OSPF Authentication on Cisco Routers
Contributed by abanks 
February 18, 2004 
Tagged: Cisco router
Configure OSPF authentication to prevent unauthorized routing updates.
In this example, Router A and Router B are connected with a WAN link. The WAN is addressed as 192.168.0.0/30, Router A has 10.1.1.0/24 and Router B has 10.2.2.0/24 on their respective FastEthernet interfaces.
Configure OSPF for basic routing:
Router A:router ospf 100
network 10.1.1.0 0.0.0.255 area 10.1.1.0
network 192.168.0.0 0.0.0.3 area 0.0.0.0
Router B:router ospf 100
network 10.2.2.0 0.0.0.255 area 10.2.2.0
network 192.168.0.0 0.0.0.3 area 0.0.0.0
Now, the routing table in both routers should show OSPF routes.
Configure OSPF for authentication:
Router A:interface serial 0/0
ip ospf message-digest-key 10 md5 PASSWORD
router ospf 100
area 0 authentication message-digest
Router B:interface serial 0/0
ip ospf message-digest-key 10 md5 PASSWORD
router ospf 100
area 0 authentication message-digest
Now, the two routers should be authenticating updates.