Hello @rookie R ,
>> Why the type 5 LSA is the only LSA flooded throughout the entire AS? Does the type 3 LSA didn't flood throughout the whole AS?
LSA type 3 have actually a single area scope , but other ABR routers can re-generate them for flooding to another area.
I had your same doubt at the beginning of my studies and I did a lab test and then examined the results in the OSPF database.
If we have AREA 1 ------- R1 ABR(0,1) ----- area 0 ----------- R2 ABR (0,2) ---- area 2 ---- R3
what happens is the following:
R1 as ABR between area 0 and area 1 can create one or more LSA type 3 that describe prefixes in area 1.
For example 10.1.1.1.0/24. This LSA is flooded in area 0 and reach all routers in area 0.
R2 is an ABR between area 0 and area 2. Area 2 is a standard area so inter area routing information needs to be propagated to area 2.
Here comes the interesting part the original LSA originated by R1 is not propagated into area 2 directly, R2 creates a new LSA type 3 describing the prefix 10.1.1.0/24 and originated by R2 with the purpose to send it into area 2.
R3 a router internal to area 2 will receive the LSA but it will never know that R1 is the true originator. R3 will look at how to reach R2 and it is enough to make inter area routing to work correctly.
This can be seen by using show ip ospf database summary 10.1.1.0 on R2 or on R3.
This is the reason wly only LSA type 5 have scope global in the OSPF domain.
Hope to help
Giuseppe
