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, performance degradation can be noticed.
In half−duplex environments, it is possible for both the switch and the connected device to sense the wire and transmit at exactly the same time and result in a collision. Collisions can cause runts, FCS, and alignment errors, caused when the frame is not completely copied to the wire, which results in fragmented frames.
When operating at full−duplex, FCS, cyclic redundancy checks (CRC), alignment errors, and runt counters are probably minimal. If the link operates at full−duplex, the collision counter is not active. If the FCS, CRC, alignment, or runt counters increment, check for a duplex mismatch. Duplex mismatch is a situation in which the switch operates at full−duplex and the connected device operates at half−duplex, or the other way around. The result of a duplex mismatch is extremely slow performance, intermittent connectivity, and loss of connection. Other possible causes of data link errors at full−duplex are bad cables, a faulty switch port, or NIC software or hardware issues.
When you troubleshoot NIC performance issues, view the output of the show port mod/port command and the show mac mod/port command, and note the counter information.
Table 2Explanation of CatOS show port Command Counters
Counter | Description | |
| ||
Alignment | Alignment errors are a count of the number of | |
Errors | ||
frames received that do not end with an even | ||
| ||
| number of octets and have a bad CRC. | |
| FCS error count is the number of frames that were | |
FCS | transmitted or received with a bad checksum (CRC | |
| value) in the Ethernet frame. These frames are | |
| dropped and not propagated onto other ports. | |
|
| |
Xmit−Err | This is an indication that the internal transmit | |
| buffer is full. | |
|
| |
Rcv−Err | This is an indication that the receive buffer is full. | |
| ||
UnderSize | These are frames that are smaller than 64 bytes, | |
| which includes FCS, and have a good FCS value. | |
|
| |
Single | Single collisions are the number of times the | |
Collisions | ||
transmitting port had one collision before | ||
| ||
| successfully transmitting the frame to the media. | |
Multiple | Multiple collisions are the number of times the | |
Collisions | ||
transmitting port had more than one collision before | ||
| ||
| successfully transmitting the frame to the media. | |
Late | A late collision occurs when two devices transmit | |
Collisions | 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 sends until after it puts the entire packet | |
| on the network. Late collisions are detected by the | |
| transmitter after the first time slot of the | |
|
|