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| 9.9 |  | Redundancy, Symmetry, and Load Balancing |
| 9.9.1 |  | Issues with redundancy, symmetry, and load balancing |

Redundancy, symmetry, and load balancing are crucial needs facing anyone in the process of implementing a high throughput connection to the Internet. ISPs and their large customers require adequate control over how traffic enters and exits their respective autonomous systems.

Redundancy is achieved by providing multiple alternate paths for the traffic. This occurs by having multiple connections to one or more autonomous systems. Symmetry exists if traffic leaves the AS from a certain exit point and returns through the same point. Load balancing, as has been seen, results in the division of traffic optimally over multiple links. Together, these three requirements can be difficult to meet in a BGP environment because 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 solution, however, depends on the needs and unique configuration of an individual network. The following sections examine the general design issues of redundancy, symmetry, load balancing, as well as configuration models for successful implementation.