In this topology, if AS 200 becomes partitioned (R10 goes down), R1 will not be able to send 1.1.1.1/32 prefix through the ISP router over to R2, because the route will have AS 200 in the path.
Therefore, if the network does become partioned, we may want to configure on R1 and R2 towards the ISP router option allowas-in with the command:
neighbor <ISP> allowas-in
