Choosing the right optic
A decision guide for picking the right transceiver for a link you're building.
Connectivity Planner
Already know the link? Skip the decision tree
The Connectivity Planner answers the four questions below for you. Pick the two devices, the fiber, and the distance — it picks the reach class, fiber sub-grade, and platform-coded SKU automatically.
The four questions
Every transceiver decision reduces to four questions, in this order:
- What speed does the link need to run at?
- What kind of fiber is in the ground / cable tray?
- How far apart are the two endpoints?
- What's the switch platform on each end?
Answer those four and you have a SKU. The rest is logistics.
1. Speed
Match the lowest-bandwidth end of the link. A 25G NIC connected to a 100G switch port runs at 25G; you need 25G optics on both ends or a breakout cable. A 10G server NIC connected to a 40G QSFP+ port runs at 10G, you need either a 10G transceiver with a QSFP+ adapter, or a 40G-to-4×10G breakout.
| Use case | Speed |
|---|---|
| IoT, access points, branch printers | 1G |
| Server access (older), branch core | 10G |
| Modern server NICs | 25G |
| Storage interconnect, spine-leaf (legacy) | 40G |
| Data-center fabric (current) | 100G |
| AI/ML clusters, hyperscaler spines | 200G / 400G / 800G |
| Inter-data-center, metro DCI | 100G / 400G ZR |
2. Fiber type
Look at the fiber jacket color, patch-panel labeling, or cabling drawings.
| Jacket color | Fiber type | Module family |
|---|---|---|
| Aqua | OM3 multi-mode | SR / SR4 / BiDi (limited reach) |
| Pink | OM4 multi-mode | SR / SR4 / BiDi |
| Lime green | OM5 wideband multi-mode | SR / SWDM4 |
| Yellow | OS1/OS2 single-mode | LR, ER, ZR, DWDM |
| Orange | OM1 / OM2 (legacy) | SR with limited reach; LX with mode-conditioning |
If you have no fiber in the ground yet:
- In a building, pull OM4 if you'll stay at 10G–40G–100G inside a single floor, or OS2 single-mode if you might extend the link to a neighboring building.
- Between buildings or floors, OS2 single-mode is almost always the right answer. It's cheaper than OM4, and short single-mode optics (LR, FR4) are now competitively priced.
3. Distance
Measure or estimate the actual length, including patch-panel slack. Then add 20% margin.
| Reach class | Max distance | Fiber type | Typical wavelength |
|---|---|---|---|
| SR | 300 m (OM3) / 400 m (OM4) | Multi-mode | 850 nm |
| LRM (10G) | 220 m on OM1/2/3 (with EDC) | Multi-mode | 1310 nm |
| LR / LR4 | 10 km | Single-mode | 1310 nm |
| ER / ER4 | 40 km | Single-mode | 1550 nm |
| ZR / ZR4 | 80 km | Single-mode | 1550 nm |
| Coherent ZR | 80–120 km+ | Single-mode | C-band DWDM |
| BiDi | 10–40 km | Single-mode (single strand) | 1270/1330 or 1490/1550 nm |
Picking reach
- ≤ 70 m on multi-mode → any SR module
- 100–300 m on multi-mode → SR (OM3) or SR (OM4)
- 300 m–10 km on single-mode → LR / LR4
- 10–40 km → ER / ER4
- 40–80 km → ZR / ZR4
- 80 km+ → coherent ZR or DWDM
Don't overshoot
Running an ER (high transmit power) into a short fiber run can overload the receiver and cause CRC errors. If you need 1 km of single-mode, an LR is the right answer, not an ER. If you absolutely need to send a high-power optic over a short link, add an inline attenuator (3 dB or 5 dB).4. Switch platform
The optical performance doesn't depend on the switch, but whether the switch enables the module does. Each major OEM has its own coding requirements:
| OEM | Coding requirement |
|---|---|
| Cisco IOS-XE / NX-OS | Vendor ID + part number + security signature |
| Juniper Junos | Vendor ID + part number |
| Arista EOS | Vendor ID; permissive in most releases |
| Dell SmartFabric / OS10 | Vendor ID + part number |
| HPE Aruba (CX, AOS-S) | Vendor ID; warns on non-Aruba |
| Mellanox / NVIDIA Cumulus | Permissive |
| Extreme EXOS | Vendor ID for some platforms |
When you order from NetAPI, tell us the exact switch model and the OS version so we can program the EEPROM correctly. Examples:
- "Cisco Catalyst 9300-48T, IOS-XE 17.9.4"
- "Juniper QFX5120-48Y, Junos 22.4R1"
- "Arista 7280R3-48C8, EOS 4.30.4M"
- "Dell S5232F-ON, OS10 10.5.5.0"
Connector type
| Connector | Where it's used |
|---|---|
| LC duplex | Default for SFP/SFP+/SFP28 and most QSFP single-mode (LR4, ER4) |
| MPO/MTP (12-fiber) | QSFP+ SR4, QSFP28 SR4, breakout patch panels |
| MPO/MTP (8-fiber) | 40GBASE-SR4 (4 lanes × 2 fibers each) |
| MPO/MTP (16-fiber) | 100GBASE-SR8, some 400GBASE-DR8 variants |
| RJ45 | 1000BASE-T, 10GBASE-T copper modules |
Before ordering MPO modules
Confirm the polarity (Type A, B, or C) of your trunk cables. Mismatched polarity is the #1 cause of "the SR4 module won't link up" tickets.Worked example, an easy one
I'm running a new link between two MDFs in adjacent buildings, about 600 meters apart, with single-mode fiber already pulled. Switches are Arista 7050X3 on each end.
- Speed: 25G ✓
- Fiber: OS2 single-mode (already pulled) ✓
- Distance: 600m, within reach of any 25G single-mode optic ✓
- Switch: Arista EOS, no vendor lock-in concerns ✓
Answer: 25GBASE-LR (1310 nm, 10 km single-mode), Arista-coded, two units. LC duplex connector. Standard launch power.
Worked example, a harder one
I need to connect a new 100G data-center spine across town to our DR site. There's lit dark fiber from the carrier, 35 km path, no amplifiers, single-mode.
- Speed: 100G ✓
- Fiber: single-mode ✓
- Distance: 35 km, beyond LR4 (10 km), within ER4 (40 km) ✓
- Switch platform: ask the customer (Cisco Nexus? Juniper QFX?)
Answer: 100GBASE-ER4 (4-channel CWDM, 1295/1300/1305/1310 nm, 40 km single-mode), coded for the target platform. Verify receive power at install: ER4 modules transmit at higher power and a 35 km run may need a small attenuator if the fiber is unusually clean. If the link runs long-term at > 100 km or needs to share fiber with other channels, switch to coherent 100ZR.
Ready to build the link?: Open the Connectivity Planner and have the SKU in your cart in 30 seconds.