SFP28 vs QSFP28: 25G Access and 100G Aggregation Interface Selection

SFP28 and QSFP28 serve distinct roles in a network. SFP28 carries a single 25Gbps lane and is used for server access, Top-of-Rack connections, and 5G fronthaul. QSFP28 aggregates four 25Gbps lanes into 100Gbps and is used for uplinks, aggregation, leaf-spine fabric, and 4x25G breakout. The two form factors are physically incompatible, and the selection should be driven by the port type, lane architecture, breakout requirements, power budget, and the network layer being designed.

1. What Is SFP28?

SFP28 is a single-lane 25Gbps optical module form factor built on the SFP+ physical footprint. It carries one 25Gbps electrical lane using NRZ modulation and is widely deployed for 25G server access, Top-of-Rack switching, enterprise aggregation, storage networking, and 5G fronthaul[reference:0].

SFP28 modules typically consume around 1.5W to 2W depending on reach and optics type, with short-reach and AOC variants often below 1W and longer-reach single-mode variants approaching 2W[reference:1][reference:2]. The form factor is backward compatible with SFP+ cages at 10G, but an SFP+-only port generally cannot operate an SFP28 module at 25G.

In 5G networks, SFP28 is the workhorse for eCPRI and O-RAN fronthaul over 25GbE, providing the symmetric uplink and downlink capacity required for centralized and cloud RAN architectures[reference:3].

2. What Is QSFP28?

QSFP28 stands for Quad Small Form-factor Pluggable 28. It combines four 25Gbps lanes in a single module to deliver 100Gbps aggregate bandwidth. QSFP28 is the mainstream 100G interface for switch uplinks, aggregation layers, leaf-spine fabric, and high-density 100G links in cloud and enterprise data centers[reference:4][reference:5].

QSFP28 modules typically consume around 3W to 5W depending on reach and optics type, with short-reach variants near 3.5W and longer-reach ER-class optics approaching 5W[reference:6][reference:7]. While the per-module power is higher than SFP28, a single 100G QSFP28 link can replace four 25G SFP28 links at the uplink level, which often improves power efficiency per 100G of bandwidth.

A defining capability of QSFP28 is breakout support. A single 100G QSFP28 port can be split into four independent 25G interfaces using a breakout cable and compatible SFP28 modules on the downstream end. This 100G-to-4x25G breakout is common in leaf-spine architectures, where spine switches use QSFP28 ports to connect to multiple leaf switches with 25G server access[reference:8][reference:9].

3. Key Specifications

4. Physical Incompatibility and Breakout Adapters

5. Power, Density, and Cost Perspective

A published comparison notes that a direct QSFP28 100G link can be more cost-effective and significantly more power-efficient than four separate SFP28 25G links for the same 100G aggregate, with reported figures around 5.5W for QSFP28 versus roughly 14W for 4×SFP28[reference:12]. Actual values depend on the specific modules, reach, and platform design.

6. Application Fit

6.1 SFP28 for 25G Access and Fronthaul

SFP28 is the default 25G interface for server access, Top-of-Rack switching, storage networking, virtualized clusters, and 5G fronthaul. In a data center, servers connect to leaf switches through SFP28 ports at 25G. In 5G, SFP28 carries eCPRI and O-RAN fronthaul traffic over 25GbE, supporting the bandwidth and latency requirements of centralized and cloud RAN[reference:13].

6.2 QSFP28 for 100G Aggregation and Leaf-Spine

QSFP28 is used for switch uplinks, aggregation layers, and spine-leaf fabric. In a typical leaf-spine design, leaf switches provide 25G SFP28 server access and use QSFP28 uplinks at 100G to connect to spine switches. Spine switches use QSFP28 ports for 100G interconnect and can break out those ports into 4 × 25G to connect multiple leaf switches efficiently[reference:14][reference:15].

6.3 Breakout and Mixed Design

QSFP28 breakout allows one 100G port to serve four 25G downstream connections through a breakout cable and SFP28 modules at the far end. This is common in leaf-spine fabrics and in racks where a 100G uplink is distributed to multiple 25G server ports. The switch must support breakout mode, and the cabling and optics must be matched to the breakout configuration.

7. Selection Guidance

Start with the port type on the switch or NIC and the required per-port speed. For 25G server access, storage, or fronthaul, SFP28 provides a low-power, cost-effective solution with a mature optics portfolio. For 100G uplinks, aggregation, or spine interconnect, QSFP28 provides the required bandwidth and can support 4 × 25G breakout to connect downstream 25G devices.

Verify whether the switch port supports breakout, the correct breakout cable or adapter is available, and the far-end optics are compatible. Do not attempt to insert an SFP28 module into a QSFP28 cage or vice versa. Use the switch datasheet, optics compatibility list, and breakout configuration guide as the basis for selection.

8. Frequently Asked Questions

9. Conclusion

SFP28 and QSFP28 are complementary interfaces at different levels of the network. SFP28 provides single-lane 25G connectivity for server access, ToR, storage, and 5G fronthaul. QSFP28 aggregates four 25G lanes into 100G for uplinks, aggregation, spine-leaf fabric, and breakout applications. They are not physically interchangeable, but they can be connected through breakout cables and adapters. The correct choice depends on the port type, required bandwidth, breakout needs, power budget, and network layer.