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 Figure 1. Serial 0/0/0 is up, line protocol is up, indicates that the line is up and functioning; encapsulation HDLC, indicates that the default serial encapsulation (HDLC) is enabled.

The show interfaces serial command returns one of six possible states:

Of the six possible states, there are five problem states. Figure 2 lists the five problem states, the issues associated with that state, and how to troubleshoot the issue.

The show controllers command is another important diagnostic tool when troubleshooting serial lines, as shown in Figure 3. The output indicates the state of the interface channels and whether a cable is attached to the interface. In the figure, interface serial 0/0/0 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 displays 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 command shows that the interface and line protocol are down.