just wanted to understand the need of LSA#4 once again …
I have attached one simple topology and in this context, I am still not able to understand why LSA#4 would be vital here for R2 to reach OE2 routes coming from R4 …
My understanding is that R4 will share LSA#1 (along with LSA#5) with R5 and thus R5 learns the RID of R4 from LSA#1 and also learns the path to reach OE2. right?
now, R5 will propagate LSA#1, LSA#3 and LSA#5 in Area25 and thus R2 will learn the RID of R4 via LSA3 coming from R5 and from LSA#5 will learn that R4 is advertising OE2 routes … So, with these 3 LSAs R2 can still find a path to reach OE2 routes … So, where is the need for LSA#4 in this case?
My further understanding is that LSA#4 might come into requirement if R5 start performing Route summarization and then R2 won’t be able to know where is R4 RID located, as it will only receive a summary route from R5 in LSA#3 … So here need of LSA#4 will come in action and through that R5 will explicitly inform R2 that RID of ASBR is XYZ and I am the one advertising it, so consider me your next hop to reach ASBR RID …
Is this overall understanding correct? Pls correct me if my understanding is incorrect and add your comments?
Take a look at this note that explains the use of Type4 more thoroughly.
I hope this has been helpful!