Type 1 LSAs, or Router LSAs, are originated by every OSPF-enabled router within an area. They describe the state and cost of the router’s directly connected links (interfaces) and are used for intra-area route calculations. These LSAs are confined to the area in which they are generated and do not propagate beyond it.
Type 2 LSAs, or Network LSAs, are generated by the Designated Router (DR) on multi-access networks, such as Ethernet. They contain information about all the routers in the segment and the subnet mask. Each multi-access segment has one Type 2 LSA, assuming a DR is present, which helps in intra-area routing within that specific network segment.
Type 3 and Type 4 LSAs, collectively called Summary LSAs, are generated by Area Border Routers (ABRs). Type 3 LSAs describe network prefixes in one area and summarize them to other areas. Type 4 LSAs specifically describe the location of Autonomous System Boundary Routers (ASBRs), allowing routers in other areas to reach external destinations. These LSAs propagate across OSPF areas to share inter-area routing information.
Type 5 LSAs, or External LSAs, are originated by ASBRs to represent external networks that are redistributed into OSPF from other protocols or sources. They propagate throughout the OSPF domain, except in stub or totally stubby areas, which restrict external route advertisement. Type 5 LSAs are essential for sharing information about networks outside the OSPF domain.
Type 7 LSAs, or NSSA (Not-So-Stubby Area) External LSAs, are unique to NSSAs. These are originated by ASBRs within an NSSA to advertise external routes. Unlike Type 5 LSAs, Type 7 LSAs are confined to the NSSA and are translated into Type 5 LSAs by the ABR when needed, for propagation outside the NSSA. This functionality allows external connectivity while preserving the benefits of a stub area.
