SFP link down / Loss of Signal (LOS) Networking Origin: Network OSI layer 1 · Physical Last updated 5 Nov 2026

Networking: Interface down — SFP transceiver Loss of Signal / no receive light

A fibre or SFP link is down because the transceiver receives no light.

Exact message you may see SFP link downLoss of Signal LOSRx power -40 dBmInterface down: no light%PM-4-ERR_DISABLE: Unsupported SFP
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Seen on: Linux Networking

Meaning

Clean and reseat both SFPs, check the fibre polarity (TX goes to RX), confirm both ends use the same wavelength and fibre type (SR vs LR, multimode vs single-mode) and read the optical power with show interfaces transceiver or ethtool -m. Replace a failed transceiver or damaged patch cable.

Common causes

  • Dirty or damaged fibre connector
  • TX/RX polarity swapped
  • Mismatched SFP type, wavelength or fibre (SR on single-mode)
  • Failed or unsupported transceiver (vendor lock)
  • Fibre cut or too much attenuation

⚡ Quick fix

  1. Clean connectors and reseat the SFPs
  2. Swap TX/RX at one end
  3. Read optical power: ethtool -m eth1 or show interfaces transceiver

Detailed fix by platform

Linux

  1. sudo ethtool -m eth1 | grep -iE "rx power|tx power|laser"

How to diagnose

  1. Rx power — Within range?
  2. Polarity — TX to RX?
  3. SFP types — Matching?

🔧 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…Cannot assign requested address (EADDRNOTAVAIL) — Networking: connect: Cannot assign requested address — ephemeral port range exhausted by TIME_WAIT connections (Networking)Cannot assign request…Duplex mismatch: late collisions / CRC errors — Networking: Duplex mismatch — late collisions, runts and CRC errors on the switch port (Networking)Duplex mismatch: late…Many sockets in CLOSE_WAIT — Networking: Thousands of sockets in CLOSE_WAIT — the application is not closing its side of the connection (Networking)Many sockets in CLOSE…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…Port forwarding not working (CGNAT / double NAT) — Networking: Port forwarding configured but the port stays closed — carrier-grade NAT (CGNAT) or double NAT (Networking)Port forwarding not w…SFP link …

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