9.3
Issues of Redundancy, Symmetry and Load Balancing
9.3.2 Why redundancy is desired
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 connection to the Internet involves the router, the channel service unit/data service unit (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 New York -- traffic leaving San Francisco might come back through New York. 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 when implementing routing policies.

Companies might also feel geographical pressure to implement redundancy; many contemporary companies are national, international, or multinational in nature, and their AS 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 the figure, the San Francisco office of AS1 connects to the San Francisco POP of ISP1, and the New York office connects to the New York POP of ISP2. In this environment, traffic can take a shorter path to reach a destination by traveling via the geographically adjacent POP.

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 that can be used to provide backup routes in case primary connections fail. The next section discusses the different aspects of default routing and how it can be applied to achieve simple routing scenarios.