How to Connect QSFP to SFP Port? - Store.QSFPTEK

How to Connect QSFP to SFP Port? - Multilane Transceivers, QSA or Direct Attach Splitter Cables

The high-speed and high-density application drives the growth of QSFP port deployments on system equipment, such as 40G QSFP+ ports and 100G QSFP28 ports on switches and server network interface cards (NICs). However, the legacy devices are in 10Gb or gigabit access. How to migrate your network to 40G or 100G while reusing the existing 10G network infrastructure? That means you need to connect the QSFP to SFP port.

Case Background:

Two switches are located in adjacent layers on the same rack, one spine switch has 40G QSFP+ ports only, and the other switch is a gigabit switch with 4 SFP+ ports. How to connect the QSFP+ port to the SFP+ ports? This article will introduce three solutions to this problem.

Solution 1: Using QSFP to SFP Adapter (QSA) Module

QSFP to SFP adapter (QSA) is a transceiver with the integrated function of QSFP to SFP conversion. The converter module has a QSFP interface at the finger duct end for inserting into the QSFP port on a switch or a network interface card (NIC), and an SFP connector at the optical end for connecting to the outward SFP form factor optics. 

using 40G QSFP+ adapter for QSFP+ to SFP+ conversion

Product Preparation:

  1. A QSFP+ to SFP+ adapter module
  2. Two 10GBASE-SR SFP+ modules
  3. A matched multimode fiber patch cable: OM3 or OM4 MMF

You may be wondering how the quad lane QSFP converts to a single lane SFP. The QSA adapter is mechanically designed with four channels on the QSFP connector, however, only one channel is connected to the SFP receptacle. The remaining three channels will keep in silent. 

QSA Types

Division by connector type, the QSA adapter modules come with QSFP+ to SFP+ adapter (QSA+) and QSFP28 to SFP28 adapter (QSA28). 

The 40G to 10G QSFP+ adapter provides the option to link 10G SFP+ transceivers or DAC/AOC cables to the 40G QSFP+ port of switch or NIC. 100G to 25G QSFP28 adapter enables 25G SFP28 transceivers or cables to connect to the 100G platform port. Noted that QSA-40 also supports 1G SFP optics and QSA-100 also supports 10G SFP+ and 1G SFP optics in most cases.

QSFP to SFP adapter is either with EPROM configuration or without EPROM in I2C design. EPROM offers the adapter the self-ability to tell the host device the transceiver information, which is also known as DOM/DDM. Otherwise, the host will rely on the SFP transceiver that is connected to the QSA adapter to read DOM. 

QSA Advantages

  • Zero power consumption due to QSA is passive
  • Few attenuation signal loss and Low latency & crosstalk due to design
  • 10G/1G or 25G/10G/1G connectivity option for 40G or 100G only equipment
  • Seamless migration to 40G or 100G while having lower-speed access
  • Wide compatibility with 10G SFP+/1G SFP or 25G SFP28/10G SFP+/1G SFP transceivers and cables that compliant with SFP MSA

Solution 2: Using QSFP Multichannel Transceivers and Harness Fiber Cable

Compared to the SFP module, the Quad Small Form-factor Pluggable (QSFP) transceivers have four bi-directional channels, each channel integrates the capability of receiving and transmitting signals. While in breakout mode, the QSFP can separate into four parallel channels with each channel carrying ¼  of the total bandwidth. Therefore the multilane QSFP transceiver can be used for QSFP to SFP connection. 

using 40G QSFP+ transceiver and mtp to 4 LC breakout fiber cable for QSFP+ to SFP+ connectivity

Product Preparation:

  1. A 40GBASE-SR4 QSFP+ module
  2. Four 10GBASE-SR SFP+ modules
  3. A matched multimode harness fiber cable, such as OM4 MTP to 4 LC breakout MMF

QSFP Transceiver Types

As applications for higher bandwidth always exist, the original QSFP transceivers have developed into multiple variants. By form factors, there are QSFP, QSFP+ and QSFP28, QSFP56, and the latest derived QSFP-DD. 

Breakdown by single-lane data rate, QSFP breakout mode is most used for 4G QSFP to 4x 1G SFP, 40G QSFP+ to 4x 10G SFP+, 100G QSFP28 to 4x 25G SFP28, etc. 

QSFP Transceiver Limitations

  • A fraction of QSFP transceivers are not capable of breakout transceivers, such as BiDi QSFP, transceivers using multiple wavelengths in simplex fiber, transceiver breakout is banded by ASIC.
  • Some switch and NICs QSFP ports don’t support breakout mode

Solution 3: Using QSFP to SFP Breakout Direct Attach Cable

QSFP to SFP Direct Attach cable has pretermitted a QSFP connector at one end of the cable, and at the other end, the cable has split into four strands with an SFP connector terminated at each end. 

Using QSFP to SFP Breakout Direct Attach Cable

Product Preparation:

  1. A QSFP+ to SFP+ DAC cable, or
  2. A QSFP+ to SFP+ AOC cable

QSFP to SFP Breakout Direct Attach Cable Types

Considering cable transmission media, QSFP splitter cables have the option of fiber or copper, which is the Active Optical Cable (AOC) cable or direct attach copper (DAC) cable. 

Considering connector type, QSFP to SFP breakout cables are popular for 40G QSFP+ to 4x SFP+ splitter cable and 100G QSFP28 to 4x SFP28 splitter cable, both available for breakout DAC and AOC cables.

QSFP to SFP Breakout Direct Attach Cable Types

Breakout High-Speed Cale Advantages

  • Plug and play, no extra installation 
  • Simple maintenance
  • Low power consumption
  • Cost-effective
  • Higher reliability (no risk of optical interface pollution)

Breakout High-Speed Cale Limitations:

  • Reach Distance is restricted for short-distance connectivity


QSFP to SFP adapter, breakout capable multilane QSFP modules, and direct-attach breakout cable are three products to realize QSFP to SFP port connectivity. Specific cases may be 40G QSFP+ to SFP+ conversion, and 100G QSFP28 to SFP28 conversion between the switch to switch or switch to server cards.

The QSA adapter solution has the optimal interoperation for wide-range compatibility with most transceivers. Direct-attach breakout cable is the most cost-effective and easy solution but is limited for short-reach connections. Transceivers with harness fiber patch cable combination offer flexibility but are costly. Take application requirements such as transmission distance, existing cable media, and budget into consideration for making the right choice.

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