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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.
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