InfiniBand optics & cables

InfiniBand uses the same pluggable housings as high-speed Ethernet, but the choice of cable versus transceiver — and the tolerance for third-party modules — follows its own rules shaped by HPC and AI clusters.

Cable or transceiver, by generation

Generation	Port	Copper DAC	Active optical cable	Transceiver + fibre
EDR 100G	QSFP28	up to ~3–5 m	up to 100 m	SR4 (MPO-12 MMF), LR4
HDR 200G	QSFP56	up to ~2 m	up to 100 m	SR4/DR4
NDR 400G	OSFP (twin-port 2×400G)	~1–2 m	limited	DR4/DR8 on MPO-12 APC single-mode

The pattern: as rates rise, passive copper reach collapses and fabrics move to optics. At NDR the norm is single-mode DR optics with MPO-12 APC connectors — the angled polish is mandatory, and mixing APC with flat UPC connectors is a common installation fault.

Twin-port OSFP

NDR switch ports are commonly twin-port OSFP: one OSFP module carries two independent 400G links (2×DR4), doubling faceplate density. A finned integrated heat sink handles the power. See OSFP.

Signalling and FEC

EDR: NRZ 25G lanes; HDR: PAM4 50G; NDR: PAM4 100G.
RS-FEC is mandatory from HDR onward. FEC mismatch or a module that does not

advertise the expected FEC is a frequent "link never trains" cause.

Vendor policy

InfiniBand fabrics are dominated by a single switch/NIC vendor, and that vendor's platforms validate cables and optics by identity. A module that is not recognised may read correctly yet fail to link with a status such as "speed and type: Not supported" — see Vendor lock. Operators therefore stock qualified optics and, where third-party modules are used, code them to the expected identity.

Operating at scale

Thousands of optics per cluster: even a small annual failure rate means a steady

stream of replacements. Track per-lane DDM to catch a dying lane before it slows a collective operation.

Cleanliness on MPO end-faces is the dominant cause of flaps — see

CMIS issues for 400G specifics.

In CodingBox

CodingBox reads EDR/HDR modules per SFF-8636 and NDR OSFP/QSFP-DD per CMIS: identity, module state, per-lane DDM, and the identity fields that determine whether a fabric will accept the module.