Troubleshoot › Error Decoder › %CDP-4-DUPLEX_MISMATCH

What does %CDP-4-DUPLEX_MISMATCH mean?

Duplex mismatch between two connected devices

%CDP-4-DUPLEX_MISMATCH: duplex mismatch discovered on GigabitEthernet1/0/1 (not half duplex), with Switch GigabitEthernet0/2 (half duplex)

Platform: Cisco IOS / IOS XE. Values shown are the documented defaults — confirm them on your platform with the check commands below.

What it means

The two ends of one cable disagree about how to talk. One side is running full duplex (send and receive at the same time) while the other runs half duplex (one direction at a time, like an old walkie-talkie). The link stays 'up' but performs terribly: the half-duplex side sees the other side's perfectly legal transmissions as collisions, backs off, and errors pile up. This is one of the classic 'link is up, network is slow' faults.

Likely causes, in order

• One end was hard-set (for example 'speed 100, duplex full') and the other end was left on auto-negotiation. A hard-set side stops advertising, so the auto side falls back to half duplex. This is the cause almost every time.
• An old device that genuinely cannot do full duplex was connected.
• A failing cable pair can occasionally corrupt negotiation, but check the settings first.

Check, in order

1. Copyshow interfaces Gi1/0/1 statusThe duplex and speed this side is actually running — compare both ends of the link.
2. Copyshow interfaces Gi1/0/1 counters errorsLate collisions and runts on the half-duplex side confirm the mismatch is doing damage.

The fix

• Best practice: set BOTH ends to auto-negotiation and let them agree — modern gear negotiates reliably.
• If a device truly needs a fixed setting, set the exact same speed and duplex on both ends. Never hard-set one side only.
• Re-check error counters after the change; they should stop climbing immediately.

Source: Cisco CDP and Ethernet negotiation documentation

More error messages