I used GNS3.
From R2, we enable only auth message-digest. No keys, no md5 password.
If R2 has: Only R2(config-if)#ip ospf authentication message-digest
R1#debug ip ospf adj
*Apr 2 16:28:26.855: OSPF-1 ADJ Gi1/0: Rcv pkt from 192.168.2.15 : Mismatched Authentication Key - No message digest key 0 on interface
R1#
-------1st test:
From R2, If we match only the "md5" password but, use a different "key 2":
R2(config-if)#ip ospf message-digest-key 2 md5 cisco
R1#debug ip ospf adj
*Apr 2 16:30:38.723: OSPF-1 ADJ Gi1/0: Rcv pkt from 192.168.2.15 : Mismatched Authentication Key - No message digest key 2 on interface
*Apr 2 16:30:39.691: OSPF-1 ADJ Gi1/0: Send with youngest Key 1
R1#
----2nd test:
From R2, If we match only the "key 1" but, stil use a different md5 password:
R2(config-if)#ip ospf message-digest-key 1 md5 cisco1
R1#debug ip ospf adj
*Apr 2 16:35:14.011: OSPF-1 ADJ Gi1/0: Rcv pkt from 192.168.2.15 : Mismatched Authentication Key - Message Digest Key 1
R1#
*Apr 2 16:35:15.191: OSPF-1 ADJ Gi1/0: Send with youngest Key 1
-----3rd test:
From R2, once we match the same "Key 1" and md5 password, the adj comes up.
R2(config-if)#ip ospf message-digest-key 1 md5 cisco
R1#debug ip ospf adj
*Apr 2 16:37:04.843: OSPF-1 ADJ Gi1/0: Rcv LS UPD from 192.168.2.15 length 64 LSA count 1
*Apr 2 16:37:04.847: %OSPF-5-ADJCHG: Process 1, Nbr 192.168.2.15 on GigabitEthernet1/0 from LOADING to FULL, Loading Done
----Then, option D should be correct.
In MD5 authentication The packet carries the hash value, along with a key ID.
The key ID allows the routers to reference multiple passwords. This makes password migration easier and more secure.
Here is and example taked from Cisco debug where there is a key mismatch:
#debug ip ospf adj
00:59:43: OSPF: Send with youngest Key 1
00:59:53: OSPF: Send with youngest Key 2
!--- Informs that this router is also configured !--- for Key 2 and both routers now use Key 2
Source:
https://www.cisco.com/c/en/us/support/docs/ip/open-shortest-path-first-ospf/13697-25.html
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