In regards to your last comment I'll post as a separate answer because modifying existing answer changes it a bit. Let me break it the following way:
First:
So clear me up on this. If say, Router A needs to form an adjacency with Router B, then it has to exchange it's LSDB summary which is contained in DBD packet.
I'm not sure if this is just your wording "exchange it's LSDB summary" or if you meant Type 3 LSA's, however let us recall that OSPF routers in the same area need to have identical LSDB, by this I mean that they will share all LSA's they know about and which are permitted as per area type (i.e Stub, Totally Stub, NSSA, Totally NSSA).
Second:
And DBD packet contains LSA headers which in turn contains LSA type(Router LSAs or Network LSAs, etc).
This process indeed takes effect with the usage of DBD packets as well as Type 3, 4 and 5 packets. let's see what each one contains:
Database description - Type 2
- OSPF Header
- Sequence Number
- LSA Header
Link-state request - Type 3
- OSPF Header
- Link-state type
- Link-State ID
- Advertising Router
Link-state update - Type 4
- OSPF Header
- Number of Advertisements
- Link-State Advertisements
Link-state acknowledgement - Type 5
- OSPF Header
- List of LSA Headers
Third:
So if A and B routers are exchanging DBD ,then isn't the LS type always Type 1 or Type 2 as 2 routers from different area cannot communicate with each other directly.
Remember that each LSA has a flooding scope, in the case of Router LSA (Type 1) and Network LSA (Type 2) they have an Area scope, this means they are not flooded across an area boundary. For this matter a Summary LSA (Type 3) will be used to represent those LSA's into other areas.
From the previous answer you can see the information that uniquely identifies the LSA Header which is carried in the DBD packet.
Forth:
And also, when we say Router A sends router LSA to Router B, does that mean only the Routers Link state info or does it send the LSA it has received from other router?
Router LSA consists of the standard LSA header plus the following below (I will purposefully omit some which don't matter in regards the concern):
- Number of links
- Link ID
- Link data
- Link type
- Metric
Strictly speaking LSA T1 has only area scope so say for example routers in A0 will all eventually receive the Router LSA, now for that information to be carried to other area like A1 you would need a T3.