Which two authentication types does OSPF support? (Choose two.)
A . plaintext
B . MD5
C . HMAC
D . AES 256
E . SHA-1
F . DES
Answer: A,B
Explanation:
These are the three different types of authentication supported by OSPF + Null Authentication-This is also called Type 0 and it means no authentication information is included in the packet header. It is the default.
Plain Text Authentication–This is also called Type 1 and it uses simple clear-text passwords.
MD5 Authentication–This is also called Type 2 and it uses MD5 cryptographic passwords.
Source: http://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13697-25.html