Which two OSPF network scenarios require OSPF virtual link configuration? (Choose two.)
A. to connect an OSPF non-backbone area to area 0 through another non-backbone area
B. to connect an NSSA area to an external routing domain
C. to connect two parts of a partitioned backbone area through a non-backbone area
D. to enable route leaking from Level 2 into Level 1
E. to enable route leaking from Level 1 into Level 2
F. to enable OSPF traffic engineering
