In the topology
Router R3 receives 1.1.1.1/32 route information from R2
This route gets next into the routing table
But what if R2 installed another protocol or source same route (same subnet and mask) to R1 loopback0 network ?
R2 starts poisoning the R1 route in its updates to R3
And the route is deleted from the RIP database and routing main table
For a RIP learnt route to be advertised downward it has to be in the routing table first