Troubleshooting of interfaces on Cisco Catalyst switches

When data transmission on the LAN fails, it is often difficult to determine whether it is a link fault or a port fault. Therefore, troubleshooting and isolation are required to fix faults. The following describes the simple judgment process:
Take the 3700/2900 series switch as an example. On the 3700/2900 switch, it is already an IOS operating system. If a problem occurs during port transmission, run the following command on the vswitch:
Sh controller Ethernet-controller <interface-number>
This command can help you determine whether the problem is caused by a switch port fault or a cable fault or an interface card error.
When you execute this command, if the number of errors increases continuously, it may be physical layer faults. You need to check the transmission media. Frame Check Sequence is one of the common errors on the Ethernet interface. It indicates that the Ethernet Frame received by the switch from the adjacent device is wrong with the CRC of the Frame. Check whether data frames are damaged during transmission. If the switch finds that the data frame has an exception of the FCS, it will discard the data frame and Add 1 to the port's FCS counter.
Another common error is alignment, which indicates that the data frame processed by the port of the switch is misplaced. This is usually a problem with the network port. Alignment Error indicates that the bit of the data frame received from the adjacent device is not a multiple of 8, and it is accompanied by an exception of the FCS. We know that the transmission of data frames is in bytes, and one byte is equal to 8 bits. Therefore, the total length of each data frame (in bits) should be a multiple of 8. If the data frame length is not a multiple of 8, the alignment error occurs. After the switch receives an alignment error, it will also discard the data frame and Add 1 to the alignment counter of the port.
The failure of the FCS and alignment usually indicate physical layer errors, such as cable faults, port faults, and nic faults, but may also be caused by mismatch of the duplex mode.
The following is the result of this command for your reference:
IOSwitch01 # sh controller Ethernet-controller fa0/1
Transmit Receive
5755681 Bytes 1335670 Bytes
78090 Frames 16070 Frames
75520 Multicast frames 19456789 fc s errors
1588 Broadcast frames 6011 Multicast frames
0 Pause frames 34 Broadcast frames
0 Single defer frames 0 Control frames
0 Multiple defer frames 0 Pause frames
0 1 collision frames 0 Unknown opcode frames
0 2-15 collisions 1000 Alignment errors
1 Late collisions 0 Length out of range
0 Excessive collisions 0 Symbol error frames
0 Total collisions 8 False carrier errors
0 Control frames 0 Valid frames, too small
0 VLAN discard frames 0 Valid frames, too large
0 Too old frames 0 Invalid frames, too small
72639 Tagged frames 0 Invalid frames, too large
1 Aborted Tx frames 0 Discarded frames

Transmit and Receive
5017 Minimum size frames
87837 65 to 127 byte frames
1030 128 to 255 byte frames
265 256 to 511 byte frames
0 512 to 1023 byte frames
10 1024 to 1518 byte frames
0 1519 to 1522 byte frames

The preceding command is used on a low-end switch. For a catalyst 6000 or catalyst 4000 Switch, you can run the 'show interface counters errors 'command to check the FCCs error or alignment error.