[Day 10] 10. HDLC and SDLC

• SDLC (Synchronous Data Link Control): A data link layer protocol developed by IBM. It is a proprietary protocol designed for synchronous communication.

• HDLC (High-Level Data Link Control): Developed by IEEE as an alternative to SDLC. While it serves a similar purpose, HDLC lacks the Logical Link Control (LLC) layer present in SDLC. This means HDLC focuses solely on the data link layer without the additional functionality provided by LLC.

Key Points About HDLC

1. Default on Cisco's WAN Interfaces: HDLC is the default encapsulation method for WAN interfaces on Cisco devices. This means that unless otherwise configured, Cisco routers will use HDLC for WAN communications.

2. Vendor Specificity: HDLC is vendor-specific, meaning that if two routers are using HDLC to communicate, they must be from the same vendor (e.g., both must be Cisco routers). This is due to potential differences in how different vendors implement the protocol.

3. Synchronous Links: HDLC operates only on synchronous links, where both ends of the communication are synchronized in terms of timing (clocking is provided). The terms DT (Data Terminal) and DC (Data Circuit) refer to the timing signals used in these synchronous connections.

Command to Deploy HDLC on Cisco Routers

To configure HDLC on a Cisco router interface, you can use the following command:

A(config-if)# encapsulation hdlc

However, since HDLC is the default encapsulation method on cisco routers, this command is not strictly necessary unless you're changing from a different encapsulation type.