Cisco HDLC is the default encapsulation method used by Cisco devices on synchronous serial lines.
You use Cisco HDLC as a point-to-point protocol on leased lines between two Cisco devices. If you are connecting to a non-Cisco device, use synchronous PPP.
If the default encapsulation method has been changed, use the encapsulation hdlc command in privileged mode to re-enable HDLC.
There are two steps to enable HDLC encapsulation:
Step 1. Enter the interface configuration mode of the serial interface.
Step 2. Enter the encapsulation hdlc command to specify the encapsulation protocol on the interface.
The output of the show interfaces serial command displays information specific to serial interfaces. When HDLC is configured, "Encapsulation HDLC" should be reflected in the output, as highlighted in the figure.
The show interface serial command returns one of five possible states. You can identify any of the following five possible problem states in the interface status line:
Serial x is down, line protocol is down
Serial x is up, line protocol is down
Serial x is up, line protocol is up (looped)
Serial x is up, line protocol is down (disabled)
Serial x is administratively down, line protocol is down
The show controllers command is another important diagnostic tool when troubleshooting serial lines. The output indicates the state of the interface channels and whether a cable is attached to the interface. In the figure, serial interface 0/0 has a V.35 DCE cable attached. The command syntax varies, depending on the platform. Cisco 7000 series routers use a cBus controller card for connecting serial links. With these routers, use the show controllers cbus command.
If the electrical interface output is shown as UNKNOWN instead of V.35, EIA/TIA-449, or some other electrical interface type, the likely problem is an improperly connected cable. A problem with the internal wiring of the card is also possible. If the electrical interface is unknown, the corresponding display for the show interfaces serial