Topology of AS 10:
R1 advertises BGP route 188.8.131.52 to R2 and R4. R5 learns IBGP route 184.108.40.206 from RR R2 and RR R4 in 2 different clusters. R5 chooses the best path which is reflected by R2.
R5#sh ip bgp 220.127.116.11
BGP routing table entry for 18.104.22.168/16, version 4
Paths: (2 available, best #2, table Default-IP-Routing-Table)
Not advertised to any peer
22.214.171.124 (metric 21) from 126.96.36.199 (188.8.131.52)
Origin IGP, metric 0, localpref 100, valid, internal
Originator: 184.108.40.206, Cluster list: 220.127.116.11
18.104.22.168 (metric 21) from 22.214.171.124 (126.96.36.199)
Origin IGP, metric 0, localpref 100, valid, internal, best
Originator: 188.8.131.52, Cluster list: 184.108.40.206
If I change the configuration to make R2 and R4 have same cluster ID, R2 becomes primary RR, and R4 becomes the redundant RR. Still R5 chooses the best path which is reflected by R2.
What is the advantage of the 2nd configuration of making R4 the redundant RR?
when both RR are members of the same cluster they do not exchange routes coming from route-reflector clients assuming all clients have valid iBGP sessions with both.
When RR are in different cluster they exchange NLRI of their own clients even if most or all of them are in common.
This second design pays in terms of unnecessary routing information exchanged by the RR, but provides protection from the case where a client X has lost iBGP connection to RR1 and a client Y has lost connection to RR2. With this second design this particular fault scenario is covered and services between clientX and client Y are still operational (think of an MPLS L3 VPN for example)
Hope to help