Content

| 9.9 |  | Redundancy, Symmetry, and Load Balancing |
| 9.9.2 |  | Redundancy, symmetry, and load balancing |

Although corporations and ISPs prefer uninterrupted connectivity, disruptions still occur for a variety of reasons. Connectivity is not the responsibility of one entity. A connection to the Internet may involve a router, a CSU/DSU, premise wiring, the provider's physical layer, switching equipment, and numerous administrators. Each of these elements has influence over different parts of the connection. At any time, end-to-end connectivity can be jeopardized by human error, software errors, physical errors, or adverse unforeseen conditions, such as bad weather or power outages.

For these reasons, redundancy is generally desirable, but finding the optimal balance between redundancy and symmetry is crucial. Redundancy and symmetry can be conflicting design goals. The more redundant links a network has, the more unpredictable the entrance and exit points for a packet become. A customer may have multiple connections, for example, one to a point of presence (POP) in San Francisco and another to a POP in New York. Therefore, 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 geographic 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 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 by way of the geographically adjacent POP.

Because redundancy refers to the existence of alternate routes to and from a network, additional routing information needs to be kept in the routing tables. To avoid this 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.