 either method, you must have a local user configured at least on the receiving (called) router, although as you'll see, the requirements vary. Configuring non-AAA authentication for PPP is covered here. To read about AAA PPP authentication using PAP and CHAP, see the section "Applying Method Lists" at the end of the chapter. PAP can be configured for one-way or two-way authentication. One-way authentica- tion indicates that only one side initiates a challenge. Two-way authentication indicates that both sides of the link authenticate each other. One-way authentication. With one-way authentication, the calling router sends a user- name and password, which must match a username and password configured on the called router. The calling router must be configured as a callin router with the ppp authentication pap callin command. The calling router must also be configured to send a username and password with the ppp pap sent-username command. Here are some example configurations. Configuration entries not specific to PPP authentication are not shown, for clarity: Calling side: interface BRI1/0 encapsulation ppp ppp authentication pap callin ppp pap sent-username Bob password 0 ILikePie Basic (Non-AAA) Authentication Called side: username Bob password 0 ILikePie interface BRI1/0 encapsulation ppp ppp authentication pap Two-way authentication. With two-way authentication, the callin keyword is not nec- essary. Because the authentication is done in both directions, both routers must be configured with usernames/passwords and the ppp pap sent-username command. The end result is that both sides are configured the same way: Calling side: username Bob password 0 ILikePie interface BRI1/0 encapsulation ppp ppp authentication pap ppp pap sent-username Bob password 0 ILikePie Called side: username Bob password 0 ILikePie interface BRI1/0 encapsulation ppp ppp authentication pap ppp pap sent-username Bob password 0 ILikePie Debugging PPP authentication. Debugging PPP authentication is done with the debug ppp authentication comman