HDLC is an ISO standard, bit-oriented, data-link layer protocol that encapsulates data on synchronous serial data links. Standard HDLC does not inherently support multiple protocols on a single link because it does not have a way to indicate which protocol it is carrying. HDLC specifies a data encapsulation method on synchronous serial links using frame characters and checksums.
Cisco offers a proprietary version of HDLC. The Cisco HDLC frame uses a proprietary-type field that acts as a protocol field, which makes it possible for multiple network layer protocols to share the same serial link.
HDLC does not provide link authentication.
Router(config-if)# encapsulation hdlc
By default, Cisco devices use the Cisco HDLC serial encapsulation method on synchronous serial lines. However, if the serial interface is configured with another encapsulation protocol and you want to change the encapsulation back to HDLC, enter the interface configuration mode of the interface that you want to change. Use the encapsulation hdlc interface configuration command to specify HDLC encapsulation on the interface. Cisco HDLC is a PPP that can be used on leased lines between two Cisco devices. When communicating with a device from another vendor, synchronous PPP is a more viable option.