BigIron RX Series Configuration Guide 75353-1001986-01Configuring confederations 26FIGURE 114 Example BGP4 confederationIn this example, four routers are configured into two sub-ASs, each containing two of the routers.The sub-ASs are members of confederation 10. Routers within a sub-AS must be fully meshed andcommunicate using IBGP. In this example, routers A and B use IBGP to communicate. Routers Cand D also use IBGP. However, the sub-ASs communicate with one another using EBGP. Forexample, router A communicates with router C using EBGP. The routers in the confederationcommunicate with other ASs using EBGP.Routers in other ASs are unaware that routers A – D are configured in a confederation. In fact,when routers in confederation 10 send traffic to routers in other ASs, the confederation ID is thesame as the AS number for the routers in the confederation. Thus, routers in other ASs see trafficfrom AS 10 and are unaware that the routers in AS 10 are subdivided into sub-ASs within aconfederation.Configuring a BGP confederationPerform the following configuration tasks on each BGP router within the confederation:• Configure the local AS number. The local AS number indicates membership in a sub-AS. AllBGP routers with the same local AS number are members of the same sub-AS. BGP routersuse the local AS number when communicating with other BGP routers within theconfederation.• Configure the confederation ID. The confederation ID is the AS number by which BGP routersoutside the confederation know the confederation. Thus, a BGP router outside theconfederation is not aware and does not care that your BGP routers are in multiple sub-ASs.BGP routers use the confederation ID when communicating with routers outside theconfederation. The confederation ID must be different from the sub-AS numbers.• Configure the list of the sub-AS numbers that are members of the confederation. All therouters within the same sub-AS use IBGP to exchange router information. Routers in differentsub-ASs within the confederation use EBGP to exchange router information.Router A Router BRouter C Router DIBGPIBGPEBGPSub-AS 64513Sub-AS 64512EBGPConfederation 10 AS 20This BGP4 router sees alltraffic from Confederation 10as traffic from AS 10.Routers outside the confederationdo not know or care that the routersare subdivided into sub-ASs within aconfederation.
