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.
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.
The following table describes the structure of a cHDLC frame on the wire.[citation needed]
| Address | Control | Protocol Code | Information | Frame Check Sequence (FCS) | Flag |
|---|---|---|---|---|---|
| 8 bits | 8 bits | 16 bits | Variable length, 0 or more bits, in multiples of 8 | 16 bits | 8 bits |
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.
| Address | Control | Protocol Code | SLARP Op-Code | Address | Mask | Reserved | Frame Check Sequence (FCS) | Flag |
|---|---|---|---|---|---|---|---|---|
| 8 bits | 8 bits | 16 bits (0x8035) | 32 bits | 32 bits | 32 bits | 16 bits | 16 bits | 8 bits |
The following table shows the structure of a SLARP cHDLC keep-alive frame.
| Address | Control | Protocol Code | SLARP Op-Code | Sequence Number (Sender) | Sequence Number (Last Received) | Reliability | Frame Check Sequence (FCS) | Flag |
|---|---|---|---|---|---|---|---|---|
| 8 bits | 8 bits | 16 bits (0x8035) | 32 bits | 32 bits | 32 bits | 16 bits | 16 bits | 8 bits |