OSPF : Configure with Highest Level Security !

=> Must use md5 authentication instead of the three types available ( null, normal,md5)
i.e
router ospf 1
area 0 authentication message-digest

int f0/0
ip ospf message-digest 1 md5 CISCO

OR

int f0/0
ip ospf authentication message-digest
ip ospf message-digest 1 md5 CISCO