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QSFP+ (Quad Small Form-Factor Pluggable Plus) is the workhorse optical form factor for 40 GbE compact, hot-pluggable, and built around 4×10 Gb/s electrical lanes per direction. In data centers, it enables short-reach MMF fabrics, long-reach SMF leaf–spine, and simple 4×10G breakouts.
This guide demystifies QSFP+ types (SR4/CSR4/PLR4/LR4/ER4, BiDi, UNIV, LR4-Lite), clarifies LC vs MPO choices, and compares QSFP+ with CFP so you can pick the right optic the first time.
Product Overview
QSFP+ in One Minute (What it is & How it Works)
- Form factor: Hot-pluggable QSFP+; mechanical/electrical per SFF-8436 (4×10 Gb/s lanes). Typical body ~72.4 × 18.4 × 13.5 mm.
- Where it’s used: 40GBASE-X Ethernet (and 4×10G breakout), aggregation, router/switch uplinks.
- Connector styles:MPO-12 (a.k.a. MTP) for parallel optics (SR4/CSR4/PLR4/PLRL4). LC duplex for serial/WDM optics (BiDi, UNIV, LR4/LR4L/ER4).
- MPO-12 (a.k.a. MTP) for parallel optics (SR4/CSR4/PLR4/PLRL4).
- LC duplex for serial/WDM optics (BiDi, UNIV, LR4/LR4L/ER4).
Quick Check
| Situation | Pick | Why |
| 40G short-reach in-row (≤100 m OM3 / 150 m OM4/OM5) | QSFP-40G-SR4 (MPO-12) | Lowest cost MMF workhorse; broad multi-vendor support. |
| Need ~300–400 m on MMF | QSFP-40G-CSR4 (MPO-12) | Extended SR; 300 m (OM3) / 400 m (OM4/OM5). |
| Re-use existing duplex MMF (10G plant) | QSFP-40G-SR-BiDi (LC) | 40G over one duplex MMF (100 m OM3 / 150 m OM4/OM5). |
| Mixed fiber types / fastest turn-up | QSFP-40G-UNIV (LC) | Works on MMF and SMF; typically 150 m MMF / up to ~2 km SMF (vendor-dependent). |
| Long-reach campus/metro (10 km) | QSFP-40G-LR4 (LC, SMF) | CWDM4 over duplex SMF to 10 km. |
| Very long reach (40 km) | QSFP-40G-ER4 (LC, SMF) | 40 km over SMF with proper link budget. |
| 40G port → 4×10G fan-out | SR4/PLR4 + MPO breakout | Parallel optics support 4×10G on separate fibers. |
40G Optics Lineup & Distances Tables
| Type | Connector | Fiber | Wavelength | Max Distance (typical) | 4×10G Breakout? |
| SR4 | MPO-12 | MMF | 850 nm | 100 m (OM3), 150 m (OM4/OM5) | Yes |
| CSR4 | MPO-12 | MMF | 850 nm | 300 m (OM3), 400 m (OM4/OM5) | Yes |
| PLRL4 | MPO-12 | SMF | 1310 nm (CWDM) | 1–1.4 km | Yes |
| PLR4 | MPO-12 | SMF | 1310 nm (CWDM) | 10 km | Yes |
| BiDi (SR-BD) | LC duplex | MMF | ~832–918 nm pair | 100 m (OM3), 150 m (OM4/OM5) | No |
| UNIV | LC duplex | MMF/SMF | 1310 nm (internal mux) | ~150 m MMF, 0.5–2 km SMF (by vendor) | No |
| LR4-Lite (LR4L/iLR4/IR4) | LC duplex | SMF | 1310 nm | 2 km | No |
| LR4 | LC duplex | SMF | 1310 nm (4 λ) | 10 km | No |
| ER4 | LC duplex | SMF | 1310 nm (4 λ) | 40 km | No |
Connector Choice in Practice: LC vs MPO
- MPO-12 (SR4/CSR4/PLR4/PLRL4): highest fiber-count density, native 4×10G breakout support, ideal for leaf–spine trunks and fan-outs. Requires proper Type-B polarity and clean MPO ferrules.
- LC duplex (BiDi/UNIV/LR4/ER4): simpler jumpers, easier day-2; UNIV lets you standardize on LC while bridging MMF↔SMF.
Model Mapping You Can Order: Cisco | Huawei | Ruijie
| Type (What you need) | Cisco (examples) | Huawei (examples) | Ruijie (examples) |
| SR4 (100 m/150 m) | QSFP-40G-SR4 / -SR4-S | QSFP-40G-SR4 | RG 40GBASE SR4 (series) |
| CSR4 (300 m/400 m) | QSFP-40G-CSR4 / -CSR-S | Availability varies by region | Availability varies |
| BiDi (duplex MMF) | QSFP-40G-SR-BD / QSFP-40G-BD-RX | Use SR4 on MMF where BiDi not offered | Availability varies |
| UNIV (LC on MMF/SMF) | QSFP-40G-UNIV | — | — |
| LR4-Lite ~2 km | WSP-Q40GLR4L | iLR4 / IR4 class variants | RG 40G iLR4 (series) |
| LR4 10 km | QSFP-40G-LR4 / -LR4-S | QSFP-40G-LR4 | 40G-QSFP-LR4-SM1310 (10 km) |
| ER4 40 km | QSFP-40G-ER4 | QSFP-40G-ER4 | RG 40G ER4 (series) |
| 4×10G breakout (SMF) | QSFP-4X10G-LR-S / PLR4 class | SR4 supports 1→4 split to 10G-SR | — |
Deployment Notes
- Polarity matters (MPO): Use Type-B for SR4/CSR4 trunks; ensure pinned/unpinned components match your cassettes.
- Breakout planning: SR4/PLR4 can fan-out to 4×10G; check switch port mode/partitioning.
- Link budgets: Long-reach (LR4/ER4/PLR4) need careful accounting for connectors, splices, and patch leads.
- Cleanliness first: MPO/LC end-faces must be inspected per IEC 61300-3-35—parallel optics are unforgiving.
- UNIV variability: The SMF reach is vendor-specific (commonly 0.5–2 km); confirm the datasheet used for your BOM.
QSFP+ or CFP? Picking the Right Form Factor
| Aspect | QSFP+ | CFP (incl. CFP2/CFP4) |
| Target speeds | 40 GbE (4×10G) | Originally 100 GbE; some 40 G variants |
| Size & density | Much smaller, high front-panel density | Larger; lower port density |
| Typical connectors | MPO-12 or LC | SC/LC (earlier CFP LR/FR used SC) |
| Common 40G flavors | SR4/CSR4, BiDi, UNIV, LR4/LR4L, ER4 | Fewer 40G options; more common at 100G era |
| Today’s usage | Dominant for 40G in DCs | Mostly historical for 40/100G; newer builds favor QSFP28/QSFP-DD |
Answers to Common Questions
Q1: Is 40G BiDi the same performance as SR4?
A: Reach is similar (100 m OM3 / 150 m OM4/OM5) but BiDi uses duplex LC over existing MMF; SR4 uses MPO-12 and supports 4×10G breakout.
Q2: Can I run 40G 10 km on MPO?
A: Yes, PLR4 achieves ~10 km over SMF using MPO-12, often chosen for parallel 4×10G over SMF or specific cabling topologies.
Q3: What’s LR4-Lite (LR4L/iLR4/IR4)?
A: A lower-power, shorter-reach LR4 variant, commonly ~2 km on SMF via LC. Check the vendor label (LR4L/iLR4/IR4) and reach table.
Q4: Why pick QSFP+ over CFP at 40G?
A: QSFP+ is smaller, cheaper, denser with more 40G choices; CFP was early-generation and mostly seen in legacy builds.
Bottom Line & Build Checklist
For 40G inside the rack/row, start with SR4; stretch to CSR4 for 300–400 m on MMF. If you’re re-using duplex MMF, pick BiDi; if you want one optic everywhere, UNIV bridges MMF↔SMF. For campus/metro, use LR4 (10 km) or ER4 (40 km). Plan MPO polarity, verify breakout behavior, and always run a link budget—then your 40G fabric will be plug-and-go.
Pre-flight checks:
- Distances vs fiber class verified
- MPO polarity & pinning documented
- Clean/inspect end-faces before turn-up
- Switch port modes (40G vs 4×10G) confirmed
- BOM SKUs matched to platform/OS support
Cheat Sheet: QSFP+ Types & Distances
| Type | Fiber | Connector | MMF Reach | SMF Reach | Notes |
| SR4 | MMF | MPO-12 | 100 m (OM3) / 150 m (OM4/OM5) | — | Parallel 4×10G capable |
| CSR4 | MMF | MPO-12 | 300 m (OM3) / 400 m (OM4/OM5) | — | Extended SR |
| BiDi | MMF | LC | 100 m (OM3) / 150 m (OM4/OM5) | — | Uses duplex MMF plant |
| UNIV | MMF/SMF | LC | ~150 m (MMF) | ~0.5–2 km (SMF) | Vendor-specific; verify datasheet |
| PLRL4 | SMF | MPO-12 | — | 1–1.4 km | Parallel LR-Lite |
| PLR4 | SMF | MPO-12 | — | 10 km | 4×10G LR over MPO |
| LR4-Lite | SMF | LC | — | 2 km | Also called iLR4/IR4 |
| LR4 | SMF | LC | — | 10 km | CWDM4 |
| ER4 | SMF | LC | — | 40 km | Long-haul 40G |
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