Duplex Mismatch:

Duplex & Speed mismatch occurs when Autonegotiation fails because of Duplex&Speed mismatched settings.

• Because of IEEE Autonegotiation rule which says "IF AUTONEGOTIATION FAILS SET 10Mbps SETTING & FOR 10/100Mbps SET HALF DUPLEX SETTINGS "

  ONLY ETHERNET & FAST ETHERNET MAY FACE DUPLEX MISMATCH BUT GIGABIT & + PORTS WILL NEVER FACE IT.

  WHY IT IS NOT RECOMMENDED TO CHANGE DUPLEX/SPEED SETTINGS:

• The reason why it is not recommended to change the duplex/speed settings is because when you change the duplex/speed settings on a Switch (E.g. SW2) then that port's Manual Autonegotiation will stop working (as it is configured by an Admin Statically).

• And if you set Duplex Full BUT Speed 100 & the neighbor Switch (E.g. SW1) tries to Autonegotiate it won't receive any Autonegotiation replies & the SW1 will follow the default steps:

  1. Sense the maximum speed of the neighbor port (SW2's port) (100Mbps).

     • Will set it's own interface's speed to 100Mbps

  2. Will follow the IEEE Autonegotiation default Duplex Settings:

     • "On 10/100 ports - set Half Duplex setting"

• SW1 will put it's port to Half Duplex but SW2's port is manually in Full Duplex. This leads to DUPLEX MISMATCH

• If on one side Half Duplex is used & Full duplex on the other Full Duplex port by default will keep sending & receiving frames at the same time whereas Half Duplex by default who uses CSMA/CD will wait until the line is clear before sending the frame & realize collision is occurring & will back off.

• All these collisions & re-transmission are using up all the Bandwidth on the Half Duplex side. You may be able to PING but won't get full Bandwidth.

  (Verifying Duplex Mismatch)

Reference:

CCNA 200-301 OCG, Volume 1, Pg. 140-141, 161, 165 - Wendell Odom.