During data center upgrades, 100G aggregation switches and 25G servers, storage systems, or access switches often coexist for an extended period. In this case, a 100G QSFP28 port can be broken out into four independent 25G SFP28 links, improving port utilization and supporting a smoother network transition.

However, breakout deployment involves more than connecting an MPO breakout cable. Port capability, the optical architecture of the 100G module, fiber type, and FEC settings can all determine whether the links come up successfully.

Confirm That the Port Supports 4x25G Breakout

The host side of a QSFP28 interface typically contains four 25G electrical lanes, but not every switch port supports breakout mode. Before deployment, confirm:

• The target port supports 4x25G operation.
• The port must be manually switched from 100G mode to 4x25G mode, if required.
• The device clearly maps the four logical ports to the physical breakout branches.

If the port remains in 100G mode, the four 25G branches will usually not come up as independent links.

Which 100G Modules Can Be Broken Out?

Breakout support depends on the optical architecture of the module, not only on the QSFP28 form factor.

Three Common Deployment Options

1. QSFP28 SR4 to Four SFP28 SR Modules

This option is suitable for short-reach multimode fiber links. A 100G SR4 module connects through an MPO/MTP-to-4xLC duplex breakout cable to four 25G SFP28 SR interfaces. Check MPO polarity, fiber type, and lane mapping before deployment.

2. Parallel Single-Mode 100G to Four 25G Single-Mode Links

This option is used for longer single-mode links. The 100G module presents four independent optical lanes through an MPO interface, which are connected to four 25G single-mode modules through an MPO-to-4xLC duplex breakout assembly. In addition to reach, verify transmit power, receiver sensitivity, and the total link budget.

3. 100G-to-4x25G DAC or AOC

This is often the most direct option for in-rack or adjacent-rack connectivity. DAC is typically preferred for very short links where cost and power are the main priorities, while AOC is lighter and can simplify cabling and airflow management. In both cases, the switch port must still support 4x25G breakout mode.

Key Checks Before Deployment

1. Confirm that the switch port supports 4x25G breakout.
2. Use a 100G module with a parallel optical architecture.
3. Match the fiber type, connector type, and MPO polarity.
4. Verify the reach and optical power budget of the 25G modules.
5. Keep the port speed, FEC mode, and interface settings consistent at both ends.
6. Validate module and cable compatibility on the target equipment before volume deployment.

Common causes of link failure include leaving the switch port in 100G mode, using a non-breakout-capable LR4 or CWDM4 module, incorrect MPO polarity, and mismatched FEC settings.

How to Choose

Breaking out one 100G port into four 25G links is well suited to networks where 25G access and 100G aggregation coexist. The key is not the cable alone, but whether the port mode, module architecture, fiber system, and device settings are properly matched.

ETERN Optoelectronics provides 25G SFP28 transceivers, 100G QSFP28 parallel optical modules, and related high-speed optical interconnect products for different reaches and cabling environments.