Exam Essentials
Remember the default serial encapsulation on Cisco routers. Cisco routers use a proprietary High-Level Data-Link Control (HDLC) encapsulation on all of their serial links by default. You cannot see that a serial interface is configured with HDLC with a show running-config command because if a serial interface is set to the default, it doesn't display the encapsulation method of HDLC. To verify HDLC is enabled, use the show interface command.
Remember the PPP Data Link layer protocols. The three Data Link layer protocols are Network Control Protocol (NCP), which defines the Network layer protocols; Link Control Protocol (LCP), a method of establishing, configuring, maintaining, and terminating the point-to-point connection; and ...
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