First, you need to create a key chain if you don't have one already. To create a key chain named "my-key-chain" from the config prompt, type:
key chain my-key-chain
Next, type the number of the key you want to create on the chain. The first key should be key 1. You can set up multiple keys if you have multiple adjacent routers and you want different keys for each, but that's just a management hassle. You can use the same key for all the routers. Type:
Now, you need the key string, which is the password. Type:
You should now have a key chain created. Now to enable the chain for RIP, you need to type the following command from the config interface prompt:
ip rip authentication key-chain my-key-chain
ip rip authentication mode md5
Instead of sending your password in clear text over the wire, now you'll send an md5 hash of the password. Note that earlier this year, a flaw was found in the md5 algorithm so it's no longer considered "secure," but it's enough to deter most people.
Obviously, you need to type these same commands on each pair of router interfaces in your network. However, the key numbers do not have to be the same. For more information, you can search for key chains on Cisco's Website.
This was first published in September 2005