100G and 400G Optics: LR4, ER4 and FR4 Reach Guide

Choosing an optic is a four-variable problem — speed, form factor, reach class and connector — and getting any one wrong produces a link that either does not come up or comes up and then flaps. The suffix after the lane count is the part that decides everything about distance and fibre type. This article lays out the reach classes for 100G and 400G, the connector each uses, and the signal-integrity details (FEC, power budget) that decide whether a marginal link is stable.

The naming convention, decoded

[speed][form factor]-[reach][lanes]
100G   QSFP28   - LR4            (4 x 25G)
400G   QSFP-DD  - FR4            (8 x 50G PAM4)

The distinction between FR4 and LR4 matters commercially: both are 4-wavelength WDM over duplex single-mode, but FR4 uses CWDM4 wavelengths to 2 km while LR4 uses LAN-WDM to 10 km and costs noticeably more. If your run is under 2 km, FR4 is the better value.

Typical link budgets and power draw

100G SR4    850 nm      budget ~1.9 dB     ~2.5 W    4x25G
100G CWDM4  1271-1331   budget ~5.0 dB     ~3.5 W    4x25G
100G LR4    LAN-WDM     budget ~8.5 dB     ~3.5-4.5 W
100G ER4    LAN-WDM     budget ~18 dB      ~4.5-5.0 W
400G DR4    1310 nm     budget ~3.0 dB     ~10-12 W   8x50G PAM4
400G FR4    CWDM4       budget ~4.0 dB     ~10-12 W
400G LR4    LAN-WDM     budget ~6.3 dB     ~12 W

These are indicative engineering figures; always confirm against the specific part number. The pattern to notice is that longer reach buys budget, and the budget is consumed by fibre attenuation, connectors and splices — exactly the same arithmetic as any other optical link.

FEC, and why some links need it

• 100G NRZ classes (SR4, LR4) generally work without FEC, though RS-FEC is often enabled for margin on long or lossy runs.
• 400G PAM4 classes practically require RS-FEC: the signal-to-noise ratio penalty of PAM4 makes it mandatory rather than optional.
• Both ends must agree. A host that expects RS-FEC facing a host with FEC disabled will not bring the link up, and the interface counters will show nothing useful — check show interfaces ... transceiver and the switch's FEC configuration first.

Reading DOM values before you blame the optics

! Arista EOS
show interfaces Ethernet1/1 transceiver
show interfaces Ethernet1/1 transceiver detail

! Cisco IOS / IOS XE
show interfaces GigabitEthernet1/1 transceiver detail
show interfaces GigabitEthernet1/1 transceiver | include Rx|Tx

! Nokia / generic SFP families
show port 1/1/1 transceiver

Receive power is the number to read first. A value inside the specification but within a dB of the minimum is a link that will drop out in summer, when the cabinet gets hot. TX power outside the window indicates a failing module, and a 0.00 dBm reading on one lane of an MPO module usually means a dirty or unterminated lane in the breakout.

Failure patterns worth recognising

• Link comes up, then flaps intermittently — marginal receive power, temperature, or a mismatched FEC setting. Check DOM history, not just the current value.
• Link never comes up on an MPO-connected port — polarity. A straight-through MPO patch cord on a link that needs a crossover will produce exactly nothing on the receive lane.
• One of four lanes down on a breakout — a single fibre in the ribbon, a dirty connector, or a module fault. Swap the breakout to distinguish.
• Works at 100G, not at 400G on the same fibre — budget. PAM4 has a lower tolerance and the extra insertion loss of the MPO path may be the difference.
• Vendor coding / compatibility lockout — some platforms refuse third-party modules. Verify before ordering in volume.

Connector and cable discipline decides more of these links than any configuration: see QSFP-DD vs OSFP form factors for the mechanical side, and copper cable certification for the DAC twinax equivalent, where the same loss arithmetic applies at much shorter distances.

原文链接：https://sanoc.com.tw/2026/06/23/qsfp-qsfp28-transceiver-guide-en