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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, premises
wiring, the provider's physical layer, the provider's switching equipment,
and numerous administrators -- each with 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. If a customer has multiple
connections -- for example, 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 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 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, 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.
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