If an AS has multiple BGP speakers, it could be used as a transit service for other ASs. As you see below, AS200 is a transit AS for AS100 and AS300.
It is necessary to ensure reachability for networks within an AS before sending the information to external ASs. This is done by a combination of internal BGP (iBGP) peering between routers inside an AS and by redistributing BGP information to Internal Gateway Protocols (IGPs) running in the AS.
As far as this paper is concerned, when BGP is running between routers belonging to two different ASs, we call this exterior BGP (eBGP). When BGP is running between routers in the same AS, we call this iBGP.
It is necessary to ensure reachability for networks within an AS before sending the information to external ASs. This is done by a combination of internal BGP (iBGP) peering between routers inside an AS and by redistributing BGP information to Internal Gateway Protocols (IGPs) running in the AS.
As far as this paper is concerned, when BGP is running between routers belonging to two different ASs, we call this exterior BGP (eBGP). When BGP is running between routers in the same AS, we call this iBGP.
