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.
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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.
cHDLC frame structure
The following table describes the make up of a cHDLC frame on the wire.
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.
SLARP Keep-Alive frame structure
The following table shows the makeup of a SLARP cHDLC keep-alive frame.