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
Explanation:
http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/47866-ospfdb7.html
All areas in an Open Shortest Path First (OSPF) autonomous system must be physically connected to the backbone area (Area 0). In some cases, where this is not possible, you can use a virtual link to connect to the backbone through a non-backbone area. You can also use virtual links to connect two parts of a partitioned backbone through a non-backbone area. The area through which you configure the virtual link, known as a transit area, must have full routing information. The transit area cannot be a stub area.
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Use the area virtual-link command to establish a virtual link from a remote area to the backbone area. In OSPF, all areas must be connected to a backbone area. If the connection to the backbone is lost, it can be repaired by establishing a virtual link.
You must configure both sides of a virtual link with the same area ID and the corresponding virtual link neighbor router ID.
You cannot configure a virtual link on a not-so-stubby (NSSA) area.