Cisco IOS Cookbook, 2nd Edition

Enabling OSPF Authentication

Problem

You want to authenticate your OSPF neighbor relationships to ensure that no unauthorized equipment is allowed to affect routing.

Solution

To enable OSPF MD5 authentication, you need to define the encryption key, which is essentially just a password, on an interface. And you also must enable authentication for the entire area. For the first router, you could do this as follows:

Router1#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router1(config)#interface Serial0/1
Router1(config-if)#ip ospf message-digest-key 1 md5 oreilly
Router1(config-if)#exit
Router1(config)#router ospf 55
Router1(config-router)#area 2 authentication message-digest
Router1(config-router)#exit
Router1(config)#end
Router1#

Similarly, you must enable OSPF authentication on other routers in the area, as well as making sure that the authentication keys match on all interfaces that share the same network segment:

Router2#configure terminal 
Enter configuration commands, one per line.  End with CNTL/Z.
Router2(config)#interface Serial0/0
Router2(config-if)#ip ospf message-digest-key 1 md5 oreilly
Router2(config-if)#exit
Router2(config)#router ospf 12
Router2(config-router)#area 2 authentication message-digest 
Router2(config-router)#exit
Router2(config)#end
Router2#

Discussion

RFC 2328, which defines OSPF Version 2, includes three different types of authentication for OSPF: null authentication, simple password authentication, and cryptographic authentication. ...