Found this graphic of condensed definitions of error descriptions when running a command for interface gi1/1 as “show interfaces gi 1/1 counter errors” when troubleshooting a switches interface for errors.
A very good full explanation of all the different output can be found at the youtube channel I stole the graphic from (shame on me) at this link, so please go give it a watch (10 minutes long) for some really good detailed info.
Though the graphic does convey the full extent of the possible error causes!
I would highly advise looking over these definitions at least once to know all POSSIBLE causes for the counter errors shown in the output, as some of them have some rather unique possibilities that is very possible to see on exam day!
Though generally these will be the result of duplex mismatches, as duplex and speed of interfaces should be set to auto-detect on each side at all times, however here is the full listing of possible causes for errors for exam day and on the job:
- Align-Err – An alignment error occurs when frames do not end with an even number of octets, while simultaneously having a bad cycling redundancy check (CRC). An alignment error normally suggests a Layer 1 issue, such as cabling or port (Switchport or NIC) issues
- FCS-Err – A frame check sequence (FCS) error occurs when a frame has an invalid checksum, although the frame has no framing errors. Like the Align-err error, an FCS-err often points to a Layer 1 issue, but can also occur when there is a duplex mismatch
- Xmit-err – A transmit error occurs when a ports transmit buffer overflows. A speed mismatch between inbound and outbound links often results in a transmit error
- Rcv-Err – A receive error ocurs when a ports receive buffer overflows, which may be caused by congestion on the switches “Backplane” (switches physical port interconnection) could cause the buffer to fill to capacity, but is most likely caused by a duplex mismatch
- Undersize – An undersize frame is a frame with a valid checksum but a size less than 64 bytes. This issue suggests that a connected host is sourcing invalid frame sizes
- Single-Col – A Single-Col error occurs when a single collision occurs before a port successfully transmits a frame. Common reasons for a Single-Col error include high bandwidth utilization on an attached link or a duplex mismatch
- Multi-Col – A Multi-Col error occurs when more than one collision occurs before a port successfully transmits a frame. Similar to the Single-Col error, common reasons include high bandwidth utilization and duplex mismatch on attached link
- Late-Col – A late collision is a collision that is not detected until well after the frame has begun to be forwarded. While Late-Col error could indicate that the connected cable is too long, it is extremely common error for a mismatched duplex
- Excess-Col – The Excess-Col error occurs when a frame experiences 16 successive collisions, after which the frame is dropped. This error could result from high bandwidth utilization, a duplex mismatch, or too many devices on a segment
- Carrier-Sen – The Carrier-Sen counter is incremented when a port wants to send data on a half-duplex link. This is normal and expected on a half-duplex port, because the port is checking the wire to make sure that no traffic is present prior to sending a frame. This operation is the carrier sense procedure described the Carrier Sense Multiple Access with Collision Detect (CSMA/CD) operation used on half-duplex connections. Full-duplex connections however do not use CSMA/CD
- Runts – A runt is a frame that is less than 64 bytes in size and has a bad CRC. A runt could result from a duplex mismatch or a Layer 1 issue
- Giants – A giant is a frame size greater than 1518 bytes (assume that the frame is not a jumbo frame) that has a bad FCS. Typically a giant is caused by a problem with the NIC in an attached host. The jumbo frame has a frame size greater than 1518 bytes but has a valid FCS.
I would try to remember some of the more niche causes (cable too long, Giant vs Jumbo frame type, etc) for exam day.
That is it for this one, wanted this definitions readily available for exam day and on the job, until next time!