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A customer connected to multiple providers is
considered to be multihomed to different providers. Redundancy and
geographical restrictions are strong motivations for multihoming.
In this case, the customer can follow
defaults toward the provider. One link will be used as primary, and the
second link as backup. The figure illustrates a relevant situation.
The customer can either configure static
default routes to the two providers or can dynamically learn a default
route from both providers. For static default routes, the administrative
distance can be used to prefer one default over the other, while one
dynamically learned routes can be preferred using the local preference.
One good method of pointing defaults to
both providers is to accept the same network from both providers and then
statically configure a default towards that network. The customer can
manipulate the local preference of this prefix as it is learned from both
providers to choose one link over the other. If one default goes away
because of a link failure toward one provider, the other default will take
its place.
Configuration Example: Multihoming to
Two Providers
In the Figure to the left, the customer is pointing
the default toward the prefix 192.213.0.0/16 it is receiving from both
providers and setting the local preference on the New York link to be
higher (200). The New York link will be the primary link, and the San
Francisco link will be the backup.
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