9.3
Issues of Redundancy, Symmetry and Load Balancing
9.3.1 Introduction to 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 and returns 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 provide solutions for all of these requirements. On the Internet, multiple providers can control and manipulate traffic that transits any AS -- any provider between the source and destination of a packet can affect its path.

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 network. While you might not see your exact network configuration in these examples, the general issues and implementation methods they raise provide a model for your routing analysis and design. Before examining specific network scenarios, it is necessary to establish some basic concepts and definitions concerning redundancy.