Configuring Multiple Autonomous Systems
If an Autonomous System (AS) has multiple BGP routers, the AS can serve as a transit service for other ASs. When BGP runs between routers in different ASs, it uses exterior BGP (eBGP). When BGP runs between routers in the same AS, it uses interior BGP (iBGP).
In the following diagram, AS 200 is a transit AS for AS 100 and AS 300.
Multiple Autonomous Systems Configuration
To configure multiple ASs as shown in the above diagram, configure routers RTA, RTB, and RTC as follows: