Chapter 7. Redundancy, Symmetry, and Load Balancing

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 way to build 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 often a design goal of routing architecture.

• Load balancing— Dividing traffic over multiple links for optimal network perfomance

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

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

Redundancy is achieved by providing multiple alternative 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 to give 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.