Hi Neiby.
Export and imports could be quite confusing. First, think that a router should propagate only accurate routing information. This is the main point to flood information from the forwarding table instead of information from the routing protocols. A router should inform other routers only about active routing information, this avoids loops and wrong path selection. And only advertise in each protocol the active routing information for that protocol.
But OSPF is quite different to other routing protocols.
Under OSPF, you dont have to configure any export policy, because all neighbors should have the same OSPF Link State Database, i dont said route database.
OSPF is a link state protocol that uses the Djistra algorithm. This use a topological database for path or route selection. In order to avoid loops and wrong route selection, all the ospf routers under the same domain (not area) should have the same database, this is because you can't filter ospf internal routes, so the default import or export policy is a "flood all". All routers should have the same network view, diagram, topology.
This ospf database is not a route database, is a topology or "network diagram". If there is a link failure, all routers have the topology knowledge to select another path across this topology database, so dont have to wait for a route update like other routing protocols.
ospf use a information unit called lsa for information flooding, not route flooding.
Under a link failure, there is only a lsa update about the failing link, so every router know how to react to this failure and select other path if available for every destination network across that link.
This way the network dont have to flood new routing information for ¨1000¨ routes, only have to flood the updated lsa and every router compute the changes.
The others protocols, RIP, EIGRP, BGP, support route filtering, because they dont need a complete network view, and this view is not a network topology. So you could manipulate its routing information as needed, this is why these use export and import policies.
ISIS is near like OSPF, so it also uses a default flood all.
RIP export policy is reject everything, have to configure a export policy to flood routing information to other neighbors.
BGP default is flood all.
The default import for all is import everything. After import the routing information, the router have to go through a route selection process, selects from all the routing protocols the best routes, using best preferences values, longest matches, etc, and build the forwarding table.