Dennis HilkLinux Fix LabDE

Networking, DNS & Wi-Fi

Ethernet has no carrier or repeatedly loses link

Check physical Ethernet carrier and negotiated link mode before DHCP or DNS changes when a cable connection never becomes operational or keeps dropping.

Editorial source review: Start with the evidenceShare this guide
On this page
  1. Symptoms & scope
  2. Possible causes
  3. Diagnose safely
  4. Evidence-guided next steps
  5. References & review
  6. Related problems

Symptoms & scope

  • The interface is administratively up but has NO-CARRIER or link detected: no.
  • Repeated link-down/up events coincide with connection drops or speed renegotiation.

Relevant environment

Physical Ethernet NICs; virtual interfaces, bridges and optical links have different operational-state and module requirements.

Recognizable messages (synthetic examples)
e1000e 0000:00:19.0 enp0s25: NIC Link is Down

It can also occur during deliberate interface closure. Only unexplained drops correlated with loss of connectivity justify physical-link investigation.

Possible causes

These are possible explanations, not a confirmed diagnosis. Several independent faults can coexist.

  • Cable pairs, connectors, a switch port, power or a transceiver can prevent the lower-layer link from working.
  • An unsupported optical module or incompatible link-mode configuration can matter on particular NICs; lack of an IP address alone is not a carrier diagnosis.

Diagnose safely

Run one command at a time in the relevant session. Read the explanation first. Uppercase placeholders need your own values; tools and privileges vary by distribution. These commands are displayed here and never executed by the website.

Check 1

Replace IFACE with the physical Ethernet NIC. Read administrative flags, carrier-related flags, state and master membership without bringing it up or down.

ip -details link show dev IFACE

Interpret the result: UP means administratively enabled; LOWER_UP indicates the driver’s lower-layer signal. An interface enslaved to a bridge may receive its IP configuration on the bridge instead.

Evidence-guided next steps

Compare cable and switch port separately

If carrier is absent or flaps without intentional disconnects, compare one known-good appropriate cable on the same port, then a known-good switch port in a separate test. Check switch power and link indicators to determine whether the fault follows the path.

Precautions: Record VLAN and port policy before moving the cable; the alternate port may belong to another network. Physical changes interrupt active sessions.

Recovery / rollback: Reconnect the original cable and port if the comparison loses the expected VLAN or management path.

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References & review

This guide was prepared from primary project or distribution sources and reviewed on the date shown. This is an editorial source check, not evidence that a fix was reproduced on your hardware. Diagnostic log examples are synthetic fixtures. Version-dependent details must be checked against your installed release.

Related hardware and driver context

These profiles explain relevant driver paths. They do not establish that your device has this fault. Identify your device and binding first.

Identify hardware locally