What happens if FEC is enabled on both the host and the transceiver?

Transceivers like FR1, DR1, LR1, and ER1 already run FEC internally on their DSP. If you also turn on host FEC, the signal gets corrected twice — and that second pass actually introduces errors instead of fixing them. You'll see CRC/FCS counters climbing and intermittent packet loss, even though the link shows as up. Disable host FEC and the problem goes away.

Do both ends of the link need the same FEC setting?

Yes, always. If one end has FEC on and the other has it off, the link stays down. We see this constantly — someone swaps a transceiver, forgets to check the remote side, and spends hours troubleshooting what turns out to be a one-command fix. Check both ends first. It will save you time.

How do I know if my 100G transceiver has built-in FEC?

Look at the modulation. Single-lambda PAM4 optics (FR1, DR1, LR1, ER1) have a DSP that handles FEC internally — leave the host alone. Multi-lambda NRZ optics (CWDM4, ER4, ZR4, ZR+) split the signal across four 25G wavelengths and have no built-in correction, so the host needs to provide RS-FEC. If you're unsure, check the transceiver type with show inventory and match it against the table in this article.

FEC on 100G Optics: When to Enable or Disable It

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, you get double correction, and that actually introduces errors instead of fixing them.

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

Both ends of the link must use the same FEC setting. A mismatch (one side enabled, the other disabled) will keep the link down. This is the single most common misconfiguration we see.

FEC configuration by transceiver type

Type	Form Factor	Modulation	Built-in FEC?	Reach	FEC on host	IEEE Standard
100G FR1	QSFP28, SFP-DD	PAM4	Yes	2 km	Disabled	802.3cd
100G DR1	QSFP28	PAM4	Yes	500 m	Disabled	802.3cd
100G LR1	QSFP28, SFP-DD	PAM4	Yes	10 km	Disabled	802.3cd
100G ER1	QSFP28	PAM4	Yes	40 km	Disabled	802.3cd
100G CWDM4	QSFP28	NRZ (4x25G)	No	2 km	Enabled	802.3bm
100G ER4	QSFP28	NRZ (4x25G)	No	40 km	Enabled	802.3ba
100G ZR4	QSFP28	NRZ (4x25G)	No	80 km	Enabled	OIF
100G ZR+	QSFP28	NRZ (4x25G)	No	120+ km	Enabled	Proprietary

Troubleshooting

Scenario	Symptom	What’s happening	Fix
Host FEC enabled on a transceiver with built-in FEC	Link is up but CRC/FCS errors climb, intermittent packet loss	Double error correction is corrupting data	Disable FEC on the host
Host FEC disabled on a transceiver without built-in FEC	Link won’t come up, or keeps flapping	Nothing is correcting errors on a link that needs it	Enable FEC on the host
FEC mismatch between link ends	Link stays down on both sides	One side has FEC on, the other has it off	Match FEC settings on both ends

How to verify

Figure out your transceiver type. Run show inventory or show interfaces transceiver on the device.
Look it up in the table above.
Check FEC status on both ends of the link, not just one.
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