High-Level Data Link Control (HDLC) Encapsulation

High-Level Data Link Control (HDLC) is a bit-oriented, code-transparent, synchronous data link layer protocol. It is widely used for reliable delivery of data frames over communication links. HDLC provides essential functions such as:

• Framing
• Data transparency
• Error detection and correction
• Flow control

Note: In HDLC communication, Primary stations send commands containing the address of secondary stations & Secondary stations reply with responses addressed back to the primary station.

HDLC Encapsulation Protocol

Encapsulation in HDLC refers to the way frames are structured to carry data across serial links.

Standard HDLC Frame Structure

A standard HDLC frame typically contains six fields:

1. Start/End Flag – Marks the beginning and end of the frame (01111110).
2. Address Field – Identifies the secondary station.
3. Control Field – Used for flow and error control.
4. Information Field – Carries the actual payload (optional in control frames).
5. FCS (Frame Check Sequence) – Error detection using CRC.
6. End Flag – Same as start flag, ensuring frame boundary.

Cisco HDLC (cHDLC)

Cisco developed its own extension of HDLC, known as Cisco HDLC (cHDLC), which adds an extra field to support multiple protocols.

• Standard HDLC -> supports only one protocol.
• Cisco HDLC -> supports multiple protocols, enabled by an additional Protocol Field in the header.

Fields in cHDLC Frame

• Address Field: Identifies packet type. For example, 0x0F (Unicast), 0x8F (Broadcast).
• Control Field: Always set to 0x00.
• Protocol Field: Identifies the encapsulated protocol. For example, 0x0800 for IP packets.

Key Point: The Protocol Field makes cHDLC suitable for multi-protocol environments (unlike standard HDLC).

Verifying HDLC Encapsulation

By default, Cisco routers use HDLC encapsulation on serial interfaces. Since it is the default, it does not appear in the running configuration. To verify encapsulation:

Use the command

Router# show interfaces serial 0/0/0

Example Output

Serial0/0/0 is administratively down, line protocol is down (disabled)
Hardware is HD64570
MTU 1500 bytes, BW 1544 Kbit, DLY 20000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation HDLC, loopback not set, keepalive set (10 sec)

Here, The line “Encapsulation HDLC” confirms that HDLC is in use.

Troubleshooting HDLC Encapsulation

HDLC troubleshooting often involves checking the status of serial interfaces. Useful commands include:

• show ip interface brief
• show interfaces [interface]
• show controllers (to check physical interface and cable status).

Common Issues

• Serial x is up, Line Protocol is up, Normal condition -> No action needed.
• Serial x is down, Line Protocol is down (DTE mode), Indicates a physical layer issue.

Possible causes:
1. Faulty cable -> Replace/swapping cables.
2. Hardware failure -> Try a different port or card.

• Serial x is up, Line Protocol is down (DTE mode), Indicates a misconfiguration or remote-side issue.

Possible causes:
1. Mismatch in encapsulation (remote side using PPP instead of HDLC).
2. No clock rate from DCE to DTE.
3. CSU/DSU or modem issues.
Solution: Place CSU/DSU in local loopback mode and run show interface serial to check if line protocol recovers.

Other Reasons for Protocol Down

• Remote device is a non-Cisco router (not supporting cHDLC).
• Remote side is using a different encapsulation protocol (like PPP).
• Internal wiring/card issues.
• Unsupported electrical interfaces.