If the routes to be redistributed are subnetworks of major networks, the subnets keyword is necessary to redistribute routes into Open Shortest Path First (OSPF).
Another possible cause for routes not being redistributed is the use of filters with the redistribute configuration command. These filters do not permit the routes in question.
To resolve this issue, perform these steps:
If the route to be redistributed is a default route, then other options need to be taken into consideration.
For more information, refer to How Does OSPF Generate Default Routes?.