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Redundancy, symmetry, and load balancing are
crucial issues facing anyone implementing high-throughput connections to
the Internet. ISPs and corporations connected to ISPs require adequate
control over how traffic enters and exits their respective ASs.
Redundancy is achieved by providing
multiple alternate paths for the traffic, usually by having multiple
connections to one or more ASs. Symmetry means having traffic that leaves
the AS from a certain exit point and returns through the same point. Load
balancing is the capability to divide traffic optimally over multiple
links. Putting these three requirements together, you can imagine how
challenging it is to achieve an optimal routing solution.
No single switch exists that you can turn
on to provide solutions for all of these requirements. On the Internet,
multiple providers can control and manipulate traffic that transits any AS
-- any provider between the source and destination of a packet can affect
its path.
The general design problem of how best to
implement redundancy, symmetry, and load balancing is common to every
network. The specific answer, however, depends on the needs and
configuration of each network. While you might not see your exact network
configuration in these examples, the general issues and implementation
methods they raise provide a model for your routing analysis and design.
Before examining specific network scenarios, it is necessary to establish
some basic concepts and definitions concerning redundancy.
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