BGP confederations are another alternative for avoiding a full mesh of BGP sessions inside an Autonomous System. A BGP confederation is a group of Autonomous Systems managed by a single technical administration that appear as a single AS to BGP routers outside the confederation; the single externally visible AS is called the confederation ID. Each AS in the group is called a member AS and the ASN of each member AS is visible only within the confederation. For this reason, member ASNs are often private ASNs.
Within a confederation EBGP-type sessions can be setup between BGP routers in different member AS. These confederation-EBGP sessions avoid the need for a full mesh between routers in different member ASes. Within each member AS the BGP routers must be fully-meshed with IBGP sessions or route reflectors must be used to ensure routing consistency.
In SR OS, a confederation EBGP session is formed when the ASN of the peer is different from the local ASN and the peer ASN appears as a member AS in the confederation command. The confederation command specifies the confederation ID and up to 15 member AS that are part of the confederation.
When a route is advertised to a confederation-EBGP peer the advertising router prepends its local ASN, which is its member ASN, to a confederation-specific sub-element in the AS_PATH that is created if it does not already exist. The extensions to the AS_PATH are used for loop detection but they do not influence best path selection (that is, they do not increase the AS Path length used in the BGP decision process). The MED, NEXT_HOP and LOCAL_PREF attributes in the received route are propagated unchanged by default. The ORIGINATOR_ID and CLUSTER_LIST attributes are not included in routes to confed-EBGP peers.
When a route is advertised to an EBGP peer outside the confederation the advertising router removes all member AS elements from the AS_PATH and prepends its confederation ID instead of its local/member ASN.