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

Explanation:

http://www.cisco.com/en/US/docs/ios/12_2t/12_2t8/feature/guide/ospfshmk.html