In OSPF, the topology data is stored in the form of LSAs in each router. Every router in OSPF area contains a link state database which has these link state advertisements. In ospf all the routers in one area should have the identical topology information.
This LSA is created by every router in the area to represent itself. Type 1 LSA gives the information about the RID of the router and its interface IP address which belongs to that area.
When more than two routers are connected to the same subnet, the OSPF uses the Type 2 LSA to represent this network or subnet. One router is elected as DR among these routers and that DR will generate the Network LSA.
In OSPF domain storing Type 1 and Type 2 LSAs of all the areas in a router requires a lot of memory and compute resources. So OSPF router does not store the Type 1 and Type 2 LSAs of other areas. It means that ABRs do not forward Type 1 and Type 2 LSAs from one area into another area and vice versa. As a result of this, the LSDBs for each area are smaller. ABRs share the information about one area to the other area using Type 3 summary LSA. For each subnet, a Type 3 LSA is generated by the ABR and advertised into other areas. Any router which wants to reach the subnet listed in the Type 3 LSA should reach the ABR first using their best route and then it can reach the desired subnet.
LSA Type 4 is described as an ASBR Summary LSA. It is generated by an ABR. It represents the ABR's reachability to ASBRs in other areas wherein it includes cost but hides the ABR's actual path to the destination.
In OSPF routes from non-ospf routing protocols are known as external routes. ASBR is the router where the redistribution happens and the external routes are induced into OSPF domain as Type 5 LSA(External LSA). So Type 5 LSAs are generated by ASBR for external routes injected into OSPF.
Type 7 LSA is generated by the ASBR in the NSSA area. Type 7 LSA can only exist in the NSSA area. Type 7 LSA is converted into Type 5 LSA by the NSSA area border router(ABR), which will be induced into the OSPF domain.
Type1 LSA/Router LSA
This LSA is created by every router in the area to represent itself. Type 1 LSA gives the information about the RID of the router and its interface IP address which belongs to that area.
Type 2 LSA/ Network LSA
When more than two routers are connected to the same subnet, the OSPF uses the Type 2 LSA to represent this network or subnet. One router is elected as DR among these routers and that DR will generate the Network LSA.
Type 3 LSA/Summary LSA
In OSPF domain storing Type 1 and Type 2 LSAs of all the areas in a router requires a lot of memory and compute resources. So OSPF router does not store the Type 1 and Type 2 LSAs of other areas. It means that ABRs do not forward Type 1 and Type 2 LSAs from one area into another area and vice versa. As a result of this, the LSDBs for each area are smaller. ABRs share the information about one area to the other area using Type 3 summary LSA. For each subnet, a Type 3 LSA is generated by the ABR and advertised into other areas. Any router which wants to reach the subnet listed in the Type 3 LSA should reach the ABR first using their best route and then it can reach the desired subnet.
Type 4 LSA/ASBR Summary LSA
LSA Type 4 is described as an ASBR Summary LSA. It is generated by an ABR. It represents the ABR's reachability to ASBRs in other areas wherein it includes cost but hides the ABR's actual path to the destination.
Type 5 LSA /External LSA
In OSPF routes from non-ospf routing protocols are known as external routes. ASBR is the router where the redistribution happens and the external routes are induced into OSPF domain as Type 5 LSA(External LSA). So Type 5 LSAs are generated by ASBR for external routes injected into OSPF.
Type 7 LSA /NSSA External LSA
Type 7 LSA is generated by the ASBR in the NSSA area. Type 7 LSA can only exist in the NSSA area. Type 7 LSA is converted into Type 5 LSA by the NSSA area border router(ABR), which will be induced into the OSPF domain.
