Core issue
The user needs to understand Catalyst OS (CatOS) show port command output on Catalyst switches, including alignment, Frame Check Sequence (FCS), transmit and receive errors, runts, giants and various collisions counters.
Resolution
Many performance issues with switches can be related to data link errors. Excessive errors usually indicate a problem. When operating at a half-duplex setting, some data link errors such as FCS, alignment, runts, and collisions are normal.
Generally, a one percent ratio of errors to total traffic is acceptable for half-duplex connections. If the ratio of errors to input packets is greater than two or three percent, then the performance degradation may be noticed as a result of this configuration.
In half-duplex environments, it is possible for both the switch and the connected device to sense that the wire is not busy and transmit at exactly the same time, which results in a collision. Collisions can cause runts, FCS and alignment errors due to the frame not being completely copied to the wire. This results in fragmented frames being on the wire.
When operating at full-duplex, FCS, Cyclic Redundancy Checks (CRCs), alignment errors and runt counters should be minimal. If the link is operating at full-duplex, the collision counter is not active. If the FCS, CRC, alignment or runt counters are incrementing, this is usually the result of a duplex mismatch.
A duplex mismatch is a situation where the switch is operating at full-duplex and the connected device is operating at half-duplex or the other way around. Duplex mismatches can cause extremely slow performance, intermittent connectivity and a loss of connection. Other possible causes of data-link errors when configured for full-duplex are bad cables, faulty switch ports or Network Interface Card (NIC) software or hardware issues.
A speed mismatch is a situation where the switch is operating at a different speed when compared to the connected device. Speed determination issues may result in no connectivity. Symptoms of a failed link include FCS and alignment errors. A Network Interface Card (NIC) driver or misconfiguration of speed settings cause autonegotiation to fail.
When solving switch performance issues, view the output of the show port <mod#>/<port#> command.
To determine if the counter is incrementing, issue the show port command.
These are descriptions of the error counters and the possible causes of the errors.
To evaluate the show port command, refer to the Output Interpreter tool, as shown in this example:
| Counter | Description | Possible Causes |
| Alignment Errors (Align-Err) | Alignment errors are the number of frames received that do not end with an even number of octets and have a bad CRC. | Alignment errors are due to the frame not being completely copied to the wire, which results in fragmented frames. Alignment errors are the result of collisions at half-duplex, a duplex or speed mismatch, bad hardware (NIC, cable or port) or connected device generating frames that do not end with an octet and have a bad FCS. |
| FCS (FCS-Err) | 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. | FCS errors are the result of collisions at half-duplex, a duplex or speed mismatch, bad hardware (NIC, cable or port) or a connected device generating frames with bad FCS. |
| Xmit-Err | This indicates that the internal transmit buffer is full, which could lead to packet loss. | To verify this, issue the show counters <mod>/<port> command and paste the output into the Output Interpreter tool. |
| Rcv-Err | This is an indication that the internal receive buffer is full, which could lead to packet loss. | To verify this, issue the show counters <mod/port> command and paste the output into the Output Interpreter tool. |
| UnderSize | These are the total number of packets received that were less than 64 octets long (excluding framing bits, but including FCS) and have a good FCS value. | This is an indication of a bad frame generated by the connected device. Verify that the connected device is operating correctly. |
| Single Collisions | Single collisions are the number of times the transmitting port had one collision before successfully transmitting the frame to a media. | If this counter is incrementing, this is an indication that the ports are configured at half-duplex, change the duplex setting to full-duplex. |
| Multiple Collisions | Multiple collisions are the number of times the transmitting port had more than one collision before successfully transmitting a frame to the media. | If this counter is incrementing, this is an indication that the ports are configured at half-duplex, change the duplex setting to full-duplex. |
| 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 because 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 not detected by the transmitter until after the first 64-byte slot time. This is because they are only detected during transmissions of packets longer than 64 bytes. Its detection is exactly the same as for a normal collision, it just happens later when compared to a normal collision. | Late collisions are a result of incorrect cabling or a non-compliant number of hubs in the network. Bad NICs can also cause late collisions. |
| Excessive Collisions | The excessive collisions counter increases after 16 consecutive late collisions have occurred in a row. After 16 attempts have been made to send the packet, the packet is dropped and the counter increments. | If this counter is incrementing, it is an indication of a wiring problem, an excessively loaded network or a duplex mismatch. An excessively loaded network could be caused by having too many devices on a shared Ethernet. |
| Carrier Sense | Carrier sense occurs each time an Ethernet controller wants to send data. The counter is incremented when there is an error in the process. | Carrier sense errors are normal on half-duplex Ethernet segments. If the counter is incrementing on a full-duplex setting, then this is an indication of a faulty NIC, cable or switch port. |
| Runts | Runts are the number of frames received on a port that are smaller than 64 bytes and have a bad FCS value. | Runts are usually an indication of collisions, duplex mismatch, dot1q or Inter-Switch Link Protocol (ISL) configuration issues. If this counter is incrementing, it is an indication of poor cabling, a bad NIC, interference or a possible duplex mismatch. |
| Giants | Giants are frames that are greater than 1518 bytes and have a bad FCS value. | This is an indication of faulty hardware, dot1q or ISL configuration issues. |
