Duplex mismatch: late collisions / CRC errors Networking Origin: Network OSI layer 2 · Data Link Last updated 5 Nov 2026

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

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

Exact message you may see Duplex mismatchlate collisions on interfaceCRC errors input errorsHalf-duplex on a full-duplex link
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Seen on: Cisco Linux Networking

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?

🔧 Still not fixed?

Many errors look alike. If the steps above didn’t solve it, one of these is probably what you’re facing:

Where this error sits

Origin: Network. Errors that are often confused with or caused by this one (click to open):

arp -a shows (incomplete) — Networking: `arp -n` shows (incomplete) / Destination Host Unreachable — no ARP reply from the next hop (Networking)arp -a shows (incompl…High TCP retransmissions / packet loss — Networking: netstat -s: segments retransmitted / Wireshark: TCP Retransmission, TCP Dup ACK — packet loss on the path (Networking)High TCP retransmissi…MAC flapping / broadcast storm (spanning tree loop) — Networking: %SW_MATM-4-MACFLAP_NOTIF: Host is flapping between ports / broadcast storm (Networking)MAC flapping / broadc…Native VLAN mismatch / wrong VLAN, no connectivity — Networking: %CDP-4-NATIVE_VLAN_MISMATCH / device on the wrong VLAN can't reach the gateway (Networking)Native VLAN mismatch …Network cable unplugged / NO-CARRIER — Networking: Network cable unplugged / `ip link` shows NO-CARRIER state DOWN / Ethernet: Network cable unplugged (Networking)Network cable unplugg…Port err-disabled (psecure-violation) — Networking: %PM-4-ERR_DISABLE: psecure-violation error detected on Gi0/5, putting Gi0/5 in err-disable state (Networking)Port err-disabled (ps…SFP link down / Loss of Signal (LOS) — Networking: Interface down — SFP transceiver Loss of Signal / no receive light (Networking)SFP link down / Loss …Duplex mi…

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