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:
Sense the maximum speed of the neighbor port (SW2's port) (100Mbps).
Will set it's own interface's speed to 100Mbps
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.
Reference:
CCNA 200-301 OCG, Volume 1, Pg. 140-141, 161, 165 - Wendell Odom.
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