OSPF router will not advertise Type 3 LSA from non-backbone area into backbone area. You have advertised loopback address (MPLS LDP ID) in area 0.
Assume R3 advertise this as Type 1 LSA in area 0 to R4. R4 originate a Type 3 LSa and advertise to R2 via area 1. Since this is a Type 3 LSA received from area 1 (non backbone), it will not advertise back to R1. So R1 doesnt have reachability to loopback of R3.
The LDP hello exchanged between R1 and R3 will have the MPLS LDP ID which is not reachable an so the session is established.
What you have is a disjointed scenario and you may need virtual-links or a redesign.
Thanks Nagendra for your revert. But when i try it on only 2 routers i.e. i have only RTR-1 and RTR-3 in my topology then this works i.e. Loopback IP's of both the router's are part of OSPF Backbone Area but the link on which these 2 routers are connected is part of OSPF Area1 and LDP neighborship comes up betweeen them.
Also, would request, if you could propose the best solution for the topology that i have shared earlier. I need to keep the links between RTR1-RTR3 and RTR2-RTR4 in a separate area other than Area 0.
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