that it is not possible to achieve great improvements in scalability in an existing network. When scaling an existing network, you will have to deal with decisions you may have made earlier and, often more significantly, decisions made by your colleagues and predecessors. It is sometimes possible to reverse decisions but, more often than not, you will have to work within the constraints they impose.

FUNCTIONAL ELEMENTS OF SP NETWORKS

As mentioned earlier, this book is intended to be of value to all network engineers, irrespective of whether they work for a service provider (SP) or a company. However, the elements of the larger SP networks can provide a useful basis with which to demonstrate the design principles. Some, although not all, SPs have networks that are excellent examples of scalability. SP networks have, over the past decade or so, scaled at a phenomenal rate. Every possible mistake has been made at some time by one or more

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	AS1000
AS101 AS202	AS303

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ADM

AS5555

AS101

AS505

AS606

AS2000

Figure I.1 Example network used in this book

SPs and every possible workaround to almost every possible problem has been tried on someone’s network.

Throughout this book, we will be providing a number of examples. These will demonstrate aspects of scalability across the various functional elements of service provider networks. In order to do this, we will be using a single consistent network or autonomous system. This is shown in Figure I.1.

TERMS USED IN THIS BOOK

Various terms are used throughout this book to describe the different functional elements of SP networks. The terms used throughout this book are described below.

•Core (Provider/P). A core switch/router is a device that connects only to other devices in the same network. It provides high speed, large capacity connectivity between access devices. Traditionally, core devices are relatively ‘dumb’ and do not provide much functionality beyond forwarding packets really fast. Modern core devices are able to support more features without any detrimental effects on performance.

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Some SPs also use a further layer of distribution devices, perhaps to aggregate traffic from a regional network into the core or to aggregate traffic from particular types of access devices (e.g. CMTS) into the core.

•Access (Provider Edge/PE). An access or edge switch/router is a device that provides connectivity between customer edge (CE) router on customer networks and the SP core network.

•Fixed access. Fixed access refers to all access provided over fixed transport media. This includes leased lines (E1/T1, E3/DS-3, STM-x/OCy), metro Ethernet, fiber to the home/curb, etc.

•Broadband (Cable/DSL). Technically, this is a subsection of the previous section since almost all broadband services are currently provided over fixed lines (cable or telephone lines). There are, however, some wireless DSL services. However, there are particular aspects of these access methods, which require specific consideration.

•Wireless. This covers all forms of non-fixed access (GPRS/UMTS, WLAN, BFWA, etc). Wireless access presents a number of specific problems associated with security, capacity and reliability.

•Border (Provider Edge/PE). This is a switch/router that is somewhat like an access device. However, it provides access to other provider networks rather than to customer networks. This is a small but incredibly important difference and has a major impact on the configuration requirements.

•Peer networks. Peer switch/routers are border devices that connect your network to peer networks. Peer networks are other provider networks with which you want to exchange routing information while sharing the cost of the connection. No money normally changes hands between the operators of networks with a peering relationship.

•Transit. Transit switch/routers are border devices that connect your network to your upstream transit provider. Transit providers are other provider networks, which provide connectivity to the Internet or, in some cases, a particular region (e.g. Europe, America, Asia). This is a customer/provider relationship and the transit provider will charge a fee for this service.

•Customer Edge (CE). This is a switch/router that provides connectivity between a customer network and the PE on a SP network. In some respects, the Transit PE described above could be viewed as a CE device. As far as the transit provider PE is concerned, that router is a CE.

ORGANIZATION OF THIS BOOK

This book is divided into a number of chapters, each of which deals with a particular aspect of designing an IP/MPLS network.

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CHAPTER 1 HARDWARE DESIGN

In this chapter, I will take a look at some of the different choices made by hardware vendors in the design of their equipment and how each of their choices might impact the scalability of your network. It might seem at first that this is entirely out of your hands. However, an understanding of the architecture of vendors’ equipment can be exceptionally useful in enabling you to scale your IP network.

CHAPTER 2 TRANSPORT MEDIA

This chapter looks at the various transport media available and evaluates the impact on scalability of each of them. Different media have significantly differing levels of overhead just in the transportation of the packets. They also impose different loads on the network in respect of maintaining the link. It is also important to note that the routing protocols you choose and their behaviours may influence your chosen transport media, and vice versa. Routing protocols are discussed in Chapter 5.

CHAPTER 3 ROUTER AND NETWORK MANAGEMENT

This is an area which is often overlooked but can dramatically affect the administrative effort required to operate your network as it grows. Having the tools to understand and modify the behaviour of your network is essential. The greater your understanding, the more accurate your ability to modify your network behaviour.

CHAPTER 4 NETWORK SECURITY

Security is clearly a significant issue for all networks, large or small. As your network grows, the security issues become bigger. It is also an unfortunate fact of life that the frequency, variety and impact of attacks have grown significantly over the past few years. The details of these attacks are often made public and implementations published on the Internet. This makes it easy for anyone with the inclination to (effectively anonymously) mount an attack on your network from hundreds or even thousands of globally distributed sources. The proliferation of ‘always on’ connections to home users has provided these attackers with millions of potential targets and sources.

If you have a consistent and carefully prepared security plan, security will not hinder the scalability of your network. Without such a plan, you will likely spend a lot of time dealing with attempts to breach the security of your network and that of your customers. This time costs your company money and could be more profitably spent implementing other services.

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CHAPTER 5 ROUTING PROTOCOLS

This chapter covers all the main unicast routing protocols used in SP networks today. The scalability of your network is fundamentally reliant upon the scalability of your chosen routing protocols and, equally importantly, on the way in which you configure them. No matter how excellent the inherent scalability of the routing protocols you use, if you configure them poorly, your network will not scale, in any sense of the word.

CHAPTER 6 ROUTING POLICY

Routing policy is the technical means by which you implement your business policies with respect to your customers, peers and transit providers. As with the routing protocols themselves, it is critical that you develop effective routing policy to assist you in scaling your network.

CHAPTER 7 MULTIPROTOCOL LABEL SWITCHING (MPLS)

Multiprotocol label switching was initially devised as a means of improving the speed of IP forwarding (actually by doing a lookup on a fixed length label rather than a variable length IP prefix) in software-based routers. As vendors built hardware (ASIC) based routers, it became clear that the lookup performance of a variable length IP prefix was equal to that of a fixed length MPLS label. No longer needed for improvements in forwarding performance, the traffic engineering capabilities of MPLS became the focus. Traffic engineering allows network operators to make more effective and efficient use of their transmission assets.

CHAPTER 8 VIRTUAL PRIVATE NETWORKS (VPNs)

One of the major ‘fashionable’ services being offered to customers is IP-based Virtual Private Networks. It is suggested that VPNs can save vast amounts of money for enterprises when compared to the cost of private lines or even an ATM or Frame Relay-based VPN. This may indeed be the case for the enterprise but if the service is not scalable, then it is almost certain to be unprofitable and unmanageable for the service provider.

CHAPTER 9 CLASS OF SERVICE AND QUALITY OF SERVICE

The issue of CoS/QoS is sometimes closely associated with VPNs but can be applied to all network traffic. This association seems inevitable given that IP VPNs are often presented as an alternative to ATM VPNs with all of their inherent QoS functionality.

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INTRODUCTION

Adding CoS/QoS to an IP/MPLS network is a significant undertaking. It is important to understand what customers want and it is important to strike a balance between the perfect service as perceived by customers and a realistic, scalable service from the point of view of the service provider.

CoS/QoS provides the mechanism whereby SPs can offer differentiated services for which they can charge a premium. As the number of services running over a single infrastructure increases, so the importance of CoS/QoS increases. It is essential that best-effort traffic generated by low-paying customers must not overwhelm priority traffic generated by premium customers.

CHAPTER 10 MULTICAST

Multicast design can be a difficult thing to get right. One of the goals of multicast is to make the transmission of identical information to multiple receivers much more scalable. However, if used in the wrong circumstances or poorly implemented in your network, it can be detrimental to the overall scalability of all aspects of your network operations.

CHAPTER 11 IPv6

IPv6 is the latest version of the Internet Protocol. It has a number of enhancements over IPv4, which include improved security, improved mobility and vastly extended address space. This chapter will cover the differences between IPv4 and IPv6 including the modifications to the IP frame format, routing protocols and control protocols. It will also look at how the requirements imposed by IPv6 in the future can impact your network design decisions today.

CHAPTER 12 COMPLETE EXAMPLE CONFIGURATION FILES (IOS AND JUNOS SOFTWARE)

This chapter will provide complete configuration examples using both Cisco Systems IOS and Juniper Networks JUNOS software. These configurations help to combine the elements of this book and present them in a real-world format that displays each of the major elements of an SP network.