In an OSPF router LSA (LS Type 1), the Link State ID is the Router ID of the originating router. The Advertising Router used to identify the sender is also the Router ID.
So does it mean that the two parameters will always be same for every type 1 LSA. Can you point a use case where it is not?
The Link State ID identifies what is being advertised by the LSA. A router LSA is describing what a router claims is directly connected to it, so the Link State ID is the Router ID of the originating router.
The Advertising Router is the Router ID of the router that originated the LSA.
With LS Type 1 LSAs, these two pieces of information happen to be identical, but that is not the case for other LS Types. An LS Type 1 LSA with different values in those two fields would be damaged and invalid.
when a number of routers are connected to thesame area, they exchange router LSA's. The result of the different LSA's each router has received is then grouped with one LSA header (which is usually the router ID) to be sent to neighbours. In the case of a an LSA Type 1, the link ID from the output from the command "show ip ospf database" is the LSA header that contains the different LSA's the originating router has. The ADV ID is the router-ID of the router that sent the LSA. in the case of a type 1 LSA, Which is a router LSA, both the Link ID and the ADV ID would be thesame however in the case of a type 2 LSA. They would be different becasuse the router that originated the LSA would be different from the Designated router that is saddled with the responsibility of forwarding LSA's to other routers in the area
