Besides RR, BGP confederation can also reduce IBGP connections in an AS. It divides an AS into several sub-ASs. Fully meshed IBGP connections are established in each sub-AS, and fully meshed EBGP connections are established between sub-ASs.
In Figure 1, there are multiple BGP routers in AS 200. To reduce the number of IBGP connections, AS 200 is divided into three sub-ASs: AS 65001, AS 65002, and AS 65003. In AS 65001, fully meshed IBGP connections are established between the three routers.
BGP speakers outside a confederation such as Router F in AS 100, do not know the existence of the sub-ASs (AS 65001, AS 65002, and AS 65003) in the confederation. The confederation ID is the AS number that is used to identify the entire confederation. For example, AS 200 in Figure 1 is the confederation ID.
The confederation needs to be configured on each router, and the router that joins the confederation must support the confederation function.
BGP speakers need to be reconfigured when a network in non-confederation mode switches to confederation mode. As a result, the logical topology changes accordingly.
On large-scale BGP networks, the RR and confederation can both be used.