OSPF propagates routing information between areas through the use of Type 3 LSA (Link State Advertisement). When an OSPF router in a specific area receives a route update from another area, it generates a Type 3 LSA containing the summary of routes in that area. This LSA is flooded to all routers within the local area who form a database of Type 3 LSAs.
Routers in the receiving area summarize these Type 3 LSAs and flood them to other areas as Type 3 LSAs. When routers in other areas receive a Type 3 LSA, they add the routes to their routing table and generate Type 3 LSAs to flood the information on any other area.
The propagation of routing information depends on the OSPF topology and the area configuration. By default, OSPF does not allow the propagation of external routes between areas. However, certain configurations or protocols, such as OSPF Type 5 LSAs or redistribution, can allow external routes to be propagated.
Overall, OSPF propagation of routing information between areas is designed to be efficient and scalable, keeping network overhead to a minimum while ensuring optimal routing performance.
