Answer: a) it is used as a password.
Explanation:
In order to be able to exchange routing information , CISCO routers use an authentication process, so when they send a message (a HELLO message for instance) the router that receives the message confirms that the message is authentic.
There are several types of authentication in OSPF, null, plain and MD5.
The difference between them is as follows:
1) null, implies that no password is included in the packet header, it's the default mode.
2) plain: In this type of authentication, the password is sent transparently from a router to his neighbor, as a plain text which makes it vulnerable to attacks from packet inspection software, like Sniffer.
3) MD5, is a more secure mode, in which the password is not sent directly, but a hash built based on the packet contents and the password itself instead.
MD5 (Message Digest 5) is the name of the algoritm used to build the hash that receiver will compare with the sent by the sender in order to confirm authentication.
The chosen password, is placed at the end of the command, after the type of authentication (MD5 in this case), which in this case is "CISCO-123".
