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. Therefore some routes are

missing on R1 to advertise to eBGP peers.

Good reference:

https://www.ciscolive.com/c/dam/r/ciscolive/emea/docs/2015/pdf/TECRST-2310.pdf

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.