| 9.3 |  | Troubleshooting Router Issues Overview |
| 9.3.1 |  | Troubleshooting Layer 1 using show interfaces |

| The Cisco IOS contains a rich set of commands for troubleshooting. Among the more widely used are the show commands. Every aspect of the router can be viewed with one or more of the show commands. The show command used to check the status and statistics of the interfaces is the show interfaces command. Variations of this command can be used to check the status of the different types of interfaces. To view the status of the FastEthernet interfaces, use show interfaces FastEthernet. The command may also be used to view the status of one particular interface. To view the status of Serial 0/0, use show interfaces serial0/0. The status of two important portions of the interfaces is shown with the show interfaces command. They are the physical (hardware) portion and logical (software) portion. These can be related to the Layer 1 and the Layer 2 functions. The hardware includes cables, connectors, and interfaces showing the condition of the physical connection between the devices. The software status shows the state of messages such as keepalives, control information, and user information that are passed between adjacent devices. This relates to the condition of a Layer 2 protocol passed between two connected router interfaces. These important elements of the show interfaces serial command output are displayed as the line and data-link protocol status. The first parameter refers to the hardware layer and essentially reflects whether the interface is receiving the Carrier Detect (CD) signal from the other end of the connection. If the line is down, a problem may exist with the cabling, equipment somewhere in the circuit may be powered off or malfunctioning, or one end may be administratively down. If the interface is administratively down it has been manually disabled in the configuration. The show interfaces serial command also provides information to help diagnose other Layer 1 issues that are not as easy to determine. An increasing number of carrier transitions counts on a serial link may indicate one or more of the following problems: Line interruptions due to problems in the service provider network. Faulty switch, DSU, or router hardware. If an increasing number of input errors appear in the show interfaces serial output, there are several possible sources of those errors. Some of these related to Layer 1 problems are: Faulty telephone company equipment Noisy serial line Incorrect cable or cable length Damaged cable or connection Defective CSU or DSU Defective router hardware Another area to examine is number of interface resets. These are the result of too many missed keepalives. The following Layer 1 problems could be a cause of interface resets: Bad line causing carrier transitions Possible hardware problem at the CSU, DSU, or switch If carrier transitions and interface resets are increasing or if input errors are high while interface resets are increasing, the problem is likely to be a bad link or defective CSU or DSU. The number of errors should be interpreted relative to the amount of traffic that the router has processed and the amount of time that the statistics have been captured. The router tracks statistics that provide information about the interface. The statistics reflect router operation since it was started or since the last time the counters were cleared. If the show interfaces output shows the last clearing of the counters as never, use the show version command to find out how long the router has been functional. Use the clear counters command to reset the counters to zero. These counters should always be cleared after an interface problem has been corrected. Starting from zero gives a better picture of the current status of the network and will help verify that the issue has indeed been corrected.
| Lab Activity e-Lab Activity: Troubleshooting Layer 1: Using show interfaces In this lab, the students will verify that the physical serial 0/0 link is up, reset the interface counters, and check the interface again to see what happens after a counter reset.
| Web Links show ip interfaces Command http://www.cisco.com/en/US/products/sw/ iosswrel/ps1835/ products_command_reference_ chapter09186a0080087387.html#1020817