Selected Answer: B It looks like: "Rollover in progress, 1 neighbor(2) using old key(s): key id 1 algorithm MD5" This indicates the neighbor is using MD5 authentication. To fix it, set the MD5 authentication config back: CSR103(config)#interface fastEthernet 0/0 CSR103(config-if)#ip ospf message-digest-key 1 md5 cisco CSR103(config-if)#ip ospf authentication message-digest The above will automatically "remove" the wrongly configured "ip ospf authentication key-chain ospf"

Agreed.

C is currently configured on the router, but with a different hma-sha-256: show ip ospf interface e0/0 Cryptographic authentication enabled Sending SA: Key 1, Algorithm HMAC-SHA-1 - key chain ospf

I agree

Selected Answer: B B and D are both correct. B is the interface-based way to configure the authentication, and D is the area-based way to do it. both should work.

Selected Answer: C c looks right based on the show output