Cisco HDLC is a bit-oriented synchronous data-link layer protocol developed by the International Organization for Standardization (ISO). It specifies a data encapsulation method on synchronous serial links using frame characters and checksums.
By default, synchronous serial lines use the HDLC serial encapsulation method, which provides the synchronous framing-detection and error-detection functions of HDLC without windowing or retransmission.
Cisco HDLC monitors line status on a serial interface by exchanging keepalive request messages with peer network devices. It also enables routers to discover IP addresses of neighbors by exchanging Serial Line Address Resolution Protocol (SLARP) address-request and address-response messages with peer network devices.
The router responds to a SLARP address-request message from a remote peer with a SLARP address-response message, which indicates that it cannot participate in a SLARP session.
Cisco HDLC is compatible with Cisco Systems Cisco-HDLC protocol, the default protocol for all Cisco serial interfaces.