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LSA 4 – ASBR Summary LSA & LSA 5 (External LSA)

LSA 4 as the name suggests, is helping LSA of LSA 5 (Summary LSA). How does it help LSA 5, because as we all know LSA 5 can travel between areas? Well, Answer to this questions lies as below :-
1. LSA 5 (Summary LSA) – It tell about the prefixes which are outside of OSPF domain. It is always generated by ASBR Routers. It contains the Prefix/Cost and Cost Type. But it does not have any Info that it is the ASBR. A router is ASBR info is shared in LSA 1 (With E bit on) but a LSA 1 can not cross a area. So how does other areas knows that we have a ASBR in OSPF domain?
2. LSA 4 ( ASBR Summary LSA) – This LSA is generated by ABR (remember it for interviews). It tells other areas about the presence of ASBR and Cost to reach the ASBR from the ABR. Yes thats it, we have covered both LSA 4 and LSA 5 here. 
But lets dig down using lab.
Suppose in above topology, we did following :-
1. Made a Loopback 3 on R3 and redistributed it in OSPF domain.

R3(config)#int lo3
R3(config-if)#ip add 33.33.33.33 255.255.255.255
R3(config-router)#redistribute connected

 What will happen now. Well to answers this we will take a step by step process :-
1. As soon as we redistributed the Loopback 3 on R3, it will become ASBR. But how does it tell the R2 (area 1) – Using LSA 1 with E bit on (Wireshark capture is as below)

3. Now R2 knows that R3 is ASBR, It will generate LSA 4 for area 0 since it is ABR.
4. Now all routers knows that there is a ASBR. ASBR will share outside prefix info in LSA 5 which will travel in Area 1 and Are 0.

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