BGP Route Reflectors, Originator ID and Cluster ID

In iBGP, all routers in the same AS must be fully meshed, meaning every router forms an iBGP session with every other router. This is required because iBGP by default does not advertise routes learned from one iBGP peer to another. The full mesh ensures that every router can learn all the routes.
The problem is that in a large network with many iBGP routers, a full mesh quickly becomes unmanageable. The number of sessions grows rapidly, and you could end up with hundreds of iBGP sessions. If you have 10 iBGP routers and try to build a full mesh, you would need 45 sessions. For n routers, the number of sessions is n × (n – 1) / 2. So with 10 routers, that’s 10 × 9 / 2 = 45.
This is where route reflectors come in. A route reflector reduces the need for full mesh by allowing certain routers to reflect routes to others. With this design, you only need a few sessions instead of a full mesh, making the iBGP setup much more scalable. If you have the same 10 routers, with RR, you only need 9 sessions.