Cisco Certified Network Associate (CCNA) Practice Exam 2025 – The Comprehensive All-in-One Guide to Exam Success!

The command "ppp authentication chap" is the correct choice when configuring PPP (Point-to-Point Protocol) authentication on Router 1. This command enables CHAP (Challenge Handshake Authentication Protocol) as the method of authentication for connections using PPP.

In a PPP implementation, CHAP is commonly used because it provides a way to verify the identity of the peer in a secure manner. When this command is applied, Router 1 will respond to authentication requests from a peer device (like Router 2) using CHAP, ensuring that only devices with the correct credentials are allowed to establish a link.

The other commands presented do not directly relate to setting up PPP authentication. For instance, specifying a username and a secret is part of configuring a user account for authentication purposes, but it doesn't directly configure PPP authentication on a specific interface. The interface command is typically utilized to enter interface configuration mode but does not itself set up authentication. Lastly, the no shutdown command is used to enable an interface, ensuring it is operational; however, it does not pertain to the authentication process itself.

Thus, "ppp authentication chap" is the most relevant and effective command in the context of configuring PPP authentication.