1. Switch and NIC platforms
The Asteraix CX732Q-N is a 1U, 32-port 400G QSFP-DD switch built on the Marvell Teralynx 7 ASIC. Switching capacity is 12.8 Tbps, packet buffer is 70 MB, forwarding rate is 7,600 Mpps, and cut-through latency is sub-500 ns. AsterNOS (enterprise SONiC) is preinstalled with RoCEv2, PFC, ECN, DCBX, BGP-EVPN, VXLAN, and MLAG. Typical roles are spine and super-spine in AI/HPC fabrics, and increasingly the accept-layer switch for 400G storage nodes.
On the storage side, the two NICs in production today are NVIDIA ConnectX-7 and BlueField-3.
ConnectX-7 ships in three port configurations:
Single-port 400Gb/s, OSFP cage, RHS (riding heatsink) shell only.
Dual-port 200Gb/s each, QSFP112 cage — this is the highest-volume ConnectX-7 SKU.
Dual-port 100Gb/s or lower, QSFP56/SFP56 — outside the scope of this article.
BlueField-3 uses QSFP112 cages exclusively; it never ships with an OSFP cage. Depending on model, it appears as a single 400GbE/NDR port (e.g. B3140H) or two QSFP112 ports running 200GbE/NDR200 each, with some dual-port models supporting 400GbE per port (B3240). For this article, "BlueField-3 port" means a QSFP112 cage running at up to 400G per port.
Both ConnectX-7 and BlueField-3 use RHS (riding heatsink) cage shells only — this matters when picking modules, covered in Section 6.
2. Why a passive 400G DAC will not connect switch to NIC
CX732Q-N's QSFP-DD cage electrical interface is 8 lanes at 50G PAM4. ConnectX-7 and BlueField-3's OSFP/QSFP112 electrical interface is 4 lanes at 100G PAM4. A passive copper cable carries signal without retiming or lane remapping, so both ends need matching lane count and per-lane rate. 8×50G and 4×100G don't match, so a straight passive DAC between the switch and either NIC is not an option, regardless of reach.
Two connection methods remain:
Module pair over fiber — one optical module in the switch, one optical module (or NIC-side module) in the NIC, connected by patch fiber. Each module's internal DSP handles the lane remap on its own side.
Active cable (AOC/AEC) — a single cable with a transceiver-equivalent engine at each end, already remapped for the connector type on either side. No separate modules, no patch fiber.
3. CX732Q-N side modules (in stock)
Part | Spec |
|---|---|
OT-400G-QDD-DR4 | 400G QSFP56-DD DR4, MPO-12/APC, 1310nm, single-mode, 500m |
OT-400G-QDD-SR4 | 400G QSFP56-DD SR4, MPO-12/APC, 850nm, OM4, 100m |
OT-400G-QDD-FR4 | 400G QSFP56-DD FR4, duplex LC, 1310nm, single-mode, 2km |
These three are the switch-side half of every module-pair link described below.
4. ConnectX-7 single-port OSFP (400G)
NIC side needs an OSFP112, RHS-shell module, matched to the switch-side QSFP-DD module by reach and wavelength.
Module pair:
Reach | Switch side | NIC side |
|---|---|---|
500m, single-mode | OT-400G-QDD-DR4 | OT-400G-OSFP112R-DR4 |
100m, OM4 | OT-400G-QDD-SR4 | OT-400G-OSFP112R-SR4 |
Active cable, one cable end to end:
AOC-400G-QDD-OSFP112R, QSFP-DD end into the switch, OSFP112-RHS end into the NIC, 1–50m. No separate modules or patch fiber required.
5. ConnectX-7 dual-port QSFP112 (2×200G, highest-volume SKU)
Each of the two ports carries 200G independently. Do not treat the pair as a single 400G link, and do not plug a 400G module into either port.
Switch 400G port broken out to the NIC's two 200G ports:
AOC-400G-QDD-2*QSFP: one QSFP-DD end into the switch, breaking out to two QSFP56 ends, one into each NIC port, 1–100m.
NIC to a device with native 200G QSFP56 ports (module pair):
Reach | Both ends |
|---|---|
100m, OM4 | OT-200G-QSFP-SR4 |
2km, single-mode | OT-200G-QSFP-FR4 |
Short-reach copper:
DAC-200G-QSFP, 200G QSFP56 to 200G QSFP56, 1–3m. Only for direct connections where both ends are native 200G QSFP56 ports — this cable cannot connect to CX732Q-N's 400G QSFP-DD port (see Section 2).
6. BlueField-3 QSFP112 port (up to 400G)
NIC side needs a QSFP112 module (non-RHS shell) — do not confuse this with the OSFP112-RHS parts used for ConnectX-7's single-port OSFP variant in Section 4; the cage and shell are different even though both are "112" family optics.
Module pair:
Reach | Switch side | NIC side |
|---|---|---|
100m, OM4 | OT-400G-QDD-SR4 | OT-400G-QSFP-SR4 |
2km, single-mode | OT-400G-QDD-FR4 | OT-400G-QSFP-FR4 |
Short reach, lower cost:
OT-400G-QSFP-VR4, 50m, OM4. VR4 uses a different optical engine than SR4 and cannot mix with SR4 modules on the same link. Confirm switch-side VR4 stock before ordering.
Active cable, one cable end to end:
AOC-400G-QSFP-QDD, QSFP-DD end into the switch, QSFP112 end into the NIC, 1–50m.
7. Legacy 100G storage controllers
Some storage arrays still run 100G QSFP28 host ports. DAC-100G-QSFP (100G QSFP28 to 100G QSFP28, 1–5m) connects two such ports directly. It requires native 100G QSFP28 on both ends and cannot connect to CX732Q-N's 400G QSFP-DD port. Arrays in this category typically attach through a separate 100G leaf switch rather than directly to CX732Q-N.
8. Three deployment mistakes that correct part numbers don't prevent
Matching the part numbers in Sections 4–7 gets the cage and reach right. Three other mismatches account for most bring-up failures on this link, and none of them show up by reading the part number alone.
Heat sink and form factor: finned vs. flat-top
Switch-side OSFP cages — CX732Q-N included, along with most open network switches and NVIDIA Quantum/Spectrum switches — take OSFP modules with an integrated top heatsink, cooled by the switch chassis's own forced-air fans. This is the finned shell. ConnectX-7 and BlueField-3 sit inside a PCIe server chassis and rely on the server's own airflow instead, so their OSFP cage takes the flat-top shell — RHS, riding heatsink, the OSFP112R parts used in Section 4. A finned module does not fit a ConnectX-7 or BlueField-3 PCIe card: it causes mechanical interference, blocks full insertion, and can damage the NIC's cage. The NIC side always takes the RHS/flat-top part number; the finned part is for the switch side only.
FEC mode: RS-FEC(544,514) on both ends
400G PAM4 links depend on forward error correction to hold bit error rate within spec — unlike 25G/100G NRZ links, FEC here is not optional. CX732Q-N's port and the ConnectX-7/BlueField-3 driver both need to negotiate RS-FEC(544,514) explicitly. If one side defaults to FEC off, or negotiates a different mode such as FC-FEC or interleaved RS-FEC, the link does not hold: symptom is either an immediate link-down or a link that comes up and then drops packets continuously. Check FEC mode on both ends before treating a bring-up failure as a cabling or module problem.
Connector polish: MPO-12/APC vs. MPO-12/UPC
DR4 and SR4 links (Sections 4–7) run over MPO-12 patch cables. 400G optics are built for APC (angled physical contact, 8°) connectors on that patch, not UPC (ultra physical contact, flat). A UPC patch cable plugged into an APC transceiver creates an optical return loss mismatch — reflected light degrades PAM4 signal integrity, causes intermittent link flap, and can shorten the transmitter's service life over time. Confirm the patch cable is APC, not UPC, before connecting either end.
9. Selection reference table
NIC port | Scenario | Switch side | NIC side |
|---|---|---|---|
Single-port OSFP | 500m, single-mode | ||
Single-port OSFP | 100m, OM4 | ||
Single-port OSFP | One cable, ≤50m | same | |
Dual-port 2×200G | Breakout to switch, ≤100m | same | |
Dual-port 2×200G | Native 200G peer, 100m OM4 | OT-200G-QSFP-SR4 | |
Dual-port 2×200G | Native 200G peer, 2km single-mode | OT-200G-QSFP-FR4 | |
Dual-port 2×200G | ≤3m copper | same | |
QSFP112 (BlueField-3) | 100m, OM4 | ||
QSFP112 (BlueField-3) | 2km, single-mode | ||
QSFP112 (BlueField-3) | 50m, OM4, lower cost | confirm switch-side VR4 stock | |
QSFP112 (BlueField-3) | One cable, ≤50m | same |
What not to do
Do not connect switch and NIC with a passive 400G DAC. The electrical lane structure doesn't match on the two ends (Section 2).
Do not plug a 400G QSFP112 module into a ConnectX-7 dual-port 2×200G cage, or the reverse. Each 200G port is an independent link.
SR8 (MPO-16), ZR4, and LR4 are out of scope — neither ConnectX-7 nor BlueField-3 has a matching NIC-side module for these.
Do not mix VR4 and SR4 modules on the same link. Both use MPO-12/APC and OM4, but the optical engines are different and won't interoperate.
Do not order a finned OSFP module for the NIC side. ConnectX-7's OSFP cage takes RHS (riding heatsink) shells only; finned modules are built for switch panels, not NIC brackets.
Don't leave FEC mode unchecked after a link failure — confirm RS-FEC(544,514) is negotiated on both ends before assuming a cabling or module fault.
Don't patch an APC transceiver with a UPC cable — the mismatch degrades return loss and can damage the transmitter.
