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

**Code:** `SFP link down / Loss of Signal (LOS)` · **Category:** Networking · **Seen on:** Linux, Networking

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

## 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?

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Source: [WhatIsMyError](https://whatismyerror.com/errors/networking/sfp-link-down-no-light)