command displays information specific to serial interfaces. When HDLC is configured, “Encapsulation HDLC” should be reflected in the output. When PPP is configured, "Encapsulation PPP" should be seen in the output.

Five possible problem states can be identified in the interface status line of the show interfaces serial display:

• 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 show controllers output indicates the state of the interface channels and whether a cable is attached to the interface. In Figure , serial interface 0/0 has a V.35 DTE cable attached. The command syntax varies, depending on platform. For serial interfaces on Cisco 7000 series 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, an improperly connected cable is the likely problem. 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 <X> command will show that the interface and line protocol are down.

Following are some debug commands that are useful when troubleshooting serial and WAN problems:

• debug serial interface – Verifies whether HDLC keepalive packets are incrementing. If they are not, a possible timing problem exists on the interface card or in the network.
• debug arp – Indicates whether the router is sending information about or learning about routers (with ARP packets) on the other side of the WAN cloud. Use this command when some nodes on a TCP/IP network are responding, but others are not.
• debug frame-relay lmi – Obtains Local Management Interface (LMI) information which is useful for determining whether a Frame Relay switch and a router are sending and receiving LMI packets.
• debug frame-relay events – Determines whether exchanges are occurring between a router and a Frame Relay switch.
• debug ppp negotiation – Shows Point-to-Point Protocol (PPP) packets transmitted during PPP startup where PPP options are negotiated.
• debug ppp packet – Shows PPP packets being sent and received. This command displays low-level packet dumps.
• debug ppp – Shows PPP errors, such as illegal or malformed frames, associated with PPP connection negotiation and operation.
• debug ppp authentication – Shows PPP Challenge Handshake Authentication Protocol (CHAP) and Password Authentication Protocol (PAP) packet exchanges.

Caution: Debugging output is assigned high priority in the CPU process and can render the system unusable. For this reason, debug commands should only be used to troubleshoot specific problems or during troubleshooting sessions with Cisco technical support staff. It is good practice to use debug commands during periods of low network traffic and when the fewest users are online. Debugging during these periods decreases the likelihood that increased debug command processing overhead will affect system use.

Lab Activity Lab Exercise: Troubleshooting a Serial Interface In this lab, the students will configure a serial interface on the London and Paris routers.

   

Lab Activity e-Lab Activity: Troubleshooting a Serial Interface In this lab, the student will configure a serial interface on the London and Paris routers.