How do you configure PPP encapsulation on a serial interface?

To configure Point-to-Point Protocol (PPP) encapsulation on a serial interface, the correct command is to use "encapsulation ppp." This command tells the router to use PPP as the framing method for data being transmitted over the serial link.

PPP is designed to encapsulate multiple network layer protocols and provides robust error checking and authentication mechanisms, making it a preferred choice for point-to-point links. When you enter the command "encapsulation ppp" in the interface configuration mode, you are effectively enabling this encapsulation type, allowing the interface to handle PPP frames.

Understanding the reason for each of the other options being incorrect can also clarify this choice. "Encapsulation frame-relay" refers specifically to Frame Relay encapsulation, which is a different protocol designed for efficient data transmission in a network. The options "set encapsulation ppp" and "define ppp encapsulation" use incorrect command syntax that is not recognized in Cisco IOS. Only the "encapsulation ppp" command properly implements the configuration needed for using PPP on a serial interface.