Hello. if you have 0.0.0.0/0 and would like to leak a default route, thats a different configuration, you need a different set of checkboxes! also, if you want to leak all routes its again a different combination of checkboxes.
here is some information from the APIC online help:
When aggregation is not set, the subnets are matched exactly. For example, if 22.214.171.124/16 is the subnet, then the policy will not apply to a 126.96.36.199/24 route but it will apply only if the route is 188.8.131.52/16. However, to avoid a tedious and error prone task of defining all the subnets one by one, aggregation can be used aggregate a set of subnets into one export, import or shared routes policy. At this time, only 0/0 subnets can be aggregated. When 0/0 is specified with aggregation, all the routes are imported, exported, or shared with (leaked to) a different VRF based on the selection option below:
- Aggregate Export—Exports all routes of a subnet (0/0 subnets).
- Aggregate Import—Imports all routes of a subnet (0/0 subnets).
- Aggregate Shared Routes—When a route is learned in one VRF that needs to be leaked into another VRF, the routes can be leaked by matching the subnet exactly, or can be leaked in an aggregate way according to a subnet mask.Note: For aggregate shared routes, multiple subnet masks can be used to determine which specific route groups are leaked between VRFs (for example, 10.1.0.0/16), or 0/0 can be used to share all subnet routes across multiple VRFS.
So either you decide to advertise all routes, which require the aggregate checkboxes explained above. or you advertise a default route from OSPF in l3 out1 > ACI > l3 out2