When should FEC be enabled or disabled on 100G optics?

Symptoms

After inserting a 100G transceiver, you might see:

• •Interface stuck in “down” or “err-disabled” state
• •Link flapping repeatedly
• •High CRC or FCS error counters
• •Elevated bit error rate (BER)
• •Packet loss on a link that shows “up”

Nine times out of ten, this is an FEC mismatch between the transceiver and the host device.

Background

Forward Error Correction (FEC) adds redundant data to a signal so the receiver can detect and fix bit errors without asking for retransmission. On 100G links, whether you enable FEC on the host depends on whether the transceiver already handles it internally.

Why some transceivers have built-in FEC

Single-lambda transceivers (FR1, DR1, LR1, ER1) push the full 100G over one wavelength using PAM4 modulation. PAM4 has four signal levels instead of two, which makes it noisier. To deal with that, these transceivers have a DSP with FEC baked in. If you also turn on host FEC, the host's extra encoding breaks the framing and lane alignment the module's DSP expects, which destabilizes the link rather than improving it, often showing up as climbing CRC/FCS counters and intermittent packet loss.

Multi-lambda transceivers (CWDM4, ER4, ZR4, ZR+) split 100G across four wavelengths at 25G each using NRZ modulation. NRZ is simpler, and these transceivers don’t have their own FEC. The host device has to provide it (typically RS-FEC).

The short version

Transceiver has built-in FEC → disable FEC on the host

Transceiver does not have built-in FEC → enable FEC on the host

FEC configuration by transceiver type

Note on ER4: the row above reflects EdgeOptic's QSFP28 ER4 Lite part (100G-QSFP28-40), which needs host FEC for its full 40 km reach. The IEEE 802.3ba Clause 88 ER4 baseline itself does not require FEC at 40 km; EdgeOptic's extended-reach 100G-QSFP28-42 only needs host FEC beyond that 40 km baseline.

Troubleshooting

How to verify

1. Figure out your transceiver type. Run show inventory or show interfaces transceiver on the device.
2. Look it up in the table above.
3. Check FEC status on both ends of the link, not just one.
4. If you changed FEC, bounce the interface (shut / no shut). The change won’t take effect otherwise.

CLI examples by platform

Cisco NX-OS

! Enable FEC

interface Ethernet1/1

fec rs-fec

! Disable FEC

interface Ethernet1/1

no fec

Arista EOS

! Enable FEC

interface Ethernet1

error-correction encoding reed-solomon

! Disable FEC

interface Ethernet1

no error-correction encoding

Juniper Junos

# Enable FEC

set interfaces et-0/0/0 gigether-options fec fec91

# Disable FEC

set interfaces et-0/0/0 gigether-options fec none

SONiC

# Enable FEC

sudo config interface fec Ethernet0 rs

# Disable FEC

sudo config interface fec Ethernet0 none

Checking current FEC state