![]() ![]() |
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
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.
|