25G SFP28 and 100G QSFP28 in a Leaf-Spine Fabric

The topology in one paragraph

A leaf-spine fabric has two layers. Leaf switches connect to servers. Spine switches connect only to leaves. Every leaf connects to every spine, which is what gives the fabric its predictable, low-latency behaviour: any server can reach any other server in exactly the same number of hops. The optics question is simply how many lanes at what speed cross each of those two connection types.

Where 25G SFP28 goes

Leaf-to-server links are usually 25G. SFP28 is the natural fit because it carries a single 25G lane in the same physical footprint as 10G SFP+, letting an operator quadruple per-port bandwidth without changing the cable plant design.

Reach classes:

| Class | Wavelength | Distance | Typical use | |---|---|---|---| | SFP28 SR | 850 nm | 100 m | Within-row, multimode | | SFP28 LR | 1310 nm | 10 km | Cross-row, structured single-mode | | SFP28 ER | 1310/1550 nm | 40 km | Campus / inter-building |

Where 100G QSFP28 goes

Leaf-to-spine links are usually 100G, typically via QSFP28. The four common variants differ mainly in reach and fiber:

• QSFP28 SR4 — 100 m over multimode, MPO connector, eight fibers.
• QSFP28 CWDM4 — 2 km over single-mode duplex LC. Popular for cross-row runs where duct space matters.
• QSFP28 PSM4 — 500 m to 2 km over single-mode with MPO.
• QSFP28 LR4 — 10 km over single-mode duplex LC, the standard choice for longer spine runs and inter-building links.

Breakout: the cost lever most teams underuse

A single 100G QSFP28 port can be split into four 25G SFP28 links with a breakout cable or a breakout-capable module. In practice this means:

• One 100G port can serve four 25G servers.
• Spine ports can be used at 100G while leaf downlinks stay at 25G.
• You avoid buying switches with ports you cannot fill.

The same logic applies at 400G, where one QSFP-DD port can break out to 4×100G.

Oversubscription: plan it, do not discover it

Oversubscription is the ratio between the total bandwidth a leaf switch offers to servers and the total bandwidth it has toward the spine. A common design targets 3:1. Whatever you choose, calculate it before ordering optics — the ratio determines how many 100G spine links each leaf needs, and therefore how many QSFP28 modules you buy.

Canjin Technology supplies SFP28 and QSFP28 modules with full specification tables and supports breakout configurations on request.