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You cannot configure multiple vrfs on an ospf instance  you need a separate instance for each VRF.

If it was possible to configure multiple VRF's on an ospf instance,  using separate instances would not be wrong.

P2#sh run | section router
P2#
P2#
P2#conf t
Enter configuration commands, one per line.  End with CNTL/Z.
P2(config)#router ospf 10 vrf Site-A
P2(config-router)#
*Feb 24 07:16:28.883: %OSPF-4-NORTRID: OSPF process 10 failed to allocate unique router-id and cannot start
P2(config-router)#router-id 10.10.10.10
P2(config-router)#exit
P2(config)#
P2(config)#router ospf 10 vrf SharedSites
%VRF specified does not match existing router
P2(config)#
P2(config)#
P2(config)#
P2(config)#router ospf 20 vrf SharedSites
P2(config-router)#
*Feb 24 07:17:31.696: %OSPF-4-NORTRID: OSPF process 20 failed to allocate unique router-id and cannot start
P2(config-router)#router-id 20.20.20.20
P2(config-router)#exit
P2(config)#
P2(config)#

The question has been corrected. Thank you!

(02-24-2024, 03:52 PM)hannan Wrote: [ -> ]The question has been corrected. Thank you!

I do not believe it is best to place Site B and C in a separate area. Areas are not mentioned in this question and I believe the options to use separate areas are intentional misdirection about the function of VRFs.  

Your logic for Site-B and Site-C could easily be applied to Site-A.  But there is no option to place Site-A outside of the backbone area.

Best practice is to start with Area 0.  We do not know how large either customer is.  It is unwise to assume they are large enough to require multi-area OSPF.  

It is good network design practice to start with area 0 and then expand into other areas later on.
https://www.cisco.com/c/en/us/support/do...1793211861

The answer has been corrected. Thank you!