Wouldn't the following be an alternative solution to aggregation component of this task on R2-
router bgp 100
network 187.15.235.0 mask 255.255.255.0
aggregate-address 187.15.0.0 255.255.0.0 suppress-map RM_NO_SUPPRESS_235
ip prefix-list PL_PERMIT_235 seq 5 permit 187.15.235.0/24
route-map RM_NO_SUPPRESS_235 deny 10
match ip address prefix-list PL_PERMIT_235
route-map RM_NO_SUPPRESS_235 permit 1000
The result -
Rack15R2#show ip bgp neighbors 204.12.15.254 advertised-routes | i 0.0.0.0
*> 187.15.0.0 0.0.0.0 32768 i
*> 187.15.235.0/24 0.0.0.0 0 32768 i