LSA 4 - Points to a destination IP , which is of a ASBR.
LSA 5 - is the external networks which the ASBR advertises, but with a next hop(Forwarding Address) only known to the ASBR. So inorder to reach those networks, the packet has to be forwarded to the ASBR. For this reason inorder to identify the ASBR Type 4 LSA is used.This is my understanding on the topic. Please correct me if I am wrong.
What you said is absolutely correct. But it did not answer my question. Let us consider the following diagram.
Area 1 - | R1 | - Area 0 - | R2 | - Area 2 - | R3 |
In the above diagram, I am redistributing routes in to area 1. R2 will generate type 4 lsa to reach R1. However, it will also generate a type 3 LSA through which R1 can be reached. My question here is - why do we need type 4 LSA once again while we have type 3 LSA to reach ASBR. ASBR address is known via type 5 LSA. Let us assume ASBR IP is 10.10.10.10. R2 propagtes type 3 LSA abt 10.10.10.0 network. Right!
You are right that it can be sometimes a bit "double information", but consider that you can also filter out Type 3 LSA on ABR's but you still need to reach external Prefixes announced by ASBR, maby then the Type 4 LSA can be interesting. OK, thats whats in my mind if i try to figure out why not using the Type 3 LSA information, maby its complete rubish what I think :-)
Got your question. The difference between the ASBR and ABR summary is that Type 3 is confined to the attached areas of the ABR. But Type4 and Type 5 is advertised throughout the OSPF Domain unless its a stub.
Dear Hope that you got the ans of your question when you needed it. I m replying because there might be people like me that still had confusion on this.
THe type 4 is generated as an insurance policy to make sure that the next hop in the type 5(i.e the ASBR) is reachable from any where in the whole OSPF domain independent of any other advertisement includeing typ-3.This is done Becouse there can be such scenarios where NLRI of the ASBR itself is not available through type 3 LSA.
For example the ASBR uses a router ID of its Lo0 interface but instead of including the IP in the network statement we use redistribution of direct connected routes to advertise this IP; making it an external type advertisement. In this situation there will be no type-3 LSA and instead the ASBR will be advertising its own router ID IP through type-5 LSA. If the ABR doesn't generate a type-4 LSA then there will be no reachability to the ASBR beyond he local area.
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