Configuring PPP Authentication

After you configure your serial interface to support PPP encapsulation, you can configure authentication using PPP between routers. First, you need to set the hostname of the router, if it’s not already. Then you set the username and password for the remote router that will be connecting to your router:

Here’s an example:

Router#config t
Enter configuration commands, one per line. End with CNTL/Z.
Router(config)#hostname RouterA
RouterA(config)#username RouterB password cisco

When using the hostname command, remember that the username is the hostname of the remote router that’s connecting to your router. And it’s case sensitive too. Also, the password on both routers must be the same. It’s a plain-text password that you can see with a show run command; you can encrypt the password by using the command service password-encryption. You must have a username and password configured for each remote system you plan to connect to. The remote routers must also be configured with usernames and passwords.

Now, after you’ve set the hostname, usernames, and passwords, choose the authentication type, either CHAP or PAP:

RouterA#config t
Enter configuration commands, one per line. End with CNTL/Z.
RouterA(config)#int s0
RouterA(config-if)#ppp authentication chap pap
RouterA(config-if)#^Z
RouterA#

If both methods are configured on the same line, as shown here, then only the first method will be used during link negotiation—the second acts as a backup just in case the first method fails.