Have you ever tried to plug an optic SFP+ transceiver into an SFP+ port to discover that the connection didn’t work, i.e. traffic was very slow or there was no data transmission at all? Did you manage to diagnose the problem and find a resolution? There are several possible reasons for failure. We’ve listed the five most common ones.

First of all, let’s briefly recap what SFP and SFP+ stand for. SFPs – short for ‘small form-factor pluggable’ – are compact, hot-pluggable devices that link networking devices, like switches, routers and servers. In this article, we focus on optic transceivers, as they’re called, which deliver 1Gbps of data across single-mode or multi-mode fibers. The SFP+ is an enhanced version of the SFP that supports data rates up to 10 Gbps. Now, the difference between SFP and SFP+ is an important one when troubleshooting: the transceivers are not always interchangeable.

TIP 1: Check whether you’re using SFP or SFP+ transceivers and slots

SFP and SFP+ modules look exactly the same. And as they have the same size, your SFP transceiver will fit seamlessly into an SFP+ switch port and vice versa. However, the connection won’t work as you expect it to. Or, worse even, it won’t work at all. If you plug an SFP device into an SFP+ port, the speed will be locked at 1 Gbps. Plugging an SFP+ module into an SFP port delivers no results at all, as the 10G transceiver can never auto-negotiate to 1Gbps.

TIP 2: Ensure that the SFPs have identical wavelengths at both ends 

Data transmission implies that data is sent from one end to another. The SFP+ transceiver on one end converts electrical signals into optical signals . A built-in laser transmits light through the fiber to the other side. Here, an optical diode converts the light back into an electrical signal. To guarantee that the SFP+ at the other end is capable of doing this, the SFPs at both ends should support the same wavelength. An 1310nm transceiver, for example, will not talk to an 850 nm transceiver.

→ Here, too, look at the specs on the sticker of the modules or check out the details on the manufacturer’s website. Don’t look into the laser light ! Use your smartphone camera if you want to verify that light is coming out of the cable.

TIP 3: Use the correct single or multi-mode fiber cable 

Still in trouble even though you are sure you did not mix up SFP and SFP+ and are supporting the same wavelengths at both sides? If so, then verify if the optical transceivers on each end use the same fiber type, i.e. for single-mode or multi-mode fiber. And use the corresponding fiber cable.

  • Single-Mode Fiber (SMF): featuring a narrow core (typically around 9μm), SMF allows only a single mode (or “ray”) of light to propagate. It is mostly used to transmit data over long distances (max 2km – 120km).
  • Multi-Mode Fiber (MMF): as MMF has a much wider core (typically 50μm or 62.5μm), it allows multiple modes of light to propagate. The common MMFs are used for short distance transmissions (max 100m – 500m)

The type of fiber can be identified by use of standardized colors on the outer jacket:

TIP 4: Are both ports compatible with your SFP+ modules?

Even when using compatible SFP+s at both ends of the right cable, it is key that both of your devices support SFP+. Make sure that the SFP+ ports on your devices are compatible with the SFP+ modules you want to use. Some brands allow you to use only their own modules.

TIP 5: Is your optic cable in good shape?

Fiber optic cables are exceptionally vulnerable. Dust, dirt or tampering might cause physical damage. So, if you’re experiencing problems when connecting devices, check the connector, the module, and the module slot to make sure they’re not damaged.
To avoid physical damage, avoid extreme bends in fiber optic cables when storing them and put dust-caps on your cable ends if you disconnect them.

In summary, make sure that you know what you are doing when plugging in SFP+ modules and fiber optic cables! It may look simple, but transceivers and slots are not always compatible. Always check the specs on the sticker of your transceiver/the slot, or verify the details on the manufacturer’s website. Only when done right, using fiber optic cables that are in good shape, will you be able to transmit data at the desired speed!



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