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Cisco HDLC
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<h2 id="framing">Framing</h2> <p>Cisco HDLC <a href="/facts/Data_frame/ltGCbJIq">frames</a> uses an alternative framing structure to the standard ISO <a href="/facts/HDLC/aTndRUQ3">HDLC</a>. To support multiple protocols encapsulation, cHDLC frames contain a field for identifying the <a href="/facts/Network_protocol/5uady7cB">network protocol</a>. </p> <h2 id="structure">Structure</h2> <h3>cHDLC frame structure</h3> <p>The following table describes the structure of a cHDLC frame on the wire. </p> <table><tbody><tr><th>Address</th><th>Control</th><th>Protocol Code</th><th>Information</th><th>Frame Check Sequence (FCS)</th><th>Flag</th></tr><tr><td>8 bits</td><td>8 bits</td><td>16 bits</td><td>Variable length, 0 or more bits, in multiples of 8</td><td>16 bits</td><td>8 bits</td></tr></tbody></table> <ul><li>The Address field is used to specify the type of packet contained in the cHDLC frame; 0x0F for Unicast and 0x8F for Broadcast packets.</li> <li>The Control field is always set to zero (0x00).</li> <li>The Protocol Code field is used to specify the protocol type encapsulated within the cHDLC frame (e.g. 0x0800 for <a href="/facts/Internet_Protocol/Qq6VeEal">Internet Protocol</a>).</li></ul> <h3>SLARP address request–response frame structure</h3> <p>The Serial Line Address Resolution Protocol (SLARP) frame is designated by a specific cHDLC protocol code field value of 0x8035. </p><p>Three types of SLARP frame are defined: address requests (0x00), address replies (0x01), and keep-alive frames (0x02). </p><p>The following table shows the structure of a SLARP cHDLC address request–response frame. </p> <table><tbody><tr><th>Address</th><th>Control</th><th>Protocol Code</th><th>SLARP Op-Code</th><th>Address</th><th>Mask</th><th>Reserved</th><th>Frame Check Sequence (FCS)</th><th>Flag</th></tr><tr><td>8 bits</td><td>8 bits</td><td>16 bits (0x8035)</td><td>32 bits</td><td>32 bits</td><td>32 bits</td><td>16 bits</td><td>16 bits</td><td>8 bits</td></tr></tbody></table> <ul><li>The op-code will be 0x00 for address requests and 0x01 for address responses.</li> <li>The Address and Mask fields are used to contain a four-octet IP address and mask. These are 0 for address requests.</li> <li>The two-byte Reserved field is currently unused and undefined.</li></ul> <h3>SLARP Keep-Alive frame structure</h3> <p>The following table shows the structure of a SLARP cHDLC keep-alive frame. </p> <table><tbody><tr><th>Address</th><th>Control</th><th>Protocol Code</th><th>SLARP Op-Code</th><th>Sequence Number (Sender)</th><th>Sequence Number (Last Received)</th><th>Reliability</th><th>Frame Check Sequence (FCS)</th><th>Flag</th></tr><tr><td>8 bits</td><td>8 bits</td><td>16 bits (0x8035)</td><td>32 bits</td><td>32 bits</td><td>32 bits</td><td>16 bits</td><td>16 bits</td><td>8 bits</td></tr></tbody></table> <ul><li>The op-code is 0x02 for keep-alives.</li> <li>The sender sequence number increments with each keep-alive sent by this sender.</li> <li>The received sequence number is the last sequence number received by this sender.</li> <li>The two-byte Reliability field is required to be set to 0xFFFF.</li></ul> <h2 id="see-also">See also</h2> <ul><li><a href="/facts/Point-to-Point_Protocol/XLaU6D71">Point-to-Point Protocol</a>, an Internet Standard defined by RFC 1661 and RFC 1662 that solves the problems Cisco HDLC solves as well as many other problems.</li></ul> <h2 id="external-links">External links</h2> <ul><li><a href="https://www.cisco.com/c/en/us/td/docs/ios-xml/ios/ipaddr_arp/configuration/15-mt/arp-15-mt-book/arp-config-arp.html#GUID-020979D7-4109-4CC7-8A2B-50BC0AEE77B2">Serial Line Address Resolution Protocol</a>, <i>IP Addressing: ARP Configuration Guide, Cisco IOS Release 15M&T</i>, December 3, 2012</li> <li><a href="http://www.nethelp.no/net/cisco-hdlc.txt">http://www.nethelp.no/net/cisco-hdlc.txt</a></li> <li><a href="http://securitydigest.org/tcp-ip/archive/1990/11#000068">http://securitydigest.org/tcp-ip/archive/1990/11#000068</a></li> <li><a href="https://web.archive.org/web/20110723160051/http://www.irbs.net/internet/nanog/9610/0459.html">https://web.archive.org/web/20110723160051/http://www.irbs.net/internet/nanog/9610/0459.html</a></li></ul>