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.
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.
|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 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|
- 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. These are 0 for address requests.
- The two-byte Reserved field is currently unused and undefined.
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|
- The op-code is 0x02 for keep-alives.
- The sender sequence number increments with each keep-alive sent by this sender.
- The received sequence number is the last sequence number received by this sender.
- The two-byte Reliability field is required to be set to 0xFFFF.
- 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
- Serial Line Address Resolution Protocol, IP Addressing: ARP Configuration Guide, Cisco IOS Release 15M&T, December 3, 2012