I am in the process of designing a MPLS network. I understand that one router will become the PE, and my ABR will act as the CE. Behind CEs, there are different OSPF areas. However, these different OSPF areas have backup links connecting between them. How do I configure this backup link? Use Virtual link or Sham link or there are other ways? Please let me know. Thanks.
See....in this case , you will have to configure 2 virtual links to maintain full recahability between the routers in case of any link failures.
In case the link between PE2 and Router2 fails:-
One virtual links has to be created in the router configured in Area 2 with Transit area as Area 1 peering with the PE1.PE1 will also have a virtual link with Router2 with transit area as Area 1.
In case the link between PE1 and Router1 fails:-
One virtual link has to be created on Router1 with transit area as Area2 peering with PE2. The PE2 router will have a virtual link with transit area2 peering with Router1.
Keep the backup link on Router 1 & 2 in area 2.
This will work in your case for maintaing the redundany.