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The Cisco Discovery Protocol displays the "%CDP-4-DUPLEX_MISMATCH" error message on Catalyst fixed configuration switches running IOS software.
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.