PART 3
Effective Internet Routing Designs

You are now in a position to begin applying the attributes and functionality of BGP to practical routing problems. Chapter 6 begins this process by introducing three fundamental design criteria—redundancy, symmetry, and load balancing—that network architects frequently must implement and balance in developing their routing policies. Chapter 7 considers how to integrate BGP with interior protocols, and Chapter 8 considers how to tap BGP's potential for managing large and growing networks. Chapter 9 takes up the problem of network stability, an increasingly challenging design goal in the wake of the ever-expanding Internet. BGP includes a number of built-in functions designed to help build stability. Part 3 takes an example-oriented approach, using specific topologies and scenarios to illustrate routing design concepts and applications.

Chapter 6—Redundancy, Symmetry, and Load Balancing

Chapter 7—Controlling Routing Inside the Autonomous System

Chapter 8—Controlling Large-Scale Autonomous Systems

Chapter 9—Designing Stable Internets

This chapter covers the following key topics:

•  Redundancy
Building stability by providing alternate—default—routes in case of link failure is an important design goal of routing architecture.

•  Setting Default Routes
Configuring default routes is the fundamental method of building redundancy into network connections. When multiple default routes exist, methods of ranking them by preference are needed.

•  Symmetry
Configuring routes so that certain traffic enters and exits an AS at the same point is usually a design goal of routing architecture.

•  Specific Scenarios
Exploration is offered of several representative network designs with respect to developing redundancy, symmetry, and load balancing. Examples of attribute configuration to achieve these design goals for the different scenarios are offered.

Chapter 6
Redundancy, Symmetry, and Load Balancing

Redundancy, symmetry, and load balancing are crucial issues facing anyone implementing high-throughput connections to the Internet. ISPs and corporations connected to ISPs require adequate control over how traffic enters and exits their respective ASs.

Redundancy is achieved by providing multiple alternate paths for the traffic, usually by having multiple connections to one or more ASs. Symmetry means having traffic that leaves the AS from a certain exit point return through the same point. Load balancing is the capability to divide traffic optimally over multiple links. Putting these three requirements together, you can imagine how challenging it is to achieve an optimal routing solution.

No single switch exists that you can turn on that gives you all you need. On the Internet, multiple providers can control and manipulate traffic that transits any AS. Any provider along the way can direct the traffic. The art of balancing traffic depends on coordination between multiple entities.

The general design problem of how best to implement redundancy, symmetry, and load balancing is common to every network. The specific answer, however, depends on the needs and configuration of each particular network. This chapter considers the general design problem within the context of several specific network configurations. You might not see your exact network configuration in these examples, but the general issues and implementation methods they raise provide a model for your analysis and design of your own routing needs.

Before examining specific network scenarios, it is necessary to establish some basic concepts and definitions concerning redundancy.

Redundancy

Although corporations and providers would prefer uninterrupted connectivity, connectivity problems occur for one reason or another from time to time. Connectivity is not the responsibility of one entity. A router's connection to the Internet involves the router, the CSU/DCU, cabling, physical access line, and numerous administrators—each with influence over different parts of the connection. At any time, the connectivity can be jeopardized by human error, software errors, physical errors, or adverse unforeseen conditions (such as bad weather or power outages).

For all these reasons, redundancy is generally desirable. But finding the correct balance between redundancy and symmetry is critical. Redundancy and symmetry can be conflicting design goals—the more redundancy a network has, the more unpredictable the traffic entrance and exit points would be. If a customer has multiple connections—one to a Point Of Presence (POP) in San Francisco and another to a POP in NY—traffic leaving San Francisco might come back from NY. Adding a third connection to a POP in Dallas makes connectivity even more reliable, but it also makes traffic symmetry more challenging. These are the trade-offs that network administrators must consider in implementing routing.

Geographical Restrictions Pressure

In addition to the reliability motivation, companies might feel geographical pressure to implement redundancy. Many contemporary companies are national, international, or multinational in nature. For them, the autonomous system is a logical entity that spans different physical locations. A corporation with an AS that spans several geographical points can take service from a single provider or from different providers in different regions. In figure 6-1, the San Francisco office of AS1 connects to the San Francisco POP of ISP1, and the NY office connects to the NY POP of ISP2. In this environment, traffic can take a shorter path to reach a destination by traveling via the geographically adjacent POP.

Figure 6-1  Geographically based multihoming situation.

Because redundancy refers to the existence of alternate routes to and from a network, this translates into an additional number of routing information that needs to be kept in the routing tables. To avoid the extra routing overhead, default routing becomes an alternate practical tool. Default can provide us with backup routes in case primary connections fail. The next section attempts to define the different aspects of default routing and how it can be applied to achieve simple routing scenarios.