Runts and CRC errors increment in the show interface command output on a Cisco IOS-based switch, or FCS errors and runts increment in the show port command output on a CatOS based switch due to a duplex mismatch
If runts and Cyclic Redundancy Check (CRC) errors are incrementing in the show interface command output, or Frame Check Sequence (FCS) errors and runts are incrementing in the show port command output, check the port speed and duplex settings.
Ensure that both devices are configured identically. Both sides should be set to auto speed and auto duplex, or hard set to the same speed and duplex settings.
These are explanations of CRC errors and runts:
CRC errors indicate that the cyclic redundancy checksum generated by the originating LAN device does not match the checksum calculated from the data received. On a LAN, this usually indicates noise or transmission problems on the LAN interface or the LAN bus itself. A high number of CRCs is usually the result of collisions or a station transmitting bad data.
Runts are Ethernet packets that are less than 64 bytes and may be caused by excessive collisions.
On Cisco IOS -based switches, the interface configuration commands to set the speed and duplex to auto are speed auto and duplex auto.
To hard set the speed and duplex settings, issue the duplex and speed commands specifying the desired settings. For example, to set the interface to 100/full, issue the speed 100 and duplex full commands. Never mix a hard set and an auto command on the same interface. For example, the speed auto and duplex full commands should never be issued on the same interface.
On Cisco set-based CatOS switches, there is only one command used to change the port speed and duplex settings to auto. To change the port setting back to auto issue the set port speedmod/port auto command. To hard set the speed and duplex settings, issue the set port speed mod/portspeed and set port duplexmod/portduplex commands. For example, to set port 4/1 to 100/full, issue the set port speed 4/1 100 and set port duplex 4/1 full commands.
In CatOS there is not a command to change the duplex setting to auto, so.
If the speed and duplex settings are correct, yet the CRC or runt counters continue to increase, perform these steps:
Verify that the connected Network Interface Card (NIC) is good and has the latest drivers.
Verify that the cable connecting the devices is good, using a cable tester or by replacing it with a known good cable.
Verify that no interference is occurring on the line.
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