I am surprised to see that the SG does not filter the more specific networks of the aggregate towards the BB. I thought the question implied that the BB should receive only the aggregate and not the more specific prefixes. In my solution, i actually used the "summary-only" keyword on the aggregatem and I then used an unsuppress-map towards all internal prefix to allow for the propagation of the more specific prefixes towards the "internal" peers.
Anyone else interpreted this question that way ?
Cheers.