Choosing the right optic
A decision guide for picking the right transceiver for a link you're building.
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.