8.1 Autonomous Systems
8.1.5 When not to use BGP
In many cases, the routing policy that is implemented in an AS is consistent with the ISP's policy. 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 connecting to two ISPs, it is frequently necessary to use BGP. Some network administrators connect their enterprise to different ISPs for redundancy, load sharing, and lower tariffs at particular times during the day or night. If you have a backup link for redundancy, you can use a combination of static and default routes instead of BGP. However, if both of these connections are active at the same time, BGP is required. In addition, any time your policy requirements differ from the policy of your ISP, BGP is required.

In the figure, router A is advertising a default network into the AS through a local IGP, such as 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: In general, it is necessary to use BGP to connect to an ISP only when you have different policy requirements than the ISP.