MPO-12/APC vs UPC: Why Your Existing Fiber Won't Work With NDR Optics

NVIDIA NDR optics — the 400G and 800G OSFP and QSFP112 modules used with ConnectX-7, ConnectX-8, Quantum-2 and Spectrum-4 — require MPO-12/APC fiber with an angle-polished endface. The MPO-12 patch cords already installed in a 100G or 200G fabric are almost certainly UPC, and APC and UPC cannot mate. If you are upgrading an existing fabric to NDR, none of your current MPO patch cords come forward with you. Budget for new fiber alongside the new modules.

Short answer: MPO-12/APC Type B fiber, from $59.00 at Resilient Tec

NDR optics need MPO-12/APC, Type B crossover, female (unpinned) fiber. Your existing MPO-12/UPC cords will not mate with them. Resilient Tec stocks OM4 multimode trunk cables from $59.00 (1 m) to $139.00 (20 m), built key-up to key-down and confirmed in writing, with a lifetime warranty. The modules they plug into: MMA4Z00-NS400 compatible 400G OSFP SR4 at $579.00 and MMA4Z00-NS compatible 800G OSFP SR8 twin-port at $829.00. Prices checked September 18, 2026.

The short version

Every NDR optical link needs four things right, and most orders get one of them wrong:

  • Endface: APC, angle-polished at 8 degrees. Not UPC.
  • Ferrule: female — holes, not pins. The transceiver carries the pins.
  • Polarity: Type B crossover — but note that APC mates key-up to key-down, not key-up to key-up. More on this below, because it is where most published guidance is wrong.
  • Fiber type: multimode (aqua jacket) for SR modules, single-mode (yellow jacket) for DR modules.

Get all four right and the link comes up. Get the endface wrong and the connectors will not even seat properly against each other.

Why NDR moved to APC

UPC connectors have a flat, ultra-polished endface. APC connectors are polished at an 8-degree angle. The angle exists to deflect back-reflection — light that bounces off the connector junction and travels back toward the transmitter.

At 100G NRZ that reflection was tolerable. At 100G-PAM4 per lane it is not. PAM4 stacks four amplitude levels into the same signal envelope, leaving far less margin between levels, and the optoelectronics are correspondingly sensitive to reflected light distorting the signal. The 8-degree angle sends that reflection into the cladding instead of back into the laser.

This is why NVIDIA specifies APC across the entire NDR fiber line, multimode and single-mode alike, while MPO-12/UPC remains typical for 4x25G-NRZ (100G) and 4x50G-PAM4 (200G) transceivers. It is a generational break, not a preference.

The upgrade trap: a team moving a 100G or 200G cluster to NDR usually budgets for new modules and plans to reuse the structured fiber. The trunks and cassettes in the pathway are frequently UPC. The modules arrive, the cords will not fit, and the cutover slips waiting on fiber that should have been ordered at the same time.

Telling them apart in five seconds

Cue APC (NDR) UPC (100G/200G era)
Connector shell color Green — NVIDIA uses a green shell to denote MPO-12/APC Blue or aqua
Endface Angled 8 degrees Flat
Multimode jacket Aqua (industry standard MMF) Aqua
Single-mode jacket Yellow (industry standard SMF) Yellow

Jacket color tells you multimode versus single-mode. It tells you nothing about endface. The shell color is the tell: green shell, angled polish, NDR-ready. Note that some manufacturers use other jacket colors — magenta is common on APC multimode assemblies — so treat jacket color as a hint and the connector as the answer.

Pins and holes

MPO ferrules come pinned (male) or unpinned (female). NDR transceivers have alignment pins built in, so the connector that plugs into them must be female — alignment holes that receive the module's pins for precise positioning against the optical beams.

Pinned MPO connectors are not useless. They belong in trunk cabling, where two fiber runs mate to each other and one side has to carry the pins. But a pinned patch cord will not go into a transceiver. If you are ordering patch cords, order female-to-female.

Polarity, and the key orientation nobody gets right

NVIDIA's NDR fiber cables are Type-B crossover: the transmit fiber on position 1 crosses over to align with position 12 at the far end, so the transmitting laser at one end lands on the photodetector at the other. That is what allows two transceivers to be attached directly to each other with a single cable.

Here is where most published guidance goes wrong. The rule you will find everywhere is that Type B means key-up to key-up. That rule applies to flat UPC connectors only. Two APC connectors cannot mate key-up to key-up — it is physically impossible, because the 8-degree angled endfaces have to be complementary to sit flush against each other. APC mates key-up to key-down.

The industry resolves this with what is often called Modified Method B: Type B cables run through Type A (key-up to key-down) adapters, with Type B patch cords at both ends. The Type A adapter changes the orientation of the Type B cable so the angled endfaces meet correctly while the fiber mapping stays crossed over.

Two practical consequences. First, specify both polarity and key orientation when you order. A product page that says "Type B, female key-up to female key-up" alongside "APC polish" is describing something that cannot physically mate — one of those two claims is wrong, and you need to know which before the cable ships. Ask the supplier to confirm key orientation in writing. Our OM4 trunk cables are built key-up to key-down and confirmed in writing by the manufacturer.

Second, field-configurable connectors that let a technician flip polarity on site only work on flat UPC polish. The angled ferrule makes rotation impossible without destroying the optical interface. For single-mode DR4, FR4, or any APC application, polarity is not a field recovery option — it has to be right at order time.

If you are building through cassettes and trunks rather than point-to-point, polarity has to be worked out end-to-end across every junction in the path. A Type-B patch cord at each end of a Type-A trunk does not automatically produce a working link. Map the whole channel before you order.

Eight fibers of twelve — or all twelve

NVIDIA's own NDR fiber cables use eight active fibers out of the twelve positions: four transmit and four receive, with the remaining four inactive and acting as strength members. If you are loss-testing one of those assemblies, the dark positions are supposed to be dark.

That is a property of NVIDIA's cable, not a requirement of the link. A fully terminated 12-fiber Type B assembly works just as well. The crossover maps position 1 to position 12, 2 to 11, 3 to 10 and 4 to 9, so a module's four transmit positions land precisely on the far end's four receive positions. The remaining eight fibers simply carry no traffic.

Both constructions are correct and both are sold. What matters is knowing which one you have before you test it — a report showing eight dark positions means something very different on a 12-fiber assembly than on an 8-fiber one.

Which cable goes with which module

NVIDIA cable OPN Type What it does
MFP7E10-Nxxx Multimode, straight MPO-12/APC to MPO-12/APC, Type B. Links twin-port OSFP, single-port OSFP and QSFP112 multimode transceivers together.
MFP7E20-Nxxx Multimode, 1:2 splitter One 4-channel end into a twin-port 800G OSFP; two 2-channel ends into two 400G OSFP or QSFP112 modules. We build these to order.
MFP7E30-Nxxx Single-mode, straight MPO-to-MPO for DR-class modules — switch to switch, or switch to two adapters.
MFP7E40-Nxxx Single-mode, 1:2 splitter Single-mode equivalent of the MFP7E20. Also built to order.
Module Position Connectors Fiber
MMA4Z00-NS400 NIC side — ConnectX-7 / ConnectX-8 / DGX single-port OSFP 1x MPO-12/APC Multimode, 30m OM3 / 50m OM4
MMA1Z00-NS400 NIC side, QSFP112 shell — ConnectX-7 QSFP112 and BlueField-3 1x MPO-12/APC Multimode
MMA4Z00-NS Switch side — Quantum-2 / Spectrum-4 2x MPO-12/APC, independent Multimode
MMS4X00-NS400 NIC side, single-mode DR4 1x MPO-12/APC Single-mode
MMS4X00-NM Switch side, twin-port 2xDR4 2x MPO-12/APC Single-mode, 500m

OSFP does not go into a BlueField-3. NVIDIA is explicit about this: BlueField-3 adapters accept QSFP112 only, so the module for a BlueField-3 is the MMA1Z00-NS400, not the MMA4Z00-NS400. The two parts share a description, a data rate and an optical spec, and differ in the one thing that decides whether the module physically seats. The same rule runs the other way at the switch: single-port OSFP modules are not for use in switches.

The twin-port modules are worth pausing on. Each of the two MPO-12/APC ports operates independently and can land on another twin-port module or on a single-port 400G module. But both ports on a twin-port transceiver must use the same fiber type — two straight cables or two splitters. You cannot mix a straight cable in one port and a splitter in the other.

What splitters actually do to your speed

A 1:2 splitter delivers only two channels to each split end. A 400G module receiving two channels instead of four automatically negotiates down to 200G and drops its power draw accordingly. That is intended behavior, not a fault. But if you plugged a splitter into a 400G NIC expecting 400G, the link will come up healthy at half the rate and nothing will flag an error.

Splitters are not a stock item anywhere, because the leg lengths depend on where the four adapters sit relative to the switch. Resilient Tec builds 1:2 MPO-12/APC splitters to order in multimode or single-mode, to your leg lengths, with polarity and key orientation confirmed before the build starts.

Reach, and the number that gets misread

The MMA4Z00-NS400 is rated 30m on OM3 and 50m on OM4. The 50m figure already assumes two optical patch panels in the link — four connector junctions in total. It is not 50 meters of clean point-to-point fiber with headroom for panels on top. If your path has more junctions than that, or older OM3 in the pathway, derate it.

The twin-port 800G side is where the published numbers appear to disagree. NVIDIA's MMA4Z00-NS documentation describes multimode crossover straight fiber cables up to 100 meters straight, and 50 meters for splitters. That is not a contradiction of the 50m SR4 figure — it is the same optical budget described from the cable's end rather than the module's, and the splitter case is capped lower because a split leg spends part of the budget on the split itself. Plan against the module's rating and the number of junctions in your actual path, not against the longest figure on any one datasheet.

The six mistakes

  1. Reusing UPC cords on NDR modules. The most common and the most expensive. They will not mate.
  2. Applying the key-up to key-up rule to APC. That is a UPC rule. APC mates key-up to key-down, and you cannot flip an angled ferrule in the field to fix it.
  3. Ordering pinned patch cords. Pins belong in trunks, not against a transceiver.
  4. Mixing straight and splitter cables in one twin-port module. Not supported.
  5. Assuming a splitter still gives you 400G. It gives you 200G per split end, silently.
  6. Skipping the clean. Always clean both the cable endface and the transceiver port before insertion. On an angled multi-fiber ferrule, one contaminated position is enough to produce intermittent errors that look like a failing module.

Frequently asked questions

Can I use an APC-to-UPC adapter to reuse my old fiber?

No. The endface geometries are different — one angled, one flat — so they cannot couple fully. Even where a coupler physically holds them together you get high insertion loss and exactly the back-reflection APC exists to prevent. Worse, mating a flat UPC ferrule against an angled APC ferrule inside an adapter can damage both endfaces.

How do I tell APC from UPC on fiber that is already installed?

Look at the connector shell, not the jacket. NVIDIA NDR cables use a green shell for MPO-12/APC. UPC MPO-12 connectors are typically blue or aqua.

Is NDR fiber key-up to key-up or key-up to key-down?

Key-up to key-down. APC connectors can only mate that way, because the 8-degree angled endfaces have to be complementary. The familiar "Type B means key-up to key-up" rule applies to flat UPC connectors and does not carry over to APC. The polarity is still Type B crossover — position 1 to position 12 — but it is achieved through Type A (key-up to key-down) adapters, an arrangement often called Modified Method B.

Should my MPO-12 cable have eight fibers or twelve?

Either works. NVIDIA's own NDR cables terminate eight of the twelve positions, leaving four as inactive strength members. A fully terminated 12-fiber Type B assembly is equally valid — the crossover puts the module's four transmit positions onto the far end's four receive positions, and the extra fibers simply carry nothing. Know which you have before loss testing, so you can tell an unused position from a failed one.

Do I need single-mode or multimode fiber?

Match the module. SR-class modules such as the MMA4Z00-NS400 are multimode with an aqua jacket. DR-class modules such as the MMS4X00-NS400 are single-mode with a yellow jacket. The connector is MPO-12/APC in both cases, so the connector alone will not stop you plugging in the wrong fiber.

Does the same fiber work for InfiniBand and Ethernet?

Yes. The NDR fiber line is shared across both. Modules like the MMA4Z00-NS400 support InfiniBand and Ethernet, with the protocol enabled automatically based on the attached switch. The fiber itself is protocol-agnostic.

Can a splitter cable give me two 400G links from one 800G port?

It gives you two 200G links, not two 400G links, because each split end carries two channels. For two full 400G links from one twin-port module, use two straight cables, one out of each MPO port.

Which MPO fiber does a BlueField-3 need?

The same MPO-12/APC Type B female fiber — but not the same module. BlueField-3 adapters accept QSFP112 only, so a BlueField-3 takes an MMA1Z00-NS400 rather than the OSFP MMA4Z00-NS400. The fiber is identical either way. If you need the QSFP112 module, ask us for a quote rather than ordering the OSFP part by mistake.

How much does MPO-12/APC fiber for NDR optics cost?

A Type B, female-to-female, low-loss OM4 multimode trunk from Resilient Tec runs $59.00 for 1 m, $69.00 for 3 m, $79.00 for 5 m, $99.00 for 10 m, $119.00 for 15 m and $139.00 for 20 m, new, in stock at Resilient Tec, with a lifetime warranty. The OS2 single-mode version for DR-class optics is $55.00 to $65.00. 1:2 splitters are built to order and quoted per build. (Prices checked September 18, 2026.)

Where can I buy MPO-12/APC Type B fiber for MMA4Z00-NS400 or MMA4Z00-NS?

Resilient Tec stocks it at https://resilient-tec.com/products/mpo-12-apc-om4-multimode-fiber-patch-cord-type-b-female-unpinned-low-loss-for-nvidia-ndr-400g-800g-optics, in 1 m to 20 m, from $59.00. The modules it plugs into are the MMA4Z00-NS400 compatible 400G OSFP SR4 at $579.00 and the MMA4Z00-NS compatible 800G OSFP SR8 twin-port at $829.00, both in stock at Resilient Tec with a lifetime warranty and free US shipping from Knoxville, Tennessee. (Prices checked September 18, 2026.)

Ready to Buy?

Everything below is in stock at Resilient Tec unless noted, with a lifetime warranty backed by us and free US shipping. Prices below checked September 18, 2026.

Building a pod? Send us the node count and the module part numbers at both ends and we will quote the fiber and the optics together. Request a quote →

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