Explanation of CatOS show port command counters

Counter	Description	Possible causes
Alignment Errors	Alignment errors are a count of the number of frames received that do not end with an even number of octets and have a bad CRC.	These are the result of collisions at half-duplex, duplex mismatch, bad hardware (NIC, cable, or port), or a connected device generating frames that do not end with on an octet and have a bad FCS.
FCS	FCS error count is the number of frames that were transmitted or received with a bad checksum (CRC value) in the Ethernet frame. These frames are dropped and not propagated onto other ports.	These are the result of collisions at half-duplex, duplex mismatch, bad hardware (NIC, cable, or port), or a connected device generating frames with bad FCS.
Xmit-Err	This is an indication that the internal transmit buffer is full.	This is an indication of excessive input rates of traffic. This is also an indication of transmit buffer being full. The counter should only increment in situations in which the switch is unable to forward out the port at a desired rate. Situations, such as excessive collisions and 10 Mb ports, cause the transmit buffer to become full. Increasing speed and moving the link partner to full-duplex should minimize this occurrence.
Rcv-Err	This is an indication that the receive buffer is full.	This is an indication of excessive output rates of traffic. This is also an indication of the receive buffer being full. This counter should be zero unless there is excessive traffic through the switch. In some switches, the Out-Lost counter has a direct correlation to the Rcv-Err.
UnderSize	These are frames that are smaller than 64 bytes (including FCS) and have a good FCS value.	This is an indication of a bad frame generated by the connected device.
Single Collisions	Single collisions are the number of times the transmitting port had one collision before successfully transmitting the frame to the media.	This is an indication of a half-duplex configuration.
Multiple Collisions	Multiple collisions are the number of times the transmitting port had more than one collision before successfully transmitting the frame to the media.	This is an indication of a half-duplex configuration.
Late Collisions	A late collision occurs when two devices transmit at the same time and neither side of the connection detects a collision. The reason for this occurrence is that the time to propagate the signal from one end of the network to another is longer than the time to put the entire packet on the network. The two devices that cause the late collision never see that the other is sending until after it puts the entire packet on the network. Late collisions are detected by the transmitter after the first time slot of the 64-byte transmit time occurs. They are only detected during transmissions of packets longer than 64 bytes. Its detection is exactly the same as it is for a normal collision; it just happens later than it does for a normal collision.	This is an indication of faulty hardware (NIC, cable, or switch port) or a duplex mismatch.
Excessive Collisions	Excessive collisions are the number of frames that are dropped after 16 attempts to send the packet resulted in 16 collisions.	This is an indication of over utilization of the switch port at half-duplex or duplex mismatch.
Carrier Sense	Carrier sense occurs every time an Ethernet controller wants to send data and the counter is incremented when there is an error in the process.	This is an indication of faulty hardware (NIC, cable, or switch port).
Runts	These are frames smaller than 64 bytes with a bad FCS value.	This is an indication of the result of collisions, duplex mismatch, IEEE 802.1Q (dot1q), or an Inter-Switch Link Protocol (ISL) configuration issue.
Giants	These are frames that are greater than 1518 bytes and have a bad FCS value.	This is an indication of faulty hardware, dot1q, or an ISL configuration issue.