Interface Troubleshooting -Cisco Router and Switch -Step by Step
Router/Switch Interface troubleshooting “Step by Step”
Follow are the step by step guide to troubleshooting the router and switch Serial/Ethernet, Fastethernet and GigaEthernet port. Enter the show interfaces fastethernet, show interfaces gigaethernet command on privilege mode of Cisco router and switch#
Make sure that the interface is up.
Make sure the line protocol is up.
Make sure the interface duplex mode matches the remote interface configuration are as per requirement
Make sure the interface speed matches the speed on the remote interface
Observe the output hang status on the interface.
Observe the CRC counter.
Observe the late collision counter.
Observe the carrier signal counters.
“Step by Step” troubleshoot to find the problem on router/switch interfaces
1) Verify that the interface is UP:
Use show interface command to ensure that interface is UP. If the interface is down, perform the following corrective actions:
If the interface is administratively down, use the no shutdown interface configuration command to enable the interface.
Be sure that the cable is fully connected.
Verify that the cable is not bent or damaged. If the cable is bent or damaged, the signal will be degraded.
Verify that a hardware failure has not occurred. Observe the LEDs to confirm the failure
2) Verifying the Line Protocol Is Up:
In the output of show interfaces command, verify that the line protocol is UP. If the line protocol is down, the line protocol software processes have determined that the line is unusable. Perform the following corrective actions:
Verify that a hardware failure has not occurred. Observe the LEDs to confirm the failure.
Check the local and remote interface for misconfiguration (verify the latest change into device)
Swap/replace connected cable and verify with working one
3) Verifying the Signal coming from carrier/service provider for WAN interface case only:
In the output from the show interfaces command, observe the value of the carrier signal counters. The lost carrier counter shows the number of times that the carrier was lost during transmission. The no carrier counter shows the number of times that the carrier was not present during transmission. Carrier signal resets can occur when an interface is in loopback mode or shut down. Perform the following corrective actions if you observe the carrier signal counter incrementing outside of these conditions:
Check the interface for a malfunction.
Check for a cable problem.
3) Verifying Output Hang Status:
In the output of show interfaces command, observe the value of the output hang field.
The output hang provides the number of hours, minutes, and seconds since the last reset caused by a
lengthy transmission. When the number of hours in the field exceeds 24 hours, the number of days and hours is shown. If the field overflows, asterisks are printed. The field shows a value of never if no output hangs have occurred.
4) Verifying the CRC Counter:
In the output from the show interfaces command, observe the value of the CRC (cycle redundancy check ) counter. Excessive noise will cause high CRC errors accompanied by a low number of collisions. Perform the following corrective actions if you encounter high CRC errors:
Check the cables for damage.
Verify that the correct cables are being used for the SPA interface
5) Verifying Late Collisions:
In the output from the show interfaces command, observe the value of the late collision counter. Perform the following corrective actions if you encounter late collisions on the interface:
Verify the duplex mode on the local and remote interface match. Late collisions occur when there is a duplex mode mismatch.
Verify the length of the Ethernet cables. Late collisions result from cables that are too long.
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