Confederations

Another method to reduce the iBGP mesh within an AS is BGP confederations. With confederations, the AS is divided into smaller, private autonomous systems, and the whole group is assigned a confederation ID. The private AS numbers or identifiers are not advertised to the Internet but are contained within the iBGP networks. The routers within each private AS are configured with the full iBGP mesh. Each private AS is configured with eBGP to communicate with other semiautonomous systems in the confederation. External autonomous systems see only the AS number of the confederation, and this number is configured with the BGP confederation identifier.

In Figure 12-5, a confederation divides the AS into two. Figure 12-5 BGP Confederations

Full iBGP Mesh with No Confederation to AS 2

Full iBGP Mesh with No Confederation to AS 2

iBGP Connections Reduced with Confederation

To AS 2

iBGP Connections Reduced with Confederation

To AS 2

To AS 4

To AS 5

To AS 4

To AS 5

Routers A, B, and G are configured for eBGP between the private autonomous systems. You configure these routers with the bgp confederation identifier command. The confederation identifier number is the same for all routers in the network. You use the bgp confederation peers command to identify the AS number of other private autonomous systems in the confederation. Because Routers A and G are in AS 10, the peer confederation to Router B is AS 20. Router B is in AS 20, and its peer confederation to Routers A and G is AS 10. Routers C and D are part of AS 10 and peer with each other and with Routers A and G. Routers E and F are part of AS 20 and peer with each other and with Router B.

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