A transceiver form factor defines the mechanical and electrical interface between the pluggable module and the switch port. Not all ports accept all transceivers. Getting this wrong before you order hardware means transceivers that physically cannot be installed.
Transceiver form factors — select to compare
Form factor determines which cage a module fits in and which devices are compatible
No backward compatibility — purpose-built cage only accepts OSFP 400G modules.
The form factor hierarchy
QSFP28 supports 100G using four lanes at 25G each (NRZ modulation). This was the dominant form factor through the 100G era. A QSFP28 port is electrically identical to a QSFP+ port (40G) -- full backward compatibility.
QSFP56 supports 200G using four lanes at 50G each (PAM-4). Some 200G switch deployments use this. Backward compatible with QSFP28 and QSFP+.
QSFP-DD (Double Density) supports 400G or 800G using eight lanes. The extra four lanes (vs QSFP56) come from a second row of electrical contacts in the connector -- the "double density" in the name. QSFP-DD is backward compatible with QSFP28, QSFP56, and QSFP+. This backward compatibility is its primary advantage -- a switch bought today with QSFP-DD ports can accept 100G modules from existing inventory while being ready for 400G and 800G upgrades.
OSFP (Octal Small Form-factor Pluggable) was designed specifically for the 800G era and higher. It is physically larger than QSFP-DD and has a higher power budget (roughly 20W vs 12W for QSFP-DD). OSFP is not backward compatible with QSFP form factors. OSFP-XD (OSFP Extended Density) is the 2026-era form factor for 1.6T links.
The DGX H100 port identity
Every DGX H100 node has eight OSFP ports -- one per ConnectX-7 NIC. The switch connecting these nodes (the SN5600 leaf switch in NVIDIA's reference design) also has OSFP ports. This is not a coincidence -- NVIDIA designed the H100 DGX system and the SN5600 to use the same form factor, enabling a single cable type to connect server to switch across all eight rails.
This also means that when NVIDIA says "the same OSFP ports are used for both InfiniBand and Ethernet operation" -- they mean the same physical port, same transceiver, same cable. The mode is set in firmware on the ConnectX-7. This is the physical reality underlying Chapter 0's "same cable for IB and RoCEv2" statement.
Power budgets and thermal consequences
Form factor choice directly determines power consumption per port, which determines switch total power, which determines rack cooling requirements:
| Form factor | Speed | Module power |
|---|---|---|
| QSFP-DD | 400G | 3-4W per port |
| QSFP-DD | 800G | 8-10W per port |
| OSFP | 400G | 5-8W per port |
| OSFP | 800G | 12-15W per port |
| OSFP-XD | 1.6T | 20-25W projected |
A 64-port 800G switch with OSFP modules: 64 x 15W = 960W in optics alone. Add ~500W for the switch ASIC on a 51.2 Tbps chip, and the total switch power approaches 1,500W -- from a device roughly the width of a pizza box.
Eight such switches in a DGX BasePOD compute fabric: 12,000W from switching alone. This is before accounting for the 32 DGX nodes, storage switches, management networking, or out-of-band infrastructure. Optics are not a minor thermal contribution. They are a primary input to rack cooling design.