High-Level Data Link Control (HDLC) Encapsulation

High-Level Data Link Control (HDLC) basically provides reliable delivery of data frames over a network or communication link. HDLC provides various operations such as framing, data transparency, error detection, and correction, and even flow control. Primary stations simply transmit commands that contain address of secondary stations. The secondary station then simply transmits responses that contain its own address.

HDLC Encapsulation Protocol :
We know that each of frames of HDLC includes at least six to seven fields like start/end flag field, control field, information field, FCS (Frame Check Sequence) field. Standard HDLC Protocol contains six fields. Whereas on the other hand, Cisco HDLC (cHDLC) contains one extra protocol field. The standard protocol uses to support only one protocol whereas cHDLC protocol use to support multi-protocol environments. Supporting multiple protocols is possible due to protocol field in header, that helps in identifying different protocols. cHDLC was basically created by Cisco systems.

• Address field –
  This field is used to identify and specify type of packet that is present in cHDLC frame. It can be 0*0F for Unicast and 0*8F for Broadcast packets.
• Control field –
  This field is always set to zero i.e. 0*00.
  
  
• Protocol field –
  This field is especially required to specify and identify type of protocol that is being encapsulated withing cHDLC frame. It can be 0x0800 for Internet Protocol.

Verify HDLC Encapsulation :
We know that HDLC is generally default encapsulation method for serial interfaces on Cisco router. Therefore, it will not be listed in any of running configurations. This simply means that to verify HDLC encapsulation, we cannot even use show running-config command. So, we must use show interfaces (Interface) command identify and view type of encapsulation in interface.

Router#show interfaces serial 0/0/0
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)
  Last input never, output never, output hang never
  Last clearing of "show interface" counters never 

The output will indicate type of encapsulation in HDLC.

Troubleshoot HDLC Encapsulation :
To view and know present status of serial interface, we can use two types of commands as given below:

• show ip interface brief
• show interfaces [interface]

Show controllers command is an essential and important diagnostic tool that helps in troubleshooting serial lines. This command output also indicates state of interface channels and whether a cable is attached to interface or not. There are some reasons that are responsible for some issues arising during HDLC implementation due to which protocol status will be down.

These reasons are given below:

• Presence of Non-Cisco router at remote side.
• Usage of others protocols such as PPP by remote side router.
• Inability to providing clock rate to DTE (Data Terminal Equipment) Device by DCE (Data Circuit-Terminating Equipment) Device.
• Problem or issue with internal wiring of card.
• Unknown electrical interfaces.

Some of the serial interface issues are given below:

1. Serial x is up, Line Protocol is up –
   This command indicates that line is up and functioning properly. There is no requirement of any action.
2. Serial x is down, Line Protocol is down (DTE mode) –
   This command indicates that there is an issue. This issue can arise due to different reasons. Some of them are given below:
   • Fault in cable –
     This issue can be resolved by swapping all of fault cables.
   • Failure of hardware –
     This issue can be resolved by changing serial line to another port.
3. Serial x is up, Line Protocol is down (DTE mode) –
   This command also indicates that there is an issue. This issue can arise due to reason that a local or remote is misconfigured. This problem can be resolved by putting modem, CSU (Channel Service Unit) or DSU (Data Service Unit) in local loopback mode, and then using show interface serial command. This command indicates whether line protocol has come up or not.