Hi All,
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?
Best Regards!
Ashish JAIN
Hello Ajain
Take a look at this note that explains the use of Type4 more thoroughly.
https://notes.networklessons.com/ospf-type-4-lsa
I hope this has been helpful!
Laz