Routers R1 and R2, OSPF neighbors in area 0 over their Fa0/0 interfaces (respectively),
currently both successfully use OSPF MD5 authentication. The OSPF configuration includes
the area 0 authentication command under the router ospf 1 command. Which of the following
commands must have been configured on R1’s Fa0/0 interface? (Choose two.)

A.
ip ospf authentication null

B.
ip ospf authentication message-digest

C.
ip ospf authentication-key whatever-it-is

D.
ip ospf message-digest-key 1 md5 whatever-it-is

E.
ip ospf md5 1 key whatever-it-is

Explanation:
The area 0 authentication command tells R1 to use simple text password
authentication on all interfaces in area 0 unless overridden by an interface subcommand.
So, R1 must have configured the ip ospf authentication message-digest on its
Fa0/0 interface, enabling MD5 authentication insteaD. The other correct answer is the
command that correctly configures the MD5 authentication key. Of the two incorrect
answers that list an authentication key, ip ospf authentication-key defines the
clear-text key, and the other is not a valid IOS command.