When R2 & R5 are route reflectors (RRs), routes from R4 & R8 are advertised to R5 and R5 advertises to R2. But R2 would drop them as R2 is also a RR. Thereforesome routes are missing on R1 to advertise to eBGP peers. Route reflectors (RR) must be fully iBGP meshed so we cannot configure RR on both R1 and R5. We should choose routers at the center of the topology RRs, in this case R4 & R5.