Cisco HDLC (cHDLC) is an extension to the High-Level Data Link Control (HDLC) network protocol, and was created by Cisco Systems, Inc. HDLC is a bit-oriented synchronous data link layer protocol that was originally developed by the International Organization for Standardization (ISO). Often described as being a proprietary extension, the details of cHDLC have been widely distributed and the protocol has been implemented by many network equipment vendors. cHDLC extends HDLC with multi-protocol support.

Framing

Cisco HDLC frames uses an alternative framing structure to the standard ISO HDLC. To support multiple protocols encapsulation, cHDLC frames contain a field for identifying the network protocol.

Structure

cHDLC frame structure

The following table describes the structure of a cHDLC frame on the wire.

SLARP address request–response frame structure

The Serial Line Address Resolution Protocol (SLARP) frame is designated by a specific cHDLC protocol code field value of 0x8035. Three types of SLARP frame are defined: address requests (0x00), address replies (0x01), and keep-alive frames (0x02). The following table shows the structure of a SLARP cHDLC address request–response frame.

SLARP Keep-Alive frame structure

The following table shows the structure of a SLARP cHDLC keep-alive frame.