8.1 Autonomous Systems
8.1.8
When not to use BGP to connect to an ISP
In many cases, the routing policy that will be implemented in an AS is consistent with the policy implemented by the ISP the AS will be connecting to. In these cases, it is not necessary -- or even desirable -- to use BGP to exchange routing information with the ISP. Instead, connectivity can be achieved through a combination of static routes and default routes.

When you connect to two different ISPs, it is frequently necessary to use BGP. Redundancy, load sharing, and lower tariffs at particular times of the day or night are reasons why some network administrators connect their enterprise to two different ISPs. If you have a backup link for redundancy, you can use a combination of static and default routes instead of BGP. If both of these connections are active at the same time, however, BGP is required. Also, any time your policy requirements differ from the policy of your ISP, BGP is again required.

In the Figure, router A is advertising a default network into the AS through a local IGP, such as Routing Information Protocol (RIP). A static route affords connectivity through router B to the ISP's AS. The ISP is running BGP and is recognized by other BGP routers in the Internet.

Note: It is generally only necessary to use BGP to connect to an ISP only when you have different policy requirements than the ISP.