Verifying Duplex Mismatch

To check the Duplex Mismatch:

1. Check the duplex setting on each end of the link to see if the values mismatch.

2. You can also watch for incrementing collision and late collision counters "#sh interfaces (interface)"

3. These Duplex mismatch are corrupt frames & no not pass the Error detection by the FCS field.

4. These corrupt frames changes the values in the FCS field while reach the destination device & FCS creates a CRC Error list which further changes the interface counters which is displayed in the "#sh interfaces (interface)" command.

   Types of Error Counters:

5. RUNTS:

   • Frames that did not meet the minimum frame requirement (64 bytes)

6. GIANTS:

   • Frames that did exceeded the maximum size required (1518 bytes)

7. INPUT ERRORS:

   • Sum of RUNT+GIANTS+NO BUFFER+CRC+FRAME+OVERRUN+IGNORED COUNT

8. CRC:

   • Received frame that did not pass the FCS Math causes the CRC Errors.

9. FRAME:

   • Received frame with illegal format (ending with partial byte)

10. PACKET OUTPUT:

    • Total number of frames forwarded out the interface

11. COLLISIONS:

    • Total number of collisions while the interface is transmitting a frame.

12. OUTPUT ERRORS:

    • Sum of Frames that the switch tried to transmit but errors occurred due to some problem.

13. LATE COLLISIONS:

    • Often points to a Duplex mismatch.

    • These collisions happen after the 64th byte after the frame has been transmitted (because in a perfectly working Ethernet LAN, collision should occur within the first 64 bytes only)

Reference:

CCNA 200-301 OCG, Volume 1, Pg. 166/167 - Wendell Odom.