9.4
Various Multihomed Connections to a Single Provider and Multiple Providers
9.4.1
Multihomed connections with a single provider
Although running BGP when multihomed to the same ISP is not necessary, it is generally recommended.

Scenario: A Default Only, One Primary, and One Backup Link

In the scenario illustrated in Figure , the customer wants only default routes toward the provider, and does not want to accept partial or full Internet routes. The customer wants to use one link as the primary traffic conduit and the other as a backup in case the primary link goes down. (If there were more than two connections to the provider, the customer could set up multiple defaults with varying preference levels.)

Customer's Outbound Traffic

Where a single router is used to connect to the provider in multiple locations, multiple static default routes with different administrative distances can be used. The default with the lower distance will be the primary.

If the customer's network administrator would rather learn the default route (or few aggregate routes) dynamically from the provider, they can ask the ISP to filter the routes advertised to the minimum possible. Local preference can be used to prefer one default to the other.

Assume in Figure that the default to New York is more preferred than the default to San Francisco. In normal operations, the customer will use the New York link as the primary link and the San Francisco link as a backup.

For outbound traffic, load balancing is not an option because all traffic is sent over the primary line, and the secondary is kept as backup. Absence of load balancing is offset by the fact that the customer's router requires less memory and processing power.

Customer's Inbound Traffic

The customer can advertise its networks to the provider via BGP so the provider will have two paths to reach destinations within the customer's network. Which path it chooses affects the customer's inbound traffic. Usually, the provider's default behavior (assuming that all attributes are the same) is for traffic to flow back to the customer's AS along the path which is the shortest path between it's entry point into the provider's network and the exit point to the customer's network.

While all the previous factors are outside the customer's control, customers who want to override these influences and control incoming traffic via one path or the other can do so by advertising their routes with different metrics. The provider will direct its traffic toward the customer based on the metric value. In Figure , the customer is advertising its routes with a metric of 50 toward New York and a metric of 100 toward San Francisco. As such, traffic toward the customer will take the New York route.