What is the default encapsulation method used by serial interfaces?

The default encapsulation method used by serial interfaces on Cisco routers is High-Level Data Link Control (HDLC). HDLC is a bit-oriented synchronous data link layer protocol that is used to provide encapsulation of data over point-to-point links. It is particularly efficient for point-to-point connections because it does not introduce significant overhead compared to some other protocols.

HDLC is designed to work best in environments where there are specific conditions, such as connecting a single router to another router. Since it is simpler and has less overhead, it can provide faster data transmission.

While other encapsulation methods, such as Point-to-Point Protocol (PPP) and Frame Relay, are also used in network applications, they are generally not the default for serial interfaces on Cisco devices. Ethernet, being a local area network technology, is not applicable to serial interfaces, which are typically used for wider area network (WAN) connections. Thus, HDLC is the correct choice as it is the automatic default provided by Cisco for ease of configuration and efficiency in serial link setups.