Core issue
The %CDP-4-DUPLEX_MISMATCH messages may be generated repeatedly when Cisco Discovery Protocol (CDP) has discovered a mismatch of duplex configuration between CDP neighbors.
This message occurs when a duplex mismatch is detected between CDP neighbors. It can also occur when switches are separated by a non-Cisco switch or hub. If CDP version 2 reports the link as a half-duplex when a port or interface on a CDP-enabled device has been configured or auto negotiated to full-duplex, the alternate device may not support CDP version 2. This causes the error to occur. It is a cosmetic issue.
CDP is useful for detecting errors as well as port and system statistics on neighboring Cisco devices. CDP is Cisco proprietary and works be sending packets to a well-known MAC address 01-00-0C-CC-CC-CC.
Resolution
To resolve this issue, perform these steps:
To disable CDP version 2, issue the no cdp advertise-v2 command.
To check the CDP version running on the device, issue the show cdp command.
For more information on the CDP-4-DUPLEX_MISMATCH error message, refer to the Error Message Decoder tool.
These Cisco bug IDs also deal with the %CDP-4-DUPLEX_MISMATCH error message:
CSCdu13457 - Duplex mismatch on Ethernet between VXR & CAT4003
CSCds50946 - CDP-4-DUPLEX_MISMATCH: Duplex mismatch messages flooding the log
CSCdp30476 - %CDP-4-DUPLEX_MISMATCH messages generated constantly
This message indicates a duplex mismatch on one side of the connection:
%CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on FastEthernet5/1 (not half duplex), with TBA05071417(Cat6K-B) 4/1 (half duplex).
By default, Cisco switch ports and interfaces are set to Auto-negotiate. You must match the speed and duplex settings on both sides by setting both sides to Auto-negotiate or by hardcoding.
Note: This error message also occurs in Switches running CatOS system software.
For more information on the causes of speed and duplex mismatches, refer to the Speed/Duplex, Autonegotiation, or NIC Issues section of Troubleshooting Switch Port and Interface Problems.
