OSPF Virtual-link: MD5

 router ospf 10
   router-id 5.5.5.5
   area 0 authentication message-digest
   area 1 virtual-link 2.2.2.2 message-digest-key 1 md5 123456
   network 10.1.0.0 0.0.255.255 area 2
   network 172.16.0.0 0.0.255.255 area 1

• Must define the type of authentication for area 0 even though it's the far end of the link.
• It must be the same message-digest key number and the same md5 or no message/digest on both side.