Type 4 ASBR-Summary LSA : this is needed because Type 5 External LSAs are flooded to all areas and the detailed next-hop information may not be available in those other areas. This is solved by an Area Border Router flooding the information for the router (i.e. the Autonomous System Boundary Router) where the type 5 originated. The link-state ID is the router ID of the described ASBR for type 4 LSAs.
Hi Alok. I'll try to explain it as briefly as possible. So please bear with me.
LSA Type 4 is described as a ASBR Summary LSA.
It is generated by an ABR. It is flooded from area 0 into a non-transit area and vice versa. It represents the ABR's reachability to ASBRs in other areas where in it includes cost but hides the ABR's actual path to the destination.
As an example, refer to the following topology
R5 does redistribution which makes it an ASBR. So, when R4 looks at a calculation to reach the EIGRP routes, R5 says "to get to EIGRP, use me." As long as R4 can compute its shortest path to reach R5, it will just trust whatever R5 says to reach the external (EIGRP) domain.
However, from an inter-area point of view, R5 will be advertising the EIGRP routes to R3 which is an ABR. When R3 reports it to other routers in Area 0, it's going to say "EIGRP routes are reachable via R5 and R5 is reachable via myself". So if R1 were to reach the EIGRP routes, the following occurs
- R1 looks at the SPF that he ran to get to R3.
- R3 is advertising that it has reachability to R5 because it already ran SPF to get to R5.
- R5 says it knows how to get to the final destination
Add all of these three, and it means that R1 is able to get to the EIGRP domain. However, R1 does not need to know the "end to end details" to move from Area 0 to Area 1, and R3 does not need to know the details to move from Area 1 to the external domain.
So this is what the LSA type 4 is describing.
It is R3 telling the devices in its area
"I know how to reach the device that is doing the redistribution (R5). So I'll tell you the routes that have been redistributed. But when you try to reach them, just send the traffic to me, I'll send it to R5, and he will send it to the final destination which is the external domain."
The idea is that when OSPF is doing the calculations, the less amount of time it has to run SPF, the faster the convergence.
For a complete picture and full understanding of Type-4 LSA, I believe you would have to include another area (let's say Area 2) connected to R1 - so make R1 an ABR.
In this situation, R1 (as ABR) will generate its own Type-4 LSA that will be used by all routers in Area-2 to learn about the external routes.