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What is the difference between SFP+ and SFP28 cable?
Knowledge Base + 2023.12.08

SFP+ and SFP28 are both types of small form-factor pluggable (SFP) transceivers used in networking equipment to connect to optical fibers. The main difference between SFP+ and SFP28 cables lies in their data transfer rates.

SFP+ cables have a maximum data transfer rate of 10 Gbps (gigabits per second), while SFP28 cables have a maximum data transfer rate of 25 Gbps. This means that SFP28 cables are capable of transmitting data at higher speeds than SFP+ cables, making them suitable for more demanding networking applications that require faster data transfer rates.

Can I use SFP+ transceivers on SFP28 ports?

SFP28 ports are backward compatible with SFP+ transceivers; however, the port can only support up to 10 Gbps network speed when connected to a SFP+ transceiver.

Small form-factor pluggable (SFP) network interface modules have distinct form factors, when an SFP+ transceiver is connected to an SFP28 port, the device automatically detects the inserted module's form factor and adjusts the network speed accordingly.

SFP28 ports are designed to support up to 25 Gbps, whereas SFP+ transceivers are designed to support data rates up to 10 Gbps.

QNAP recommends contacting the transceiver manufacturer prior to using SFP+ transceivers on SFP28 ports.

SFP28 vs SFP+, What is the difference?

Before we compare SFP28 and SFP+, let us see the table below.

What is SFP+?SFP+ is an enhanced SFP with higher speed. Its typical rate is 10Gbps, but it has extended variants like 8G and 16G. The SFP+ also shares the same appearance and size as the traditional SFP. Its specifications were defined in the SFF-8431 standards.

When discussing this term, the most words are SFP+ module and SFP+ port. For more detailed information, you may read our other post.

Generally speaking, The SFP+ module includes SFP+ transceivers, SFP+ DAC, and SFP+ AOC.

Take the SFP+ transceiver as an example. In most applications, the SFP+ transceiver typically supports 10G Ethernet and SDH/SONET STM64. Meanwhile, the Fibre channel SFP+ supports 8G or 16G data rates in the SAN application.

It is smaller than the earlier 10G XENPAK, 10G X2, and XFP modules. Therefore, it provides nearly double port density and has become the market’s most popular 10G transceiver.

What is SFP28?The SFP28 form factor is similar in size and appearance to the SFP+ form factor but supports a much higher speed of 28Gbps, hence the name “SFP28”.

Simply put, SFP28 is an enhanced version of SFP+ designed for 25G or 28G connectivity. It has the same form factor and appearance as the SFP+ but supports up to 28Gbps electrical interfaces per channel.

The SFP28 products are usually classified as SFP28 modules and SFP28 ports. Generally, the SFP28 module includes an SFP28 transceiver, SFP28 DAC, and AOC cable.

Unlike the SFP28 module, the SFP28 port is typically assembled in the networking switch and BBU device in the wireless station to accept the suitable transceiver.

Frequently Asked Question

Can I use SFP28 transceivers in SFP+ ports?

Compatibility of QSFP28 and SFP+ ports

QSFP and QSFP28 are the same physical form factor - the former supports 40G (or 4x10G) while the latter supports 100G (..or 4x25G / 2x50G). For the most part QSFP's will work in QSFP28 ports but the inverse is not true.

SFP, SFP+ and SFP28 also share a common form factor (although different than QSFP, obviously). SFP was the original 1G version while SFP+ is 10G. The modules supporting 25G are sometimes just generically referred to as "25G SFP" but are actually SFP28.

So - there is compatibility...for breakouts. As an example, four independent 10G SFP+ SR's can connect to a single 40G-SR QSFP (obviously only if the associated 40G Ethernet port supports breakouts) or a QSFP28 100G can connect to 4 SFP28 25G (assuming both breakout and FEC are supported).

Outside of this, however, you generally can't connect, say, a 1G SR optic to a 10G SFP+ SR or a 25G SFP28 to a 10G SFP+. In some limited cases within certain vendors some optics allowed for multiple speeds (ex: in the Cisco world some of the workgroup-class switches can support 100- or 1000- on a TX SFP) but this is highly uncommon in modern optics.

To your question - if your Mellanox has a QSFP28 port and you wanted to connect some 10GE devices then you'd need a QSFP 40GBase-SR4 adapter and an appropriate set breakout cables with an MPO connector running to 4 LC duplex connectors (this could be via mountable cassettes, structured cable or a pre-made patch cable). The LC would connect to 10GBase-SR SFP+'s on the downstream switches. You'd need to configure the Mellanox's port to run in breakout mode and then would configure the resulting 4 10GE interfaces independently.

The answer is yes, because SFP28 is backward compatible with SFP+ ports and can be fully functional. However, you should ensure the SFP28 transceiver supports a dual rate of 10G/25G, which says it is 10G/25GbE SFP28.

Can I use SFP+ Transceivers in SFP28 ports?

No, you can not use the SFP+ transceiver in the SFP28 port. The SFP28 port supports a 25G or 32GFC data rate, whereas the SFP+ transceiver typically supports a 10G data rate.

However, if the switch allows setting the port data rate to 10Gbps, you may use the SFP+ in the SFP28 port. The SFP28 port acts as a 10G SFP+ port in this case.

Is SFP28 compatible with QSFP+?

In all Cases, QSFP28 Optics Cannot Be Used on QSFP+ Port

SFP+ can’t auto-negotiate to support SFP module, similarly, QSFP28 modules can not be used on the QSFP port, either. There is the rule about mixing optical transceivers with different speeds—it basically comes down to the optic and the port, vice versa. Both ends of the two modules have to match and the form factor needs to match as well. Additionally, port speed needs to be equal to or greater than the optic used.