9.4
Various Multihomed Connections to a Single Provider and Multiple Providers
9.4.3 Multihomed scenarios with multiple providers
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.