Explanation: Point-to-Point Protocol (PPP) and High-Level Data Link Control (HDLC) are both encapsulation methods used to transmit data over serial links. On Cisco routers, however, HDLC is the default encapsulation protocol for point-to-point serial interfaces. What is HDLC? HDLC is a bit-oriented synchronous data link layer protocol developed by the International Organization for Standardization (ISO). Cisco’s implementation of HDLC is proprietary and is used primarily for serial communication between two routers. It is designed to provide a simple and efficient means of framing packets for transmission over serial links. Why HDLC Is the Default: Simplicity: HDLC is straightforward to implement and does not require the overhead of negotiating link parameters, making it an efficient choice for point-to-point links. Efficiency: As a default protocol, HDLC has minimal overhead, which is beneficial when bandwidth is limited on serial connections. Compatibility: In Cisco networks, HDLC is widely supported, ensuring that most point-to-point serial links between Cisco devices will work correctly without additional configuration. PPP vs. HDLC: While PPP (Point-to-Point Protocol) is a widely used encapsulation method—especially because of its ability to support multiple network layer protocols and provide authentication features (using PAP or CHAP)—it is not the default on Cisco routers. PPP is typically used when additional features such as multilink, authentication, or the need to support non-Cisco devices are required. PPP is negotiated via the Link Control Protocol (LCP), which adds overhead and configuration complexity. Other Options: Frame Relay (Option C): Frame Relay is a WAN technology that encapsulates data for packet-switched networks. It is not used for direct point-to-point serial communication by default. Ethernet (Option D): Ethernet is generally used in local area networks (LANs) and is not applicable for serial point-to-point links. Real-World Implications: Understanding which encapsulation method is used is important for troubleshooting and configuration. For example, if a serial link is not working, knowing that HDLC is the default helps narrow down the issue. If a connection between devices from different vendors is required, or if specific PPP features are needed, an administrator might explicitly configure PPP using the command: encapsulation ppp However, in typical Cisco-to-Cisco serial connections, the default HDLC encapsulation usually suffices. Conclusion: Cisco routers use HDLC by default for point-to-point serial links due to its simplicity, efficiency, and compatibility within Cisco environments. Although PPP is a robust alternative when additional features are required, HDLC remains the standard default encapsulation method on Cisco serial interfaces.