An OSPF adjacency can be authenticated with MD5 (Message Digest 5) or with a clear-text password. I’m not much on clear-text passwords, and hopefully you aren’t either! Whether you’re working in the real world or the certification exam room, though, it’s always a good idea to know more than one way to do things. Let’s take a look at how to configure clear-text authentication of an OSPF neighbor relationship.

The commands we’ll use are “ip ospf authentication-key� and “ip ospf authentication�. In this example, we have preexisting adjacencies between three routers in an OSPF NBMA network. The hub router (R1) has an adjacency with two spoke routers, R2 and R3.

The password is set by the interface-level command ip ospf authentication-key. While Cisco routers will usually tell you when you’re about to try to do something that you can’t do, this password is a rare exception to the rule. Let’s set a password of passbscitest and then check the router config.

R1(config-if)#ip ospf authentication-key ?

<0-7> Encryption type (0 for not yet encrypted, 7 for proprietary)

LINE The OSPF password (key)

R1(config-if)#ip ospf authentication-key passbscitest

R1#show config

interface Serial0

ip address 172.12.123.1 255.255.255.0

encapsulation frame-relay

ip ospf authentication-key passbsci

I entered a 12-character password, but only the first eight are showing in the router configuration. The router failed to warn us that this particular password has a limit of eight characters. As of IOS 12.4, the router now warns the admin about this, but there are plenty of routers out there that aren’t running that recent a release!

Clear-text authentication is enabled with the ip ospf authentication command. IOS Help shows there is no specific command for clear-text authentication. (Null and clear-text authentication are not the same thing.)

R1(config)#int serial0

R1(config-if)#ip ospf authentication ?

message-digest Use message-digest authentication

null Use no authentication

< cr >

To set clear-text authentication, just use the basic command with no options.

R1(config-if)#ip ospf authentication

About two minutes after entering that configuration, the preexisting adjacencies go down:

R1#

00:25:38: %OSPF-5-ADJCHG: Process 1, Nbr 172.12.123.2 on Serial0 from FULL to DOWN, Neighbor Down: Dead timer expired

R1#

00:25:58: %OSPF-5-ADJCHG: Process 1, Nbr 172.12.123.3 on Serial0 from FULL to DOWN, Neighbor Down: Dead timer expired

R1#

Until we configure the spoke routers with the same config, the adjacencies will stay down – so let’s get those spokes configured!

R2(config)#interface serial0

R2(config-if)#ip ospf authentication-key passbsci

R2(config-if)#ip ospf authentication

R3(config)#interface serial0

R3(config-if)#ip ospf authentication-key passbsci

R3(config-if)#ip ospf authentication

On R1, show ip ospf neighbor verifies that the adjacencies are back up.

R1#show ip ospf neighbor

Neighbor ID Pri State Dead Time Address Interface

172.12.123.3 0 FULL/DROTHER 00:01:58 172.12.123.3 Serial0

172.12.123.2 0 FULL/DROTHER 00:01:37 172.12.123.2 Serial0

Now that you know how to configure OSPF neighbor authentication in clear text, you need to learn how to use MD5 authentication, and that just happens to be the subject of my next CCNP BSCI 642-901 exam tutorial! See you then!

Chris Bryant, CCIE #12933, is the owner of The Bryant Advantage, home of free CCNP and CCNA tutorials, The Ultimate CCNA Study Package, and Ultimate CCNP Study Packages.

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