For multi-mode SFP module devices, since the wavelength of the multi-mode is in the range of visible light, we can see the red laser from the Tx port when we plug the SFP module into the SFP slot. When the connection does not work as expected after we set it up according to the Installation Guide, we need to do some troubleshooting. Optical modules are designed specifically for one type of fiber and are not interchangeable. You can identify single-mode and multi-mode modules using several clues, from simple visual checks to technical specifications. Here's a complete guide on how to identify the type of your. Identifying Single-Mode (SMF) vs. Precise verification prevents "Ghost Links" and Mode Field Diameter (MFD) mismatches that degrade 800G AI fabric performance. I used these 10GTek media converters.