OSPF Virtual-link: Plain authentication

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

• AREA 0 AUTHENTICATION
• The router not physically connected to area 0
• The router logically connected to area 0
• It must be done on both sides
• Use the same key on both sides
• Authentication has nothing to do with the transit area, could be none/MD5
• It's only the authentication of the area 0