BGP Confederation peerings
When implementing the BGP routing protocol, when configuring BGP Confederations, iBGP routers that belong to the same sub-AS must connect to each other either directly, or via another iBGP router in the same sub-AS. Peerings between iBGP routers that can reach each other through other sub-ASes cannot create iBGP peerings.
Links:
https://forum.networklessons.com/t/bgp-confederation-explained/1130/63?u=lagapides