Which interface counter can you use to diagnose a duplex mismatch problem?

3 thoughts on “Which interface counter can you use to diagnose a duplex mismatch problem?”

1. FROM WIKI: ” In normal half-duplex operations late collisions do not occur. However, in a duplex mismatch the collisions seen on the half-duplex side of the link are often late collisions. The full-duplex side usually will register frame check sequence errors, or runt frames”.

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2. The answer is B.

   Late Collisions: The subset of all collisions that happen after the 64th byte of the frame
   has been transmitted. (In a properly working Ethernet LAN, collisions should occur within the first 64 bytes; late collisions today often point to a duplex mismatch.)

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3. 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.

   https://community.cisco.com/t5/networking-documents/certain-counters-rapidly-increase-in-the-show-port-command/ta-p/3126210

   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.

   https://www.cisco.com/c/en/us/support/docs/interfaces-modules/port-adapters/12768-eth-collisions.html#topic4

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