CSMA/CD Collision Backoff Ranges Retry

Cisco switches monitor various collision counters, as follows:

• Single

• Multiple

• Late

• Excessive

Of the four types, be wary of late and excessive collisions. Late collisions occur when two devices send data at the same time. Unlike single and multiple collisions, late collisions cause packets to be lost. Late collisions are usually indicative of the cable exceeding IEEE specifications. Cascading hubs (connecting two or more hubs to each other) can also cause the length of the collision domain to increase above specification. You can use a Time Delay Reflectometer (TDR) to detect cable fault and whether the cable is within the IEEE standard. Other factors that cause late collisions include mismatched duplex settings and bad transceivers. Example 1-1 shows the output from a switch that has detected a late collision on one of its ports.