When implementing MPLS Layer 3 VPNs with customers running OSPF as the CE-PE routing protocol, which situation will require a sham link to be implemented in the MPLS backbone?

When implementing MPLS Layer 3 VPNs with customers running OSPF as the CE-PE routing protocol, which situation will require a sham link to be implemented in the MPLS backbone?
A . to connect customer sites in different OSPF areas
B . to connect customer sites in the same OSPF area
C . to prevent OSPF routing loops when a customer site has redundant CE-PE connections
D . if there is a backdoor link between the CE routers, to ensure that the backdoor link is used only to back up the primary connection through the MPLS VPN

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Answer: D