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.
The following table describes the structure of a cHDLC frame on the wire.
The following table shows the structure of a SLARP cHDLC address request–response frame.
The following table shows the structure of a SLARP cHDLC keep-alive frame.