# Networking: Duplex mismatch — late collisions, runts and CRC errors on the switch port

**Code:** `Duplex mismatch: late collisions / CRC errors` · **Category:** Networking · **Seen on:** Cisco, Linux, Networking

One side runs full duplex and the other half duplex, so the link works but is slow and drops packets.

## Meaning

Auto-negotiation failed or one end was hard-coded. Set both ends to auto (preferred) or both to the same speed and duplex, and check counters: late collisions and CRC/FCS errors rising on the half-duplex side show the mismatch. Replace flaky cables that fail negotiation.

## Common causes

- One side hard-coded, the other on auto
- Old or faulty NIC/driver
- Bad cable causing failed negotiation
- Cheap unmanaged switch with a fixed port
- Mismatched speed settings

## Quick fix

1. Set both ends to auto-negotiation
2. Or hard-set the same speed/duplex on both
3. Read counters: `ethtool -S eth0 | grep -iE "crc|collision"`

## Detailed fix by platform

### Linux

1. `sudo ethtool eth0 | grep -E "Speed|Duplex|Auto-negotiation"; ethtool -S eth0 | grep -iE "crc|collis|align"`

## How to diagnose

1. **Speed/duplex** — Same both ends?
2. **Auto-negotiation** — Enabled?
3. **CRC errors** — Rising?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/networking/duplex-mismatch-late-collisions-crc)