What are the four fields in the Cisco proprietary HDLC header in order?

The Cisco proprietary High-Level Data Link Control (HDLC) header consists of four fields that are arranged in a specific sequence. The correct order of these fields is essential for understanding how HDLC encapsulates data for error detection and communication between devices.

The first field is the Flag field, which serves as a frame delimiter to indicate the start and end of the frame. It is imperative for marking the boundaries of the HDLC frame.

Following the Flag field is the Address field. This field specifies the destination address of the frame and can contain either a physical or logical address, which is critical for ensuring that the frame is sent to the correct recipient.

Next is the Control field, which provides control information for the HDLC protocol, including information about flow control and sequence numbers for ensuring that frames are received in the correct order and without loss.

Lastly, the Type field is used in certain implementations to define the type of the data being carried by the frame. While in Cisco's implementation, this field may not always be present, it can denote specific protocol information when required.

Understanding this order—Flag, Address, Control, Type—is crucial for correctly interpreting HDLC frame structures, especially when troubleshooting network issues or configuring devices that use HDLC for data link layer