Chapter 2: HDLC (High-Level Data Link Control) Protocol
Understanding the HDLC Protocol
Keepalives and SLARP
Introducing the High-Level Data Link Control Protocol
HDLC is a common WAN protocol that operates at the data-link layer and transmits data between network endpoints. HDLC is based on IBM’s SDLC protocol and was developed by the International Organization for Standardization (ISO). HDLC data is packed into frames and sent across the network using delivery-verification procedures. HDLC encapsulates data over synchronous point-to-point serial links using framing characters and checksums. These features provide synchronous framing and error detection without windowing or retransmission. HDLC is the default encapsulation type on Cisco router serial interfaces. Cisco HDLC, also known as cHDLC, is a proprietary extension of the HDLC protocol. cHDLC maintains connections by sending a series of keepalive messages between peers. Serial Link Address Resolution Protocol, or SLARP, is another feature used by cHDLC to discover the IP addresses of neighboring routing devices.
HDLC links are set up using two main types of network configurations:
♦ Point-to-point: Network setup consisting of two nodes. One node is the controller node, or primary node. The primary node is responsible for setting up and terminating the link, along with link management and control of a peer node, called the secondary node. Secondary ...