3.2 PPP Link Control Protocol Options
3.2.5 Configuring CHAP example
Configuring CHAP is straightforward. You have two routers, as shown in the Figure: Left and Right, connected across the cloud. Use the following steps as a guide to configuring CHAP authentication:
  1. On the interface, specify encapsulation ppp.
  2. Enable the use of CHAP authentication with the ppp authentication chap command. This entry is made because router Left asks router Right to use CHAP authentication to identify itself during the LCP negotiation.
  3. Configure the usernames and passwords.

Passwords must be identical at both ends. The username and password are case sensitive.

Enabling PAP and CHAP Authentication

You can enable both PAP and CHAP authentication on an interface. The first method specified is requested during link negotiation. If the caller refuses the first method, the second method is tried. The commands are as follows:

Router(config-if)#ppp authentication pap chap

or

Router(config-if)#ppp authentication chap pap