Cisco HDLC

Cisco HDLC (also known as cHDLC) is an extension to the High-Level Data Link Control network protocol 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 workings of cHDLC have in fact been openly and widely distributed, and the protocol has been implemented by many different network equipment vendors.

Framing

Cisco HDLC frames make use of an alternative framing structure to the standard ISO HDLC. One of the primary reasons for the creation of cHDLC was to address the HDLC protocol's inability to provide multi-protocol support. Thus, cHDLC frames contain a field for identifying the network protocol being encapsulated.

Structure

cHDLC frame structure

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

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 Address field is used to specify the type of packet contained in the cHDLC frame; 0x0F for Unicast and 0x8F for Broadcast packets.
• The Control field is always set to zero (0x00).
• The Protocol Code field is used to specify the protocol type encapsulated within the cHDLC frame (e.g. 0x0800 for Internet Protocol).

SLARP address request–response frame structure

A SLARP frame is designated by a specific cHDLC Protocol Code field value of 0x8035.

Currently only three types of SLARP frame are defined: address requests (0x00), address replies (0x01), and keep-alive frames (0x02).

The following table shows the makeup 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	8 bits	16 bits	8 bits

• The op-code will be 0x00 for address requests and 0x01 for address responses.
• The Address and Mask fields are used to contain a four-octet IP address and mask.
• The one-byte Reserved field is currently unused and is always set to 0xFF.

SLARP Keep-Alive frame structure

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

Address	Control	Protocol Code	SLARP Op-Code	Sequence Number (Sender)	Sequence Number (Last Received)	Reserved	Frame Check Sequence (FCS)	Flag
8 bits	8 bits	16 bits (0x8035)	32 bits	32 bits	32 bits	8 bits	16 bits	8 bits

See also

• Point-to-Point Protocol, an Internet Standard defined by RFC 1661 and RFC 1662 that solves the problems Cisco HDLC solves as well as many other problems.
• Various documents 1, 2, 3 documenting the Cisco HDLC frame format