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

SLARP stands for: Serial Line Address Resolution 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 16 bits 16 bits 8 bits

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) 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

See also

External links

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