Breakout & MPO cabling: polarity, Base-8/12/16, pinning

Parallel optics moved the hard part of a link from the module to the cable. A 400G DR4 port needs eight fibres arriving in the right order, with the right polish and the right gender at the connector; a 4 × 100G breakout needs the far ends to be four separate ports. Most "the link is dead but the module is fine" cases on parallel optics are cabling — this page lays out the rules.

Breakout: which ports can split

Parent port	Breakout options	Requires
40G QSFP+ (SR4, PSM4, CR4)	4 × 10G	parallel optics or breakout DAC/AOC; host support
100G QSFP28 (SR4, PSM4, CR4)	4 × 25G, 2 × 50G	as above; FEC per lane consistent
100G QSFP28 (LR4, CWDM4)	none	WDM optics cannot split
200G QSFP56 (SR4, DR4, CR4)	4 × 50G, 2 × 100G	—
400G QSFP-DD / OSFP (DR4, SR8, CR8)	4 × 100G (DR4 → 4 × 100G DR), 8 × 50G, 2 × 200G	CMIS applications listing the breakout; host breakout config
400G FR4 / LR4	none	WDM
800G OSFP (DR8, 2×DR4, 2×FR4)	2 × 400G, 8 × 100G, 4 × 200G	twin-port modules present as two logical ports

Rules: the parent must be parallel (one fibre pair per lane); the host must support breakout on that port (often only on some ports or with neighbouring ports disabled); speed and FEC per child must match the far end; for CMIS modules the breakout must be an advertised application (Speed & rate, CMIS). A DR4 broken out to four DR modules works; broken out to four FR1 modules does not (different wavelength/reach class).

MPO connector basics

Attribute	Options	Rule
Fibre count	MPO-12 (8 or 12 used), MPO-16, MPO-24	SR4/DR4/PSM4 use 8 of 12; SR8/DR8 use MPO-16 (or 2 × MPO-12)
Polish	UPC (MMF and some SMF), APC (all SM parallel: PSM4, DR4, DR8)	never mate APC to UPC
Gender	pinned (male) / unpinned (female)	transceivers are pinned → the patch cord into a module is unpinned; trunks and cassette rear ports are pinned; every mating has exactly one pinned side
Key	key-up / key-down	defines fibre position numbering; polarity methods depend on it
Base	Base-8, Base-12, Base-16	fibre grouping of trunks and cassettes; must match the optics' lane count

Connector details and polish colours: Connectors & fibre.

Polarity methods (TIA-568)

The goal is simple — Tx of lane n must arrive at Rx of lane n — but a trunk, two cassettes or adapters and two patch cords can each flip or not flip the order.

Method	Trunk	Patch cords	Cassettes / adapters	Notes
A	straight (key-up to key-down), fibre 1 → 1	one straight (A-to-B), one flipped (A-to-A) at one end	key-up/key-down adapters	duplex flip done in one patch; easy to get wrong when both patches are the same type
B	key-up to key-up, fibre 1 → 12 (pairwise reversed)	both straight	key-up/key-up	most common for parallel optics (SR4/DR4 direct MPO-to-MPO); every component identical
C	pairwise flipped (1↔2, 3↔4 …)	both straight	—	duplex breakout only; unusable for parallel 8-fibre lanes

For parallel optics (MPO to MPO, module to module) use Method B end to end: trunk Type B, patch cords Type B. Mixing a Type A patch into a Type B channel lands every lane on the wrong fibre — all four lanes dark, both directions (Physical mismatches).

Base-8 vs Base-12 vs Base-16

Base-12 — the legacy MPO-12 trunk with all 12 fibres used, meant for duplex breakout (6 pairs). SR4/DR4 use only 8 → 4 fibres wasted per trunk.
Base-8 — MPO-12 connectors, 8 fibres populated, trunks in multiples of 8; maps 1:1 onto 40G/100G/400G 4-lane optics. Conversion modules or harnesses connect Base-12 trunks to Base-8 optics.
Base-16 — MPO-16 for SR8/DR8/800G; APC for single-mode. Not compatible with MPO-12 without a harness.

Structured cabling patterns

Pattern	Components	Typical use
Direct MPO-to-MPO	one Type B trunk or patch	in-row parallel links, spine cabling
Trunk + cassettes	MPO trunk, MPO-to-LC cassettes both ends	duplex optics over a parallel backbone
Trunk + breakout harness	MPO trunk, MPO-to-4×LC-duplex harness	4 × 25G from a 100G SR4 port to four SFP28 ports
Conversion cassette	Base-12 trunk → Base-8 MPO ports	reusing 12-fibre plant for 8-fibre optics
Breakout DAC/AOC	one QSFP end, four SFP ends	in-rack breakout without fibre plant

Configuration and verification

Configure breakout on the parent port (NOS-specific; often requires a reload or disables adjacent ports) and set child speed/FEC to match the far ends.
Check the module advertises the mode (QSFP28: parallel PMD codes; CMIS: application list) — Check transceiver.
Inspect and clean every MPO end-face (APC scopes for APC).
Verify per-lane Rx power on both ends — a lane at −40 dBm with others fine is a polarity or dead-lane problem (Per-lane diagnostics).
Trace polarity with a visual fault locator through the channel if lanes are swapped.

Common failures

Symptom	Cause
All lanes dark both ways	Type A patch in a Type B channel; pinned-to-pinned mismatch
Lanes 1↔4 swapped	Method C component or mislabelled trunk
One lane dark	broken fibre, contaminated fibre in the array, one dead laser
Works on 8 fibres, fails on 16	MPO-12 cable on MPO-16 optics
High loss on all lanes	APC/UPC mismatch, dirty MPO, Base-12 ferrule with unused fibres against Base-8 optics
Only some child ports link	breakout not enabled for all lanes, FEC mismatch on one child

In CodingBox

CodingBox reads the module side of the equation: connector code (0Ch MPO-12, 28h MPO-16), lane count, breakout-capable applications and per-lane DDM on the bench. If all four lanes light on the bench and the link is dark in the rack, the cable is the suspect.

Breakout configuration syntax per NOS and the port-group rules that limit it: Port configuration recipes, Port types and cages.

How breakout children are named in each NOS and shown on the port LEDs: Port naming & LEDs.