Useful commands:

Implementing OSPF Authentication

! R1-R4
router ospf 1
area 0 authentication message-digest

!R1
int gig 1/0
ip ospf message-digest-key 1 md5 cisco123

!R2
int gig 2/0
ip ospf message-digest-key 1 md5 cisco123



!R3
int fa 4/0
ip ospf authentication 
ip ospf authentication-key cisco123


!R4
int fa 4/1
ip ospf authentication 
ip ospf authentication-key cisco123

!R2
config t
int gig 1/0
ip ospf authentication null

@R4
int gig 2/0
ip ospf authentication null