WAN Troubleshooting Facts

WAN Troubleshooting Facts

The following list of commands may help when troubleshooting router connections.

Use...	To...
router#show interfaces	List a large set of information about each interface.
router#show interface status	View summary information about the interface status.
router#show ip interfaces	View a small set of information about each IP interface.
router#show ip interfaces brief	View a single line of information about each IP interface.
router#show ip route <ip address>	View details about the route the router would match for a packet sent to the listed IP address.
router#show running-config	View the currently running configuration file.
router#show controllers <serial interface>	View the serial interface configuration, such as the type of serial cable and which end of the cable is connected to the device (DCE or DTE).
router#ping <ip address>	Test communication with a specific interface using its IP address.

You can use the interface status to understand connectivity problems and quickly see whether the link between the router and the network is operational. The following table summarizes some possible conditions indicated by the interface status:

Line status	Protocol status	Indicates...
administratively down	down	The interface is configured with the shutdown command.
down	down	There is a hardware or network connection problem (Physical layer), such as: No cable or bad cable The device on the other end of the cable is powered off or the other interface is administratively shutdown (with the shutdown command)
up	down	There is a connection or communication problem (Data Link layer), such as: No clock rate provided by the DCE device Mismatched encapsulation Incorrect authentication parameters for PPP, including: Mismatched authentication method Missing username statements Mismatched passwords
up	up	The interface is working correctly

After verifying that the interfaces have Layer 1 and Layer 2 connectivity, proceed to troubleshoot TCP/IP connectivity including:

• Verifying that devices have unique IP addresses.
• Verifying that the same subnet mask is used on all devices on the same subnet.
• Verifying that the IP address assigned to each device is on the same subnet.
• Verifying routing table entries.

Be aware of the following when troubleshooting connectivity:

• If a problem exists at Layer 1, you must correct that problem before troubleshooting Layer 2 connectivity. If a problem exists at Layer 2, you must correct that problem before you can proceed to troubleshoot upper-layer connectivity.
• Use ping and traceroute to verify Network-layer connectivity, and use Telnet to verify Application-layer connectivity and configuration.
• A failed ping or traceroute test might indicate Layer 1, Layer 2, or Layer 3 problems. Examine the interface status to rule out Layer 1 and Layer 2 problems.
• A successful Telnet test means that ping and traceroute will also be successful. A failed Telnet test only indicates a failure at the Application layer or below. By itself, it does not tell you at which layer the problem exists.
• Because some devices do not respond to ICMP messages, you might have Network-layer connectivity between devices even if ping or traceroute fail.
• A successful ping test followed by an unsuccessful Telnet test means that Network-layer connectivity exists. Troubleshoot the upper-layer configuration.
• Even if Telnet to a router fails, the router might still be routing packets. This is because routing happens at the Network layer, while Telnet happens at the Application layer.